gw-basic-2026/docs/language-reference.md

Language Reference

Data Types

Type	Suffix	Description
INTEGER	%	16-bit signed
SINGLE	!	32-bit float
DOUBLE	#	64-bit float
STRING	$	Up to 255 bytes

Operators

+, -, *, /, ^, \ (integer div), MOD, AND, OR, XOR, EQV, IMP, NOT, <, =, >, <=, >=, <>

Numeric Functions

SGN, INT, ABS, SQR, SIN, COS, TAN, ATN, LOG, EXP, RND, FIX, CINT, CSNG, CDBL

String Functions

LEN, ASC, CHR$, VAL, STR$, LEFT$, RIGHT$, MID$, SPACE$, STRING$, HEX$, OCT$, INSTR, INPUT$

File Functions

EOF, LOC, LOF

Pseudo-variables

ERL, ERR, CSRLIN, INKEY$, DATE$, TIME$

Literals

Decimal, &H hex, &O octal, D exponent (double), E exponent (single), type suffixes (%, !, #)

Statements

Category	Statements
Output	PRINT, LPRINT, PRINT USING, WRITE, CLS
Variables	LET, DIM, ERASE, SWAP, DEFINT, DEFSNG, DEFDBL, DEFSTR
Control flow	GOTO, GOSUB/RETURN, FOR/NEXT, IF/THEN/ELSE, WHILE/WEND, ON...GOTO, ON...GOSUB
Input	INPUT, LINE INPUT, DATA/READ/RESTORE, INKEY$
Program control	RUN, RUN "file", CONT, STOP, END, NEW, LIST, CLEAR
Sequential I/O	OPEN, CLOSE, PRINT#, WRITE#, INPUT#, LINE INPUT#
Random-access I/O	FIELD, LSET, RSET, PUT, GET, CVI/CVS/CVD, MKI$/MKS$/MKD$
Program I/O	SAVE, LOAD, MERGE, CHAIN
Error handling	ON ERROR GOTO, RESUME, RESUME NEXT, RESUME n, ERROR, ERR, ERL
User functions	DEF FN, RANDOMIZE
File management	KILL, NAME
Screen	LOCATE, COLOR, WIDTH, SCREEN
Graphics	PSET, PRESET, LINE, CIRCLE, DRAW, PAINT
Sound	SOUND, BEEP, PLAY (MML parser, PulseAudio backend)
Misc	POKE, KEY, TRON/TROFF, OPTION BASE, MID$ assignment, COMMON
System	SYSTEM

Graphics

Graphics mode is activated with SCREEN 1 (320x200, 4 colors) or SCREEN 2 (640x200, monochrome). Drawing commands render to an internal framebuffer and output via Sixel graphics, which works in terminals like xterm, mlterm, foot, and WezTerm.

Drawing Commands

• PSET (x,y), color / PRESET (x,y) — set/reset individual pixels
• LINE (x1,y1)-(x2,y2), color [,B|BF] — lines, boxes, filled boxes
• CIRCLE (cx,cy), r [,color [,start, end [,aspect]]] — circles and arcs
• PAINT (x,y), fill, border — flood fill
• DRAW string — turtle graphics mini-language (U/D/L/R/E/F/G/H, M, C, S, B, N)
• POINT (x,y) — read pixel color
• COLOR fg, bg — set foreground/background colors

Example

SCREEN 1
LINE (0,0)-(319,199), 1
CIRCLE (160,100), 80, 2
PAINT (160,100), 3, 2

Sound

• SOUND frequency, duration — play a tone (frequency in Hz, duration in clock ticks)
• BEEP — play the default beep
• PLAY string — Music Macro Language (MML) string for melodies

Sound output uses PulseAudio when available; commands are silently ignored otherwise.

Terminal I/O

When stdin is a terminal, the interpreter enters POSIX raw mode for real-time keyboard polling with INKEY$ and character-at-a-time input with INPUT$(n). Piped input is handled normally without raw mode. INPUT and LINE INPUT temporarily exit raw mode for cooked-mode line editing.