The following variables are pre-defined by Retro Rocket BASIC and cannot be reassigned:

• TRUE – Always contains the numeric value 1. Useful for logical expressions, e.g., IF A = TRUE THEN ....
• FALSE – Always contains the numeric value 0. Often used as the opposite of TRUE, e.g., IF A = FALSE THEN ....
• PID – Contains the process ID (integer) of the current process.
• ERR – True only while in an error handler or after an error in an EVAL.
• ERRLINE – Contains the line an error last occurred on while in error handler or after an error in an EVAL.
• ERR$ – Contains the error message which last occurred while in error handler or after an error in an EVAL.
• ARG$ – A string containing the command line arguments passed to the current process.
• PROGRAM$ – A string containing the fully qualified path of the current program.
• GRAPHICS_WIDTH – An integer containing the width of the graphics screen
• GRAPHICS_HEIGHT – An integer containing the height of the graphics screen
• GRAPHICS_CENTRE_X – An integer containing the X coordinate of the centre of the graphics screen. Also accessible via the American English spelling GRAPHICS_CENTER_X.
• GRAPHICS_CENTRE_Y – An integer containing the Y coordinate of the centre of the graphics screen. Also accessible via the American English spelling GRAPHICS_CENTER_Y.
• PI# – A real constant for π (3.141592653589793238). Useful for trigonometric or geometric calculations.
• E# – A real constant for e (2.7182818284590451). Useful for trigonometric or geometric calculations.

TRUE and FALSE

In Retro Rocket BASIC, TRUE is defined as 1 and FALSE as 0. This differs from BBC BASIC, where TRUE was -1.

Why BBC BASIC used -1 BBC BASIC, like many early BASIC dialects, used 16-bit signed integers with all bits set (11111111 11111111) representing -1.
This meant any logical NOT operation (bitwise inversion) would turn -1 into 0 (FALSE), and vice versa, which was convenient when logical values were treated as bit patterns.

Why Retro Rocket uses 1 Retro Rocket BASIC treats boolean values more like modern languages (C, Python, etc.), where TRUE is 1 and FALSE is 0.
This is more intuitive, avoids confusion when mixing booleans with arithmetic, and reflects the way most modern hardware tests nonzero values.