NQC Programmer's Guide

Version 2.5 a4, by Dave Baum


1 Introduction
2 The NQC Language

2.1 Lexical Rules
2.1.2 Whitespace
2.1.3 Numerical Constants
2.1.4 Identifiers and Keywords
2.2 Program Structure
2.2.1 Tasks
2.2.2 Functions
2.2.3 Subroutines
2.2.4 Variables
2.2.5 Arrays
2.3 Statements
2.3.1 Variable Declaration
2.3.2 Assignment
2.3.3 Control Structures
2.3.4 Access Control and Events
2.3.5 Other Statements
2.4 Expressions
2.4.1 Conditions
2.5 The Preprocessor
2.5.1 #include
2.5.2 #define
2.5.3 Conditional Compilation
2.5.4 Program Initialization
2.5.5 Reserving Storage
3.1 Sensors
3.1.1 Types and Modes RCX, CyberMaster
3.1.2 Sensor Information
3.1.3 Scout Light Sensor Scout
3.1.4 Spybotics Sensors Spy
3.2 Outputs
3.2.1 Primitive Calls
3.2.2 Convenience Calls
3.2.3 Global Control RCX2, Scout
3.2.4 Spybotics Outputs
3.3 Sound
3.4 LCD Display RCX
3.5 Communication
3.5.1 Messages RCX, Scout
3.5.2 Serial RCX2
3.5.3 VLL Scout
3.6 Timers
3.7 Counters RCX2, Scout, Spy
3.8 Access Control RCX2, Scout, Spy
3.9 Events RCX2, Scout, Spy
3.9.1 Configurable Events RCX2, Spy
3.9.2 Scout Events Scout
3.10 Data Logging RCX
3.11 General Features
3.12 RCX Specific Features
3.13 Scout Specific Features
3.14 CyberMaster Specific Features
4 Technical Details
4.1 The asm statement
4.2 Data Sources

1 Introduction

NQC stands for Not Quite C, and is a simple language for programming several LEGO MINDSTORMS products. Some of the NQC features depend on which MINDSTORMS product you are using. This product is referred to as the target for NQC. Presently, NQC supports five different targets: RCX, RCX2 (an RCX running 2.0 firmware), CyberMaster, Scout, and Spybotics.

All of the targets have a bytecode interpreter (provided by LEGO) which can be used to execute programs. The NQC compiler translates a source program into LEGO bytecodes, which can then be executed on the target itself. Although the preprocessor and control structures of NQC are very similar to C, NQC is not a general purpose language - there are many restrictions that stem from limitations of the LEGO bytecode interpreter.

Logically, NQC is defined as two separate pieces. The NQC language describes the syntax to be used in writing programs. The NQC API describes the system functions, constants, and macros that can be used by programs. This API is defined in a special file built in to the compiler. By default, this file is always processed before compiling a program.

This document describes both the NQC language and the NQC API. In short, it provides the information needed to write NQC programs. Since there are several different interfaces for NQC, this document does not describe how to use any specific NQC implementation. Refer to the documentation provided with the NQC tool, such as the NQC User Manual for information specific to that implementation.

For up-to-date information and documentation for NQC, visit the NQC Web Site

2 The NQC Language

This section describes the NQC language itself. This includes the lexical rules used by the compiler, the structure programs, statements, and expressions, and the operation of the preprocessor.

2.1 Lexical Rules

The lexical rules describe how NQC breaks a source file into individual tokens. This includes the way comments are written, then handling of whitespace, and valid characters for identifiers.


Two forms of comments are supported in NQC. The first form (traditional C comments) begin with /* and end with */. They may span multiple lines, but do not nest:

/* this is a comment */
/* this is a two
line comment */
/* another comment...
/* trying to nest...
ending the inner comment...*/
this text is no longer a comment! */

The second form of comments begins with // and ends with a newline (sometimes known as C++ style comments).

// a single line comment

Comments are ignored by the compiler. Their only purpose is to allow the programmer to document the source code.

2.1.2 Whitespace

Whitespace (spaces, tabs, and newlines) is used to separate tokens and to make programs more readable. As long as the tokens are distinguishable, adding or subtracting whitespace has no effect on the meaning of a program. For example, the following lines of code both have the same meaning:

x = 2 ;

Some of the C++ operators consist of multiple characters. In order to preserve these tokens whitespace must not be inserted within them. In the example below, the first line uses a right shift operator ('>>'), but in the second line the added space causes the '>' symbols to be interpreted as two separate tokens and thus generate an error.

x = 1 >> 4; // set x to 1 right shifted by 4 bits
x = 1 > > 4; // error

2.1.3 Numerical Constants

Numerical constants may be written in either decimal or hexadecimal form. Decimal constants consist of one or more decimal digits. Hexadecimal constants start with 0x or 0X followed by one or more hexadecimal digits.

x = 10; // set x to 10
x = 0x10; // set x to 16 (10 hex)

2.1.4 Identifiers and Keywords

Identifiers are used for variable, task, and function names. The first character of an identifier must be an upper or lower case letter or the underscore ('_'). Remaining characters may be letters, numbers, an underscore.

A number of potential identifiers are reserved for use in the NQC language itself. These reserved words are call keywords and may not be used as identifiers. A complete list of keywords appears below:


2.2 Program Structure

An NQC program is composed of code blocks and global variables. There are three distinct types of code blocks: tasks, inline functions, and subroutines. Each type of code block has its own unique features and restrictions, but they all share a common structure.

2.2.1 Tasks

The RCX implicitly supports multi-tasking, thus an NQC task directly corresponds to an RCX task. Tasks are defined using the task keyword using the following syntax:

task name()
  // the task's code is placed here

The name of the task may be any legal identifier. A program must always have at least one task - named "main" - which is started whenever the program is run. The maximum number of tasks depends on the target - the RCX supports 10 tasks, CyberMaster supports 4, and Scout supports 6.

The body of a task consists of a list of statements. Tasks may be started and stopped using the start and stop statements (described in the section titled Statements). There is also an RCX API command, StopAllTasks, which stops all currently running tasks.

2.2.2 Functions

It is often helpful to group a set of statements together into a single function, which can then be called as needed. NQC supports functions with arguments, but not return values. Functions are defined using the following syntax:

void name(argument_list)
  // body of the function

The keyword void is an artifact of NQC's heritage - in C functions are specified with the type of data they return. Functions that do not return data are specified to return void. Returning data is not supported in NQC, thus all functions are declared using the void keyword.

The argument list may be empty, or may contain one or more argument definitions. An argument is defined by its type followed by its name. Multiple arguments are separated by commas. All values in the RCX are represented as 16 bit signed integers. However NQC supports four different argument types which correspond to different argument passing semantics and restrictions:





pass by value


const int

pass by value

only constants may be used


pass by reference

only variables may be used

const int &

pass by reference

function cannot modify argument

Arguments of type int are passed by value from the calling function to the callee. This usually means that the compiler must allocate a temporary variable to hold the argument. There are no restrictions on the type of value that may be used. However, since the function is working with a copy of the actual argument, any changes it makes to the value will not be seen by the caller. In the example below, the function foo attempts to set the value of its argument to 2. This is perfectly legal, but since foo is working on a copy of the original argument, the variable y from main task remains unchanged.

void foo(int x)
  x = 2;

task main()
  int y = 1; // y is now equal to 1
  foo(y);    // y is still equal to 1!

The second type of argument, const int, is also passed by value, but with the restriction that only constant values (e.g. numbers) may be used. This is rather important since there are a number of RCX functions that only work with constant arguments.

void foo(const int x)
  PlaySound(x); // ok
  x = 1;        // error - cannot modify argument

task main()
  foo(2);   // ok
  foo(4*5); // ok - expression is still constant
  foo(x);   // error - x is not a constant

The third type, int &, passes arguments by reference rather than by value. This allows the callee to modify the value and have those changes visible in the caller. However, only variables may be used when calling a function using int & arguments:

void foo(int &x)
  x = 2;

task main()
  int y = 1; // y is equal to 1
  foo(y);    // y is now equal to 2
  foo(2);    // error - only variables allowed

The last type, const int &, is rather unusual. It is also passed by reference, but with the restriction that the callee is not allowed to modify the value. Because of this restriction, the compiler is able to pass anything (not just variables) to functions using this type of argument. In general this is the most efficient way to pass arguments in NQC.

There is one important difference between int arguments and const int & arguments. An int argument is passed by value, so in the case of a dynamic expression (such as a sensor reading), the value is read once then saved. With const int & arguments, the expression will be re-read each time it is used in the function:

void foo(int x)
  if (x==x) // this will always be true

void bar(const int &x)
  if (x==x) // may not be true..value could change

task main()
  foo(SENSOR_1);  // will play sound
  bar(2);         // will play sound
  bar(SENSOR_1);  // may not play sound

Functions must be invoked with the correct number (and type) of arguments. The example below shows several different legal and illegal calls to function foo:

void foo(int bar, const int baz)
  // do something here...

task main()
  int x;     // declare variable x
  foo(1, 2); // ok
  foo(x, 2); // ok
  foo(2, x); // error - 2nd argument not constant!
  foo(2);    // error - wrong number of arguments!

NQC functions are always expanded as inline functions. This means that each call to a function results in another copy of the function's code being included in the program. Unless used judiciously, inline functions can lead to excessive code size.

2.2.3 Subroutines

Unlike inline functions, subroutines allow a single copy of some code to be shared between several different callers. This makes subroutines much more space efficient than inline functions, but due to some limitations in the RCX, subroutines have some significant restrictions. First of all, subroutines cannot use any arguments. Second, a subroutine cannot call another subroutine. Last, the maximum number of subroutines is limited to 8 for the RCX, 4 for CyberMaster, 3 for Scout and 32 for Spybotics. In addition, if the subroutine is called from multiple tasks then it cannot have any local variables or perform calculations that require temporary variables (this restriction is lifted for the Scout and RCX2). These significant restrictions make subroutines less desirable than functions, therefore their use should be minimized to those situations where the resultant savings in code size is absolutely necessary. The syntax for a subroutine appears below:

sub name()
  // body of subroutine

2.2.4 Variables

All variables in NQC are of the same type - specifically 16 bit signed integers. Variables are declared using the int keyword followed by a comma separated list of variable names and terminated by a semicolon (';'). Optionally, an initial value for each variable may be specified using an equals sign ('=') after the variable name. Several examples appear below:

int x;     // declare x
int y,z;   // declare y and z
int a=1,b; // declare a and b, initialize a to 1

Global variables are declared at the program scope (outside any code block). Once declared, they may be used within all tasks, functions, and subroutines. Their scope begins at declaration and ends at the end of the program.

Local variables may be declared within tasks, functions, and sometimes within subroutines. Such variables are only accessible within the code block in which they are defined. Specifically, their scope begins with their declaration and ends at the end of their code block. In the case of local variables, a compound statement (a group of statements bracketed by { and }) is considered a block:

int x;     // x is global

task main()
  int y;   // y is local to task main
  x = y;   // ok
  {        // begin compound statement
    int z; // local z declared
    y = z; // ok
  y = z;   // error - z no longer in scope

task foo()
  x = 1;   // ok
  y = 2;   // error - y is not global

In many cases NQC must allocate one or more temporary variables for its own use. In some cases a temporary variable is used to hold an intermediate value during a calculation. In other cases it is used to hold a value as it is passed to a function. These temporary variables deplete the pool of variables available to the rest of the program. NQC attempts to be as efficient as possible with temporary variables (including reusing them when possible).

The RCX (and other targets) provide a number of storage locations which can be used to hold variables in an NQC program. There are two kinds of storage locations - global and local. When compiling a program, NQC assigns each variable to a specific storage location. Programmers for the most part can ignore the details of this assignment by following two basic rules:

• If a variable needs to be in a global location, declare it as a global variable.

• If a variable does not need to be a global variable, make it as local as possible. This gives the compiler the most flexibility in assigning an actual storage location.

The number of global and local locations varies by target



















2.2.5 Arrays

The RCX2 and Spybotics targets supports arrays (the other targets do not have suitable support in firmware for arrays). Arrays are declared the same way as ordinary variables, but with the size of the array enclosed in brackets. The size must be a constant.

int my_array[3]; // declare an array with three elements

The elements of an array are identified by their position within the array (called an index). The first element has an index of 0, the second has index 1, etc. For example:

my_array[0] = 123; // set first element to 123
my_array[1] = my_array[2]; // copy third into second

Currently there are a number of limitations on how arrays can be used. These limitations will likely be removed in future versions of NQC:

• An array cannot be an argument to a function. An individual array element, however, can be passed to a function.

• Neither arrays nor their elements can be used with the increment (++) or decrement (--) operators.

• Only ordinary assignment (=) is allowed for array elements. The math assignments (i.e. +=) are not allowed.

• The initial values for an array's elements cannot be specified - an explicit assignment is required within the program itself to set the value of an element.

2.3 Statements

The body of a code block (task, function, or subroutine) is composed of statements. Statements are terminated with a semi-colon (';').

    2.3.1 Variable Declaration

    Variable declaration, as described in the previous section, is one type of statement. It declares a local variable (with optional initialization) for use within the code block. The syntax for a variable declaration is:

    int variables;

    where variables is a comma separated list of names with optional initial values:


    Arrays of variables may also be declared (for the RCX2 only):

    int array[size];

    2.3.2 Assignment

    Once declared, variables may be assigned the value of an expression:

    variable assign_operator expression;

    There are nine different assignment operators. The most basic operator, '=', simply assigns the value of the expression to the variable. The other operators modify the variable's value in some other way as shown in the table below




    Set variable to expression


    Add expression to variable


    Subtract expression from variable


    Multiple variable by expression


    Divide variable by expression


    Set variable to remainder after dividing by expression


    Bitwise AND expression into variable


    Bitwise OR expression into variable


    Bitwise exclusive OR into variable


    Set variable to absolute value of expression


    Set variable to sign (-1,+1,0) of expression


    Right shift variable by a constant amount


    Left shift variable by a constant amount

    Some examples:

    x = 2;  // set x to 2
    y = 7;  // set y to 7
    x += y; // x is 9, y is still 7

    2.3.3 Control Structures

    The simplest control structure is a compound statement. This is a list of statements enclosed within curly braces ('{' and '}'):

      x = 1;
      y = 2;

    Although this may not seem very significant, it plays a crucial role in building more complicated control structures. Many control structures expect a single statement as their body. By using a compound statement, the same control structure can be used to control multiple statements.

    The if statement evaluates a condition. If the condition is true it executes one statement (the consequence). An optional second statement (the alternative) is executed if the condition is false. The two syntaxes for an if statement is shown below.

    if (condition) consequence

    if (condition) consequence else alternative

    Note that the condition is enclosed in parentheses. Examples are shown below. Note how a compound statement is used in the last example to allow two statements to be executed as the consequence of the condition.

    if (x==1) y = 2;

    if (x==1) y = 3; else y = 4;

    if (x==1) { y = 1; z = 2; }

    The while statement is used to construct a conditional loop. The condition is evaluated, and if true the body of the loop is executed, then the condition is tested again. This process continues until the condition becomes false (or a break statement is executed). The syntax for a while loop appears below:

    while (condition) body

    It is very common to use a compound statement as the body of a loop:

    while(x < 10)
      x = x+1;
      y = y*2;

    A variant of the while loop is the do-while loop. Its syntax is:

    do body while (condition)

    The difference between a while loop and a do-while loop is that the do-while loop always executes the body at least once, whereas the while loop may not execute it at all.

    Another kind of loop is the for loop:

    for(stmt1 ; condition ; stmt2) body

    A for loop always executes stmt1, then it repeatedly checks the condition and while it remains true executes the body followed by stmt2. The for loop is equivalent to:



    The repeat statement executes a loop a specified number of times:

    repeat (expression) body

    The expression determines how many times the body will be executed. Note that it is only evaluated a single time, then the body is repeated that number of times. This is different from both the while and do-while loops which evaluate their condition each time through the loop.

    A switch statement can be used to execute one of several different blocks of code depending on the value of an expression. Each block of code is preceded by one or more case labels. Each case must be a constant and unique within the switch statement. The switch statement evaluates the expression then looks for a matching case label. It will then execute any statements following the matching case until either a break statement or the end of the switch is reaches. A single default label may also be used - it will match any value not already appearing in a case label. Technically, a switch statement has the following syntax:

    switch (expression) body

    The case and default labels are not statements in themselves - they are labels that precede statements. Multiple labels can precede the same statement. These labels have the following syntax

    case constant_expression :

    default :

    A typical switch statement might look like this:

      case 1:
        // do something when X is 1

      case 2:
      case 3:
        // do something else when x is 2 or 3

        // do this when x is not 1, 2, or 3

    The goto statement forces a program to jump to the specified location. Statements in a program can be labeled by preceding them with an identifier and a colon. A goto statement then specifies the label which the program should jump to. For example, this is how an infinite loop that increments a variable could be implemented using goto:

      goto my_loop;

    The goto statement should be used sparingly and cautiously. In almost every case, control structures such as if, while, and switch make a program much more readable and maintainable than using goto. Care should be taken to never use a goto to jump into or out of a monitor or acquire statement. This is because monitorand acquire have special code that normally gets executed upon entry and exit, and a goto will bypass that code probably resulting in undesirable behavior.

    NQC also defines the until macro which provides a convenient alternative to the while loop. The actual definition of until is:

    #define until(c) while(!(c))

    In other words, until will continue looping until the condition becomes true. It is most often used in conjunction with an empty body statement:

    until(SENSOR_1 == 1); // wait for sensor to be pressed

    2.3.4 Access Control and Events

    The Scout, RCX2, and Spybotics support access control and event monitoring. Access control allows a task to request ownership of one or more resources. In NQC, access control is provided by the acquire statement, which has two forms:

    acquire ( resources ) body

    acquire ( resources ) body catch handler

    where resources is a constant that specifies the resources to be acquired and body and handler are statements. The NQC API defines constants for individual resources which may be added together to request multiple resources at the same time. The behavior of the acquire statement is as follows: Ownership of the specified resources will be requested. If another task of higher priority already owns the resources, then the request will fail and execution will jump to the handler (if present). Otherwise, the request will succeed, and the body will begin to be executed. While executing the body, if another task of equal or higher priority requests any of the owned resources, then the original task will lose ownership. When ownership is lost, execution will jump to the handler (if present). Once the body has completed, the resources will be returned back to the system (so that lower priority tasks may acquire them), and execution will continue with the statement following the acquire statement. If a handler is not specified, then in both the case of a failed request, or a subsequent loss of ownership, control will pass to the statement following the acquire statement. For example, the following code acquires a resource for 10 seconds, playing a sound if it cannot complete successfully:



    Event monitoring is implemented with the monitor statement, which has a syntax very similar to acquire:

    monitor ( events ) body

    monitor ( events ) body handler_list

    Where handler_list is one or more handlers of the form

    catch ( catch_events ) handler

    The last handler in a handler list can omit the event specification:

    catch handler

    Events is a constant that determines which events should be monitored. For the Scout, events are predefined, so there are constants such as EVENT_1_PRESSED which can be used to specify events. With RCX2, the meaning of each event is configured by the programmer. There are 16 events (numbers 0 to 15). In order to specify an event in a monitor statement, the event number must be converted to an event mask using the EVENT_MASK() macro. The Scout event constants or event masks may be added together to specify multiple events. Multiple masks can be combined using bitwise OR.

    The monitor statement will execute the body while monitoring the specified events. If any of the events occur, execution will jump to the first handler for that event (a handler without an event specification handles any event). If no event handler exists for the event, then control will continue at the statement following the monitor statement. The following example waits for 10 seconds while monitoring events 2, 3, and 4 for RCX2:

    monitor( EVENT_MASK(2) | EVENT_MASK(3) | EVENT_MASK(4) )

    catch ( EVENT_MASK(4) )
      PlaySound(SOUND_DOWN); // event 4 happened

      PlaySound(SOUND_UP); // event 2 or 3 happened

    Note that the acquire and monitor statements are only supported for targets that implement access control and event monitoring - specifically the Scout and RCX2.

    2.3.5 Other Statements

    A function (or subroutine) call is a statement of the form:


    The arguments list is a comma separated list of expressions. The number and type of arguments supplied must match the definition of the function itself.

    Tasks may be started or stopped with the following statements:

    start task_name;

    stop task_name;

    Within loops (such as a while loop) the break statement can be used to exit the loop and the continue statement can be used to skip to the top of the next iteration of the loop. The break statement can also be used to exit a switch statement.



    It is possible to cause a function to return before it reaches the end of its code using the return statement.


    Any expression is also a legal statement when terminated by a semicolon. It is rare to use such a statement since the value of the expression would then be discarded. The one notable exception is expressions involving the increment (++) or decrement (--) operators.


    The empty statement (just a bare semicolon) is also a legal statement.

2.4 Expressions

Earlier versions of NQC made a distinction between expressions and conditions. As of version 2.3, this distinction was eliminated: everything is an expression, and there are now conditional operators for expressions. This is similar to how C/C++ treats conditional operations.

Values are the most primitive type of expressions. More complicated expressions are formed from values using various operators. The NQC language only has two built in kinds of values: numerical constants and variables. The RCX API defines other values corresponding to various RCX features such as sensors and timers.

Numerical constants in the RCX are represented as 16 bit signed integers. NQC internally uses 32 bit signed math for constant expression evaluation, then reduces to 16 bits when generating RCX code. Numeric constants can be written as either decimal (e.g. 123) or hexadecimal (e.g. 0xABC). Presently, there is very little range checking on constants, so using a value larger than expected may have unusual effects.

Two special values are predefined: true and false. The value of false is zero, while the value of true is only guaranteed to be non-zero. The same values hold for relational operators (e.g. <): when the relation is false, the value is 0, otherwise the value is non-zero.

Values may be combined using operators. Several of the operators may only be used in evaluating constant expressions, which means that their operands must either be constants, or expressions involving nothing but constants. The operators are listed here in order of precedence (highest to lowest).








Absolute value

Sign of operand






++, --

Increment, decrement


variables only

x++ or ++x




Unary minus

Bitwise negation (unary)

Logical negation




constant only




*, /, %

Multiplication, division, modulo



x * y

+, -

Addition, subtraction



x + y

<<, >>

Left and right shift


constant only

123 << 4

<, >, <=, >=

relational operators



x < y

==, !=

equal to, not equal to



x == 1


Bitwise AND



x & y


Bitwise XOR


constant only

123 ^ 4


Bitwise OR



x | y


Logical AND



x && y


Logical OR



x || y

? :

conditional value



x==1 ? y : z

Where needed, parentheses may be used to change the order of evaluation:

x = 2 + 3 * 4;   // set x to 14
y = (2 + 3) * 4; // set y to 20

2.4.1 Conditions

Conditions are generally formed by comparing two expressions. There are also two constant conditions - true and false - which always evaluate to true or false respectively. A condition may be negated with the negation operator, or two conditions combined with the AND and OR operators. The table below summarizes the different types of conditions.




always true


always false


true if expr is not equal to 0

expr1 == expr2

true if expr1 equals expr2

expr1 != expr2

true if expr1 is not equal to expr2

expr1 < expr2

true if one expr1 is less than expr2

expr1 <= expr2

true if expr1 is less than or equal to expr2

expr1 > expr2

true if expr1 is greater than expr2

expr1 >= expr2

true if expr1 is greater than or equal to expr2

! condition

logical negation of a condition - true if condition is false

cond1 && cond2

logical AND of two conditions (true if and only if both conditions are true)

cond1 || cond2

logical OR of two conditions (true if and only if at least one of the conditions are true)

2.5 The Preprocessor

The preprocessor implements the following directives: #include, #define, #ifdef, #ifndef, #if, #elif, #else, #endif, #undef. Its implementation is fairly close to a standard C preprocessor, so most things that work in a generic C preprocessor should have the expected effect in NQC. Significant deviations are listed below.

2.5.1 #include

The #include command works as expected, with the caveat that the filename must be enclosed in double quotes. There is no notion of a system include path, so enclosing a filename in angle brackets is forbidden.

#include "foo.nqh" // ok
#include <foo.nqh> // error!

2.5.2 #define

The #define command is used for simple macro substitution. Redefinition of a macro is an error (unlike in C where it is a warning). Macros are normally terminated by the end of the line, but the newline may be escaped with the backslash ('\') to allow multi-line macros:

#define foo(x) do { bar(x); \
                    baz(x); } while(false)

The #undef directive may be used to remove a macro’s definition.

2.5.3 Conditional Compilation

Conditional compilation works similar to the C preprocessor. The following preprocessor directives may be used:

#if condition
#ifdef symbol
#ifndef symbol
#elif condition

Conditions in #if directives use the same operators and precedence as in C. The defined() operator is supported.

2.5.4 Program Initialization

The compiler will insert a call to a special initialization function, _init, at the start of a program. This default function is part of the RCX API and sets all three outputs to full power in the forward direction (but still turned off). The initialization function can be disabled using the #pragma noinit directive:

#pragma noinit // don't do any program initialization

The default initialization function can be replaced with a different function using the #pragma init directive.

#pragma init function // use custom initialization

2.5.5 Reserving Storage

The NQC compiler automatically assigns variables to storage locations. However, sometimes it is necessary to prevent the compiler from using certain storage locations. This can be done with the #pragma reserve directive:

#pragma reserve start

#pragma reserve start end

This directive forces the compiler to ignore one or more storage locations during variable assignment. Start and end must be numbers that refer to valid storage locations. If only a start is provided, then that single location is reserved. If start and end are both specified, then the range of locations from start to end (inclusive) are reserved. The most common use of this directive is to reserve locations 0, 1, and/or 2 when using counters for RCX2. This is because the RCX2 counters are overlapped with storage locations 0, 1, and 2. For example, if all three counters were going to be used:

#pragma reserve 0 1 2


The NQC API defines a set of constants, functions, values, and macros that provide access to various capabilities of the target such as sensors, outputs, timers, and communication. Some features are only available on certain targets. Where appropriate, a section's title will indicate which targets it applies to. The RCX2 is a superset of RCX features, so if RCX is listed, then the feature works with both the original firmware and 2.0 firmware. If RCX2 is listed, then the feature only applies to the 2.0 firmware. CyberMaster, Scout, and Spybotics are indicated by CM, Scout, and Spy respectively.

The API consists of functions, values, and constants. A function is something that can be called as a statement. Typically it takes some action or configures some parameter. Values represent some parameter or quantity and can be used in expressions. Constants are symbolic names for values that have special meanings for the target. Often, a set of constants will be used in conjunction with a function. For example, the PlaySound function takes a single argument which determines which sound is to be played. Constants, such as SOUND_UP, are defined for each sound.

3.1 Sensors

There are three sensors, which internally are numbered 0, 1, and 2. This is potentially confusing since they are externally labeled as sensors 1, 2, and 3. To help mitigate this confusion, the sensor names SENSOR_1, SENSOR_2, and SENSOR_3 have been defined. These sensor names may be used in any function that requires a sensor as an argument. Furthermore, the names may also be used whenever a program wishes to read the current value of the sensor:

x = SENSOR_1; // read sensor and store value in x

3.1.1 Types and Modes RCX, CyberMaster

The sensor ports on the RCX are capable of interfacing to a variety of different sensors (other targets don't support configurable sensor types). It is up to the program to tell the RCX what kind of sensor is attached to each port. A sensor's type may be configured by calling SetSensorType. . There are four sensor types, each corresponding to a specific LEGO sensor. A fifth type (SENSOR_TYPE_NONE) can be used for reading the raw values of generic passive sensors. In general, a program should configure the type to match the actual sensor. If a sensor port is configured as the wrong type, the RCX may not be able to read it accurately.

Sensor Type



generic passive sensor


a touch sensor


a temperature sensor


a light sensor


a rotation sensor

The RCX, CyberMaster, and Spybotics allow a sensor to be configured in different modes. The sensor mode determines how a sensor's raw value is processed. Some modes only make sense for certain types of sensors, for example SENSOR_MODE_ROTATION is useful only with rotation sensors. The sensor mode can be set by calling SetSensorMode. The possible modes are shown below. Note that since CyberMaster does not support temperature or rotation sensors, the last three modes are restricted to the RCX only. Spybotics is even more restrictive, allowing only raw, boolean, and percentage modes.

Sensor Mode



raw value from 0 to 1023


boolean value (0 or 1)


counts number of boolean transitions


counts number of boolean periods


value from 0 to 100


degrees F - RCX only


degrees C - RCX only


rotation (16 ticks per revolution) - RCX only

When using the RCX, it is common to set both the type and mode at the same time. The SetSensor function makes this process a little easier by providing a single function to call and a set of standard type/mode combinations.

Sensor Configuration
























The RCX provides a boolean conversion for all sensors - not just touch sensors. This boolean conversion is normally based on preset thresholds for the raw value. A "low" value (less than 460) is a boolean value of 1. A high value (greater than 562) is a boolean value of 0. This conversion can be modified: a slope value between 0 and 31 may be added to a sensor's mode when calling SetSensorMode. If the sensor's value changes more than the slope value during a certain time (3ms), then the sensor's boolean state will change. This allows the boolean state to reflect rapid changes in the raw value. A rapid increase will result in a boolean value of 0, a rapid decrease is a boolean value of 1.

Even when a sensor is configured for some other mode (i.e. SENSOR_MODE_PERCENT), the boolean conversion will still be carried out.

SetSensor(sensor, configuration) Function - RCX

Set the type and mode of the given sensor to the specified configuration, which must be a special constant containing both type and mode information.


SetSensorType(sensor, type) Function - RCX

Set a sensor's type, which must be one of the predefined sensor type constants.


SetSensorMode(sensor, mode) Function - RCX, CyberMaster, Spy

Set a sensor's mode, which should be one of the predefined sensor mode constants. A slope parameter for boolean conversion, if desired, may be added to the mode (RCX only).

SetSensorMode(SENSOR_1, SENSOR_MODE_RAW); // raw mode
SetSensorMode(SENSOR_1, SENSOR_MODE_RAW + 10); // slope 10

ClearSensor(sensor) Function - All

Clear the value of a sensor - only affects sensors that are configured to measure a cumulative quantity such as rotation or a pulse count.


3.1.2 Sensor Information

There are a number of values that can be inspected for each sensor. For all of these values the sensor must be specified by its sensor number (0, 1, or 2), and not a sensor name (e.g. SENSOR_1).

SensorValue(n) Value - All

Returns the processed sensor reading for sensor n, where n is 0, 1, or 2. This is the same value that is returned by the sensor names (e.g. SENSOR_1).

x = SensorValue(0); // read sensor 1

SensorType(n) Value - RCX, CM, Scout

Returns the configured type of sensor n, which must be 0, 1, or 2. Only the RCX has configurable sensors types, other targets will always return the pre-configured type of the sensor.

x = SensorType(0);

SensorMode(n) Value - RCX, CyberMaster, Spy

Returns the current sensor mode for sensor n, which must be 0, 1, or 2.

x = SensorMode(0);

SensorValueBool(n) Value - RCX

Returns the boolean value of sensor n, which must be 0, 1, or 2. Boolean conversion is either done based on preset cutoffs, or a slope parameter specified by calling SetSensorMode.

x = SensorValueBool(0);

SensorValueRaw(n) Value - RCX, Scout, Spy

Returns the raw value of sensor n, which must be 0, 1, or 2. Raw values may range from 0 to 1023 (RCX) or 0 to 255 (Scout).

x = SensorValueRaw(0);

3.1.3 Scout Light Sensor Scout

On the Scout, SENSOR_3 refers to the built-in light sensor. Reading the light sensor's value (with SENSOR_3) will return one of three levels: 0 (dark), 1 (normal), or 2 (bright). The sensor's raw value can be read with SensorValueRaw(SENSOR_3), but bear in mind that brighter light will result in a lower raw value. The conversion of the sensor's raw value (between 0 and 1023) to one of the three levels depends on three parameters: lower limit, upper limit, and hysteresis. The lower limit is the smallest (brightest) raw value that is still considered normal. Values below the lower limit will be considered bright. The upper limit is the largest (darkest) raw value that is considered normal. Values about this limit are considered dark.

Hysteresis can be used to prevent the level from changing when the raw value hovers near one of the limits. This is accomplished by making it a little harder to leave the dark and bright states than it is to enter them. Specifically, the limit for moving from normal to bright will be a little lower than the limit for moving from bright back to normal. The difference between these two limits is the amount of hysteresis. A symmetrical case holds for the transition between normal and dark.

SetSensorLowerLimit(value) Function - Scout

Set the light sensor's lower limit. Value may be any expression.


SetSensorUpperLimit(value) Function - Scout

Set the light sensor's upper limit. Value may be any expression.


SetSensorHysteresis (value) Function - Scout

Set the light sensor's hysteresis. Value may be any expression.


CalibrateSensor() Function - Scout

Reads the current value of the light sensor, then sets the upper and lower limits to 12.5% above and below the current reading, and sets the hysteresis to 3.12% of the reading.


3.1.4 Spybotics Sensors Spy

Spybotics uses built-in sensors instead of externally connected ones. The touch sensor on the front of the Spybotics brick is SENSOR_1. It is normally configured in percentage mode, so it has a value of 0 when not pressed, and a value of 100 when pressed. SENSOR_2 is the light sensor (the connector on the back of the brick that is used to communicate with a computer). It is normally configured in percentage mode, where higher numbers indicate brighter light.

3.2 Outputs 3.2.1 Primitive Calls

All of the functions dealing with outputs take a set of outputs as their first argument. This set must be a constant. The names OUT_A, OUT_B, and OUT_C are used to identify the three outputs. Multiple outputs can be combined by adding individual outputs together. For example, use OUT_A+OUT_B to specify outputs A and B together. The set of outputs must always be a compile time constant (it cannot be a variable).

Each output has three different attributes: mode, direction, and power level. The mode can be set by calling SetOutput(outputs, mode). The mode parameter should be one of the following constants:

Output Mode



output is off (motor is prevented from turning)


output is on (motor will be powered)


motor can "coast"

The other two attributes, direction and power level, may be set at any time, but only have an effect when the output is on. The direction is set with the SetDirection(outputs, direction) command. The direction parameter should be one of the following constants:




Set to forward direction


Set to reverse direction


Switch direction to the opposite of what it is presently

The power level can range 0 (lowest) to 7 (highest). The names OUT_LOW, OUT_HALF, and OUT_FULL are defined for use in setting power level. The level is set using the SetPower(outputs, power) function.

Be default, all three motors are set to full power and the forward direction (but still turned off) when a program starts.

SetOutput(outputs, mode) Function - All

Set the outputs to the specified mode. Outputs is one or more of OUT_A, OUT_B, and OUT_C. Mode must be OUT_ON, OUT_OFF, or OUT_FLOAT.

SetOutput(OUT_A + OUT_B, OUT_ON); // turn A and B on

SetDirection(outputs, direction) Function - All

Set the outputs to the specified direction. Outputs is one or more of OUT_A, OUT_B, and OUT_C. Direction must be OUT_FWD, OUT_REV, or OUT_TOGGLE.

SetDirection(OUT_A, OUT_REV); // make A turn backwards

SetPower(outputs, power) Function - All

Sets the power level of the specified outputs. Power may be an expression, but should result in a value between 0 and 7. The constants OUT_LOW, OUT_HALF, and OUT_FULL may also be used.

SetPower(OUT_A, OUT_FULL); // A full power

SetPower(OUT_B, x);

OutputStatus(n) Value - All

Returns the current output setting for motor n. Note that n must be 0, 1, or 2 - not OUT_A, OUT_B, or OUT_C.

x = OutputStatus(0); // status of OUT_A

3.2.2 Convenience Calls

Since control of outputs is such a common feature of programs, a number of convenience functions are provided that make it easier to work with the outputs. It should be noted that these commands do not provide any new functionality above the SetOutput and SetDirection commands. They are merely convenient ways to make programs more concise.

On(outputs) Function - All

Turn specified outputs on. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.

On(OUT_A + OUT_C); // turn on outputs A and C

Off(outputs) Function - All

Turn specified outputs off. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.

Off(OUT_A); // turn off output A

Float(outputs) Function - All

Make outputs float. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.

Float(OUT_A); // float output A

Fwd(outputs) Function - All

Set outputs to forward direction. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.


Rev(outputs) Function - All

Set outputs to reverse direction. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.


Toggle(outputs) Function - All

Flip the direction of the outputs. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.


OnFwd(outputs) Function - All

Set outputs to forward direction and turn them on. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.


OnRev(outputs) Function - All

Set outputs to reverse direction and turn them on. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together.


OnFor(outputs, time) Function - All

Turn outputs on for a specified amount of time, then turn them off. Outputs is one or more of OUT_A, OUT_B, and OUT_C added together. Time is measures in 10ms increments (one second = 100) and may be any expression.

OnFor(OUT_A, x);

3.2.3 Global Control RCX2, Scout, Spy

SetGlobalOutput(outputs, mode) Function - RCX2, Scout, Spy

Disable or re-enable outputs depending on the mode parameter. If mode is OUT_OFF, then the outputs will be turned off and disabled. While disabled any subsequent calls to SetOutput() (including convenience functions such as On()) will be ignored. Using a mode of OUT_FLOAT will put the outputs in float mode before disabling them. Outputs can be re-enabled by calling SetGlobalOutput() with a mode of OUT_ON. Note that enabling an output doesn't immediately turn it on - it just allows future calls to SetOutput() to take effect.

SetGlobalOutput(OUT_A, OUT_OFF); // disable output A
SetGlobalOutput(OUT_A, OUT_ON); // enable output A

SetGlobalDirection(outputs, direction) Function - RCX2, Scout, Spy

Reverses or restores the directions of outputs. The direction parameter should be OUT_FWD, OUT_REV, or OUT_TOGGLE. Normal behavior is a global direction of OUT_FWD. When the global direction is OUT_REV, then the actual output direction will be the opposite of whatever the regular output calls request. Calling SetGlobalDirection() with OUT_TOGGLE will switch between normal and opposite behavior.

SetGlobalDirection(OUT_A, OUT_REV); // opposite direction
SetGlobalDirection(OUT_A, OUT_FWD); // normal direction

SetMaxPower(outputs, power) Function - RCX2, Scout, Spy

Sets the maximum power level allowed for the outputs. The power level may be a variable, but should have a value between OUT_LOW and OUT_FULL.

SetMaxPower(OUT_A, OUT_HALF);

GlobalOutputStatus(n) Value - RCX2, Scout, Spy

Returns the current global output setting for motor n. Note that n must be 0, 1, or 2 - not OUT_A, OUT_B, or OUT_C.

x = GlobalOutputStatus(0); // global status of OUT_A

3.2.4 Spybotics Outputs

Spybotics has two built-in motors. OUT_A refers to the right motor, and OUT_B is for the left motor. OUT_C will send VLL commands out the rear LED (the one used for communication with a computer). This allows a VLL device, such as a Micro-Scout, to be used as a third motor for Spybotics. The same LED may be controlled using the SendVLL() and SetLight() functions.

3.3 Sound

PlaySound(sound) Function - All

Plays one of the 6 preset RCX sounds. The sound argument must be a constant (except on Spybotics, which allows a variable to be used). The following constants are pre-defined for use with PlaySound: SOUND_CLICK, SOUND_DOUBLE_BEEP, SOUND_DOWN, SOUND_UP, SOUND_LOW_BEEP, SOUND_FAST_UP.


PlayTone(frequency, duration) Function - All

Plays s single tone of the specified frequency and duration. The frequency is in Hz and can be a variable for RCX2, Scout, and Spybotics, but has to be constant for RCX and CyberMaster. The duration is in 100ths of a second and must be a constant.

PlayTone(440, 50); // Play 'A' for one half second

MuteSound() Function - RCX2, Scout, Spy

Stops all sounds and tones from being played.


UnmuteSound() Function - RCX2, Scout, Spy

Restores normal operation of sounds and tones.


ClearSound() Function - RCX2, Spy

Removes any pending sounds from the sound buffer.


SelectSounds(group) Function - Scout

Selects which group of system sounds should be used. The group must be a constant.


3.4 LCD Display RCX

The RCX has seven different display modes as shown below. The RCX defaults to DISPLAY_WATCH.


LCD Contents


show the system "watch"


show value of sensor 1


show value of sensor 2


show value of sensor 3


show setting for output A


show setting for output B


show setting for output C

The RCX2 adds an eighth display mode - DISPLAY_USER. User display mode continuously reads a source value and updates the display. It can optionally display a decimal point at any position within the number. This allows the display to give the illusion of working with fractions even though all values are stored internally as integers. For example, the following call will set the user display to show the value 1234 with two digits appearing after the decimal point, resulting in "12.34" appearing on the LCD.

SetUserDisplay(1234, 2);

The following short program illustrates the update of the user display:

task main()
  SetUserDisplay(Timer(0), 0);

Because the user display mode continuously updates the LCD, there are certain restrictions on the source value. If a variable is used it must be assigned to a global storage location. The best way to ensure this is to make the variable a global one. There can also be some strange side effects. For example, if a variable is being displayed and later used as the target of a calculation, it is possible for the display to show some intermediate results of the calculation:

int x;
task main()
  SetUserDisplay(x, 0);
    // display may briefly show 1!
    x = 1 + Timer(0);

SelectDisplay(mode) Function - RCX

Select a display mode.

SelectDisplay(DISPLAY_SENSOR_1); // view sensor 1

SetUserDisplay(value, precision) Function - RCX2

Set the LCD display to continuously monitor the specified value. Precision specifies the number of digits to the right of the decimal point. A precision of zero shows no decimal point.

SetUserDisplay(Timer(0), 0); // view timer 0

3.5 Communication 3.5.1 Messages RCX, Scout

The RCX and Scout can send and receive simple messages using IR. A message can have a value from 0 to 255, but the use of message 0 is discouraged. The most recently received message is remembered and can be accessed as Message(). If no message has been received, Message() will return 0. Note that due to the nature of IR communication, receiving is disabled while a message is being transmitted.

ClearMessage() Function - RCX, Scout

Clear the message buffer. This facilitates detection of the next received message since the program can then wait for Message() to become non-zero:

ClearMessage(); // clear out the received message
until(Message() > 0); // wait for next message

SendMessage(message) Function - RCX, Scout

Send an IR message. Message may be any expression, but the RCX can only send messages with a value between 0 and 255, so only the lowest 8 bits of the argument are used.

SendMessage(3); // send message 3
SendMessage(259); // another way to send message 3

SetTxPower(power) Function - RCX, Scout

Set the power level for IR transmission. Power should be one of the constants TX_POWER_LO or TX_POWER_HI.

3.5.2 Serial RCX2

The RCX2 can transmit serial data out the IR port. Prior to transmitting any data, the communication and packet settings must be specified. Then, for each transmission, data should be placed in the transmit buffer, then sent using the SendSerial() function.

The communication settings are set with SetSerialComm, and determine how bits are sent over IR. Possible values are shown below.




default settings


4800 baud


25% duty cycle


76kHz carrier

The default is to send data at 2400 baud using a 50% duty cycle on a 38kHz carrier. To specify multiple options (such as 4800 baud with 25% duty cycle), combine the individual options using bitwise or (SERIAL_COMM_4800 | SERIAL_COMM_DUTY25).

The packet settings are set with SetSerialPacket and control how bytes are assembled into packets. Possible values are shown below.




no packet format - just data bytes


send a packet preamble


follow each byte with its complement


include a checksum for each packet


standard RCX format (preamble, negated data, and checksum)

Note that negated packets always include a checksum, so the SERIAL_PACKET_CHECKSUM option is only meaningful when SERIAL_PACKET_NEGATED is not specified. Likewise the preamble, negated, and checksum settings are implied by SERIAL_PACKET_RCX.

The transmit buffer can hold up to 16 data bytes. These bytes may be set using SetSerialData, then transmitted by calling SendSerial. For example, the following code sends two bytes (0x12 and 0x34) out the serial port:

SetSerialData(0, 0x12);
SetSerialData(1, 0x34);
SendSerial(0, 2);

SetSerialComm(settings) Function - RCX2

Set the communication settings, which determine how the bits are sent over IR


SetSerialPacket(settings) Function - RCX2

Set the packet settings, which control how bytes are assembled into packets.


SetSerialData(n, value) Function - RCX2

Set one byte of data in the transmit buffer. N is the index of the byte to set (0-15), and value can be any expression.

SetSerialData(3, x); // set byte 3 to x

SerialData(n) Value - RCX2

Returns the value of a byte in the transmit buffer (NOT received data). N must be a constant between 0 and 15.

x = SerialData(7); // read byte #7

SendSerial(start, count) Function - RCX2

Use the contents of the transmit buffer to build a packet and send it out the IR port (according to the current packet and communication settings). Start and count are both constants that specify the first byte and the number of bytes within the buffer to be sent.

SendSerial(0,2); // send first two bytes in buffer

3.5.3 VLL Scout, Spy

SendVLL(value) Function - Scout, Spy

Sends a Visible Light Link (VLL) command, which can be used to communicate with the MicroScout or Code Pilot. The specific VLL commands are described in the Scout SDK.

SendVLL(4); // send VLL command #4

3.6 Timers

All targets provide several independent timers with 100ms resolution (10 ticks per second). The Scout provides 3 such timers while the RCX, CyberMaster and Spybotics provide 4. The timers wrap around to 0 after 32767 ticks (about 55 minutes). The value of a timer can be read using Timer(n), where n is a constant that determines which timer to use (0-2 for Scout, 0-3 for the others). RCX2 and Spybotics provide the ability to read the same timers with higher resolution by using FastTimer(n), which returns the timer's value with 10ms resolution (100 ticks per second).

ClearTimer(n) Function - All

Reset the specified timer to 0.


Timer(n) Value - All

Return the current value of specified timer (in 100ms resolution).

x = Timer(0);

SetTimer(n, value) Function - RCX2, Spy

Set a timer to a specific value (which may be any expression).

SetTimer(0, x);

FastTimer(n) Value - RCX2, Spy

Return the current value of specified timer in 10ms resolution.

x = FastTimer(0);

3.7 Counters RCX2, Scout, Spy

Counters are like very simple variables that can be incremented, decremented, and cleared. The Scout provides two counters (0 and 1), while the RCX2 and Spybotics provides three (0, 1, and 2). In the case of RCX2, these counters are overlapped with global storage locations 0-2, so if they are going to be used as counters, a #pragma reserve should be used to prevent NQC from using the storage location for a regular variable. For example, to use counter 1:

#pragma reserve 1

ClearCounter(n) Function - RCX2, Scout, Spy

Reset counter n to 0. N must be 0 or 1 for Scout, 0-2 for RCX2 and Spybotics.


IncCounter(n) Function - RCX2, Scout, Spy

Increment counter n by 1. N must be 0 or 1 for Scout, 0-2 for RCX2 and Spybotics


DecCounter(n) Function - RCX2, Scout, Spy

Decrement counter n by 1. N must be 0 or 1 for Scout, 0-2 for RCX2 and Spybotics


Counter(n) Value - RCX, Scout, Spy

Return the current value of counter n. N must be 0 or 1 for Scout, 0-3 for RCX2 and Spybotics.

x = Counter(1);

3.8 Access Control RCX2, Scout, Spy

Access control is implemented primarily by the acquire statement. The SetPriority function can be used to set a task's priority, and the following constants may be used to specify resources in an acquire statement. Note that the user defined resources are only available on the RCX2.








LEDs - Spybotics only


user defined - RCX2 only

SetPriority(p) - Function - RCX2, Scout, Spy

Set a task's priority to p, which must be a constant. RCX2 supports priorities 0-255, while Scout supports priorities 0-7. Note that lower numbers are higher priority.


3.9 Events RCX2, Scout, Spy

Although the RCX2, Scout and Spybotics share a common event mechanism, the RCX2 and Spybotics provide 16 completely configurable events while the Scout has 15 predefined events. The only functions common to both targets are the commands to inspect or force events.

ActiveEvents(task) Value - RCX2, Scout, Spy

Return the set of events that have been triggered for a given task.

x = ActiveEvents(0);

CurrentEvents() Value - RCX2, Spy

Return the set of events that have been triggered for the active task.

x = CurrentEvents();

Event(events) Function - RCX2, Scout, Spy

Manually triggers the specified events. This can be useful in testing event handling of the program, or in other cases simulating an event based on other criteria. Note that the specification of the events themselves is slightly different between RCX2 and Scout. RCX2 uses the EVENT_MASK macro to compute an event mask, while Scout has predefined masks.

Event(EVENT_MASK(3)); // triggering an RCX2 event
Event(EVENT_1_PRESSED); // triggering a Scout event

3.9.1 Configurable Events RCX2, Spy

Note: Spybotics events appear to be very similar to RCX2 events, although very little testing has been done for the NQC API and Spybotics. The information below was written from the perspective of RCX2, and has not been updated for Spybotics yet.

RCX2 and Spybotics provide an extremely flexible event system. There are 16 events, each of which can be mapped to one of several event sources (the stimulus that can trigger the event), and an event type (the criteria for triggering). A number of other parameters may also be specified depending on the event type. For all of the configuration calls an event is identified by its event number - a constant from 0 to 15.

Legal event sources are sensors, timers, counters, or the message buffer. An event is configured by calling SetEvent(event, source, type), where event is a constant event number (0-15), source is the event source itself, and type is one of the types shown below (some combinations of sources and types are illegal).

Event Type


Event Source


value becomes on

sensors only


value becomes off

sensors only


value goes from off to on to off

sensors only


value goes from on to off or vice versa

sensors only


value changes rapidly

sensors only


value becomes low



value becomes normal



value becomes high



value from low to high back to low



two clicks within a certain time



new message received

Message() only

The first four event types make use of a sensor's boolean value, thus are most useful with touch sensors. For example, to set event #2 to be triggered when a touch sensor on port 1 is pressed, the following call could be made:


In order for EVENT_TYPE_PULSE or EVENT_TYPE_EDGE to be used, the sensor must be configured in the SENSOR_MODE_PULSE or SENSOR_MODE_EDGE respectively.

EVENT_TYPE_FASTCHANGE should be used with sensors that have been configured with a slope parameter. When the raw value changes faster than the slope parameter an EVENT_TYPE_FASTCHANGE event will be triggered.

The next three types (EVENT_TYPE_LOW, EVENT_TYPE_NORMAL, and EVENT_TYPE_HIGH) convert an event source's value into one of three ranges (low, normal, or high), and trigger an event when the value moves from one range into another. The ranges are defined by the lower limit and upper limit for the event. When the source value is lower than the lower limit, the source is considered low. When the source value is higher than the upper limit, the source is considered high. The source is normal whenever it is between the limits.

The following example configures event #3 to trigger when the sensor on port 2's value goes into the high range. The upper limit is set for 80, and the lower limit is set for 50. This configuration is typical of how an event can be triggered when a light sensor detected a bright light.


A hysteresis parameter can be used to provide more stable transitions in cases where the source value may jitter. Hysteresis works by making the transition from low to normal a little higher than the transition from normal to low. In a sense, it makes it easier to get into the low range than get out of it. A symmetrical case applies to the transition between normal and high.

A transition from low to high back to low will trigger a EVENT_TYPE_CLICK event, provided that the entire sequence is faster than the click time for the event. If two successive clicks occur and the time between clicks is also less than the click time, then an EVENT_TYPE_DOUBLECLICK event will be triggered. The system also keeps track of the total number of clicks for each event.

The last event type, EVENT_TYPE_MESSAGE, is only valid when Message() is used as the event source. The event will be triggered whenever a new message arrives (even if its value is the same as a previous message).

The monitor statement and some API functions (such as ActiveEvents() or Event()) need to handle multiple events. This is done by converting each event number to an event mask, and then combining the masks with a bitwise OR. The EVENT_MASK(event) macro converts an event number to a mask. For example, to monitor events 2 and 3, the following statement could be used:

monitor(EVENT_MASK(2) | EVENT_MASK(3))

SetEvent(event, source, type) Function - RCX2, Spy

Configure an event (a number from 0 to 15) to use the specified source and type. Both event and type must be constants, and source should be the actual source expression.

SetEvent(2, Timer(0), EVENT_TYPE_HIGH);

ClearEvent(event) Value - RCX2, Spy

Clear the configuration for the specified event. This prevents it from triggering until it is re-configured.

ClearEvent(2); // clear event #2

ClearAllEvents() Value - RCX2, Spy

Clear the configurations for all events.


EventState(event) Value - RCX2, Spy

Return the state of a given event. States are 0: Low, 1: Normal, 2: High, 3: Undefined, 4: Start calibrating, 5: Calibrating in process.

x = EventState(2);

CalibrateEvent(event, lower, upper, hyst) Function - RCX2, Spy

Calibrate the event by taking an actual sensor reading and then applying the specified lower, upper, and hyst ratios to determine actual limits and hysteresis value. The specific formulas for calibration depend on sensor type and are explained in the LEGO SDK. Calibration is not instantaneous - EventState() can be checked to determine when the calibration is complete (typically about 50ms).

CalibrateEvent(2, 50, 50, 20);
until(EventState(2) != 5); // wait for calibration

SetUpperLimit(event, limit) Function - RCX2, Spy

Set the upper limit for the event, where event is a constant event number and limit can be any expression.

SetUpperLimit(2, x); // set upper limit for #2 to x

UpperLimit(event) Value - RCX2, Spy

Return the current upper limit for the specified event number.

x = UpperLimit(2); // get upper limit for event 2

SetLowerLimit(event, limit) Function - RCX2, Spy

Set the lower limit for the event, where event is a constant event number and limit can be any expression.

SetLowerLimit(2, x); // set lower limit for #2 to x

LowerLimit(event) Value - RCX2, Spy

Return the current lower limit for the specified event number.

x = LowerLimit(2); // get lower limit for event 2

SetHysteresis(event, value) Function - RCX2

Set the hysteresis for the event, where event is a constant event number and value can be any expression.

SetHysteresis(2, x);

Hysteresis(event) Value - RCX2, Spy

Return the current hysteresis for the specified event number.

x = Hysteresis(2);

SetClickTime(event, value) Function - RCX2, Spy

Set the click time for the event, where event is a constant event number and value can be any expression. The time is specified in increments of 10ms, so one second would be a value of 100.

SetClickTime(2, x);

ClickTime(event) Value - RCX2, Spy

Return the current click time for the specified event number.

x = ClickTime(2);

SetClickCounter(event, value) Function - RCX2, Spy

Set the click counter for the event, where event is a constant event number and value can be any expression.

SetClickCounter(2, x);

ClickCounter(event) Value - RCX2, Spy

Return the current click counter for the specified event number.

x = ClickCounter(2);

3.9.2 Scout Events Scout

The Scout provides 15 events, each of which has a predefined meaning as shown in the table below.

Event Name



sensor 1 pressed


sensor 1 released


sensor 2 pressed


sensor 2 released


light sensor "high"


light sensor "normal"


light sensor "low"


low to high to low


two clicks


counter 0 over limit


counter 1 over limit


timer 0 over limit


timer 1 over limit


timer 2 over limit


new message received

The first four events are triggered by touch sensors connected to the two sensor ports. EVENT_LIGHT_HIGH, EVENT_LIGHT_NORMAL, and EVENT_LIGHT_LOW are triggered by the light sensor's value changing from one range to another. The ranges are defined by SetSensorUpperLimit, SetSensorLowerLimit, and SetSensorHysteresis which were described previously.

EVENT_LIGHT_CLICK and EVENT_LIGHT_DOUBLECLICK are also triggered by the light sensor. A click is a transition from low to high and back to low within a certain amount of time, called the click time.

Each counter has a counter limit. When the counter exceeds this limit, EVENT_COUNTER_0 or EVENT_COUNTER_1 is triggered. Timers also have a limit, and they generate EVENT_TIMER_0, EVENT_TIMER_1, and EVENT_TIMER_2.

EVENT_MESSAGE is triggered whenever a new IR message is received.

SetSensorClickTime(value) Function - Scout

Set the click time used to generate events from the light sensor. Value should be specified in increments of 10ms, and may be any expression.


SetCounterLimit(n, value) Function - Scout

Set the limit for counter n. N must be 0 or 1, and value may be any expression.

SetCounterLimit(0, 100); // set counter 0 limit to 100

SetTimerLimit(n, value) Function - Scout

Set the limit for timer n. N must be 0, 1, or 2, and value may be any expression.

SetTimerLimit(1, 100); // set timer 1 limit to 100

3.10 Data Logging RCX

The RCX contains a datalog which can be used to store readings from sensors, timers, variables, and the system watch. Before adding data, the datalog first needs to be created using the CreateDatalog(size) command. The 'size' parameter must be a constant and determines how many data points the datalog can hold.

CreateDatalog(100); // datalog for 100 points

Values can then be added to the datalog using AddToDatalog(value). When the datalog is uploaded to a computer it will show both the value itself and the source of the value (timer, variable, etc). The datalog directly supports the following data sources: timers, sensor values, variables, and the system watch. Other data types (such as a constant or random number) may also be logged, but in this case NQC will first move the value into a variable and then log the variable. The values will still be captured faithfully in the datalog, but the sources of the data may be a bit misleading.

AddToDatalog(Timer(0)); // add timer 0 to datalog
AddToDatalog(x); // add variable 'x'
AddToDatalog(7); // add 7 - will look like a variable

The RCX itself cannot read values back out of the datalog. The datalog must be uploaded to a host computer . The specifics of uploading the datalog depend on the NQC environment being used. For example, in the command line version of NQC, the following commands will upload and print the datalog:

nqc -datalog
nqc -datalog_full

CreateDatalog(size) Function - RCX

Create a datalog of the specified size (which must be a constant). A size of 0 clears the existing datalog without creating a new one.

CreateDatalog(100); // datalog for 100 points

AddToDatalog(value) Function - RCX

Add the value, which may be an expression, to the datalog. If the datalog is full the call has no effect.


UploadDatalog(start, count) Function - RCX

Initiate and upload of count data points beginning at start. This is of relatively little use since the host computer usually initiates the upload.

UploadDatalog(0, 100); // upload entire 100 point log

3.11 General Features

Wait(time) Function - All

Make a task sleep for specified amount of time (in 100ths of a second). The time argument may be an expression or a constant:

Wait(100); // wait 1 second
Wait(Random(100)); // wait random time up to 1 second

StopAllTasks() Function - All

Stop all currently running tasks. This will halt the program completely, so any code following this command will be ignored.

StopAllTasks(); // stop the program

Random(n) Value - All

Return a random number between 0 and n. N must be a constant.

x = Random(10);

SetRandomSeed(n) Function - RCX2, Spy

Seed the random number generator with n. N may be an expression.

SetRandomSeed(x); // seed with value of x

SetSleepTime(minutes) Function - All

Set the sleep timeout the requested number of minutes (which must be a constant). Specifying 0 minutes disables the sleep feature.

SetSleepTime(5); // sleep after 5 minutes
SetSleepTime(0); // disable sleep time

SleepNow() Function - All

Force the device to go to sleep. Only works if the sleep time is non-zero.

SleepNow(); // go to sleep

3.12 RCX Specific Features

Program() Value - RCX

Number of the currently selected program.

x = Program();

SelectProgram(n) Function - RCX2

Select the specified program and start running it. Note that programs are numbered 0-4 (not 1-5 as displayed on the LCD).


BatteryLevel() Value - RCX2

Return the battery level in millivolts.

x = BatteryLevel();

FirmwareVersion() Value - RCX2

Return the firmware version as an integer. For example, version 3.2.6 is 326.

x = FirmwareVersion();

Watch() Value - RCX

Return the value of the system clock in minutes.

x = Watch();

SetWatch(hours, minutes) Function - RCX

Set the system watch to the specified number of hours and minutes. Hours must be a constant between 0 and 23 inclusive. Minutes must be a constant between 0 and 59 inclusive.

SetWatch(3, 15); // set watch to 3:15

3.13 Scout Specific Features

SetScoutRules(motion, touch, light, time, fx) Function - Scout

Set the various rules used by the scout in stand-alone mode.

ScoutRules(n) Value - Scout

Return current setting for one of the rules. N should be a constant between 0 and 4.

x = ScoutRules(1); // get setting for rule #1

SetScoutMode(mode) Function - Scout

Put the scout into stand-alone (0) or power (1) mode. As a programming call it really only makes sense to put into stand-alone mode since it would already be in power mode to run an NQC program.

SetEventFeedback(events) Function - Scout

Set which events should be accompanied by audio feedback.


EventFeedback() Value - Scout

Return the set of events that have audio feedback.

x = EventFeedback();

SetLight(mode) Function - Scout

Control the Scout's LED. Mode must be LIGHT_ON or LIGHT_OFF.

SetLight(LIGHT_ON); // turn on LED

3.14 CyberMaster Specific Features

CyberMaster provides alternate names for the sensors: SENSOR_L, SENSOR_M, and SENSOR_R. It also provides alternate names for the outputs: OUT_L, OUT_R, OUT_X. Additionally, the two internal motors have tachometers, which measure 'clicks' and speed as the motors turn. There are about 50 clicks per revolution of the shaft. The tachometers can be used, for example, to create a robot which can detect if it has bumped into an object without using any external sensors. The tachometers have maximum values of 32767 and do not differentiate between directions. They will also count up if the shaft is turned by hand, including when no program is running.

Drive(motor0, motor1) Function - CyberMaster

Turns on both motors at the power levels specified. If a power level is negative, then the motor will run in reverse. Equivalent to this code:

SetPower(OUT_L, abs(power0));
SetPower(OUT_R, abs(power1));
if(power0 < 0)
  { SetDirection(OUT_L, OUT_REV) }
  { SetDirection(OUT_L, OUT_FWD) }
if(power1 < 0)
  { SetDirection(OUT_R, OUT_REV) }
  { SetDirection(OUT_R, OUT_FWD) }
SetOutput(OUT_L + OUT_R, OUT_ON);

OnWait(motors, n time) Function - CyberMaster

Turns on the motors specified, all at the same power level then waits for the given time. The time is in 10ths of a second, with a maximum of 255 (or 25.5 seconds). Equivalent to this code:

SetPower(motors, abs(power));
if(power < 0)
  { SetDirection(motors, OUT_REV) }
  { SetDirection(motors, OUT_FWD) }
SetOutput(motors, OUT_ON);
Wait( time * 10 );

OnWaitDifferent(motors, n0, n1, n2, time) Function - CyberMaster

Like OnWait(), except different power levels can be given for each motor.

ClearTachoCounter(motors) Function - CyberMaster

Resets the tachometer for the motor(s) specified.

TachoCount(n) Value - CyberMaster

Returns the current value of the tachometer for a specified motor.

TachoSpeed(n) Value - CyberMaster

Returns the current speed of the tachometer for a specified motor. The speed is fairly constant for an unladen motor at any speed, with a maximum value of 90. (This will be lower as your batteries lose power!) The value drops as the load on the motor increases. A value of 0 indicates that the motor is stalled.

ExternalMotorRunning() Value - CyberMaster

This is actually a measure of the current being drawn by the motor. The values returned tends to fluctuate slightly, but are, on average, as follows for an unladen motor:

0 motor is floating

1 motor is off

 <=7 motor is running at around this power level. This is where the value fluctuates the most (probably related to the PWM method used to drive the motors.) In any case, you should know what power level you set the motor to in the first place.

The value increases as the load on the motor increases, and a value between 260 and 300 indicates that the motor has stalled.

AGC() Value - CyberMaster

Return the current value of the automatic gain control on the RF receiver. This can be used to give a very rough (and somewhat inaccurate) measure of the distance between the CyberMaster and the RF transmitter.

x = AGC();

4 Technical Details

This section explains some of the low-level features of NQC. In general, these mechanisms should only be used as a last resort since they may change in future releases. Most programmers will never need to use the features described below - they are mainly used in the creation of the NQC API file.

4.1 The asm statement

The asm statement is used to define almost all of the NQC API calls. The syntax of the statement is:

asm { item1, item2 ... itemN }

Where an item is one of the following


& expression

& expression : restrictor

The statement simply emits the values of each of the items as raw bytecodes. Constant items are the simplest - they result in a single byte of raw data (the lower 8 bits of the constant value). For example, the API file defines the following inline function:

void ClearMessage() { asm { 0x90 }; }

Whenever ClearMessage() is called by a program, the value 0x90 is emitted as a bytecode.

Many API functions take arguments, and these arguments must be encoded into an appropriate effective address for the bytecode interpreter. In the most general case, an effective address contains a source code followed by a two byte value (least significant byte first). Source codes are explained in the SDK documentation available from LEGO. However, it is often desirable to encode the value in some other manner - for example to use only a single byte value after the source code, omit the source code itself, or only allow certain sources to be used. A restrictor may be used to control how the effective address is formatted. A restrictor is a 32 bit constant value. The lower 24 bits form a bitmask indicating which sources are valid (bit 0 should be set to allow source 0, etc). The upper 8 bits include formatting flags for the effective address. Note that when no restrictor is specified, this is the same as using a restrictor of 0 (no restriction on sources, and a format of source followed by two value bytes). The API file defines the following constants which can be used to build restrictors:

#define __ASM_SMALL_VALUE 0x01000000
#define __ASM_NO_TYPE      0x02000000
#define __ASM_NO_LOCAL     0x04000000

#if __RCX==2
    // no restriction
    #define __ASM_SRC_BASIC   0
    #define __ASM_SRC_EXT     0
    #define __ASM_SRC_BASIC   0x000005 
    #define __ASM_SRC_EXT     0x000015

The __ASM_SMALL_VALUE flag indicates that a one-byte value should be used instead of a two-byte value. The __ASM_NO_TYPE flag indicates that the source code should be omitted. The __ASM_NO_LOCAL flag specifies that local variables are not a legal source for the expression. Note that the RCX2 firmware is less restrictive than the other interpreters, thus the definition of __ASM_SRC_BASIC and __ASM_SRC_EXT are relaxed in the RCX2 case. The API definition file for NQC contains numerous examples of using restrictors within asm statement. If you are using a command-line version of NQC, you can emit the API file by typing the following command:

nqc -api

4.2 Data Sources

The bytecode interpreters use different data sources to represent the various kinds of data (constants, variables, random numbers, sensor values, etc). The specific sources depend to a certain extent on which device you are using and are described in the SDK documentation available from LEGO.

NQC provides a special operator to represent a data source:


The value of this expression is the data source described by the constant. The lower 16 bits of the constant represent the data value, and the next 8 bits are the source code. For example, the source code for a random number is 4, so the expression for a random number between 0 and 9 would be:


The NQC API file defines a number of macros which make the use of the @ operator transparent to the programmer. For example, in the case of random numbers:

#define Random(n)  @(0x40000 + (n))

Note that since source 0 is the global variable space, the global storage locations can be referenced by number: @0 refers to storage location 0. If for some reason you need explicit control over where variables are being stored, then you should use #pragma reserve to instruct NQC not to use those storage locations, and then access them manually with the @ operator. For example, the following code snippet reserves location 0 and creates a macro for it called x.

#pragma reserve 0
#define x (@0)

Because of how sensors have been implemented it is necessary to convert the sensor's data source into a sensor index for use in macros such as SensorValueRaw(). The __sensor expression can be used to do this:

#define SensorValueRaw(n) @(0xc0000 + (__sensor(n)))