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

RND can be called with or without parentheses: RND is equivalent to RND(1).

String Functions

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

File Functions

EOF, LOC, LOF

Memory Functions

• FRE(x) — free bytes in the string space pool. FRE("") triggers a garbage collection pass before reporting; FRE(0) reports without collecting.
• VARPTR(var) / VARPTR$(var) — variable address (internal index)

Pseudo-variables

ERL, ERR, CSRLIN, INKEY$, DATE$, TIME$, TIMER, POS(0)

Literals

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

Statements

Category	Statements
Output	PRINT, LPRINT, LLIST, 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, AUTO, RENUM, DELETE, EDIT
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 (binary/ASCII), LOAD (auto-detects), MERGE, CHAIN, COMMON
Event trapping	ON TIMER(n) GOSUB, TIMER ON/OFF/STOP, ON KEY(n) GOSUB, KEY(n) ON/OFF/STOP
Error handling	ON ERROR GOTO, RESUME, RESUME NEXT, RESUME n, ERROR, ERR, ERL
User functions	DEF FN, RANDOMIZE
File management	KILL, NAME, FILES, MKDIR, RMDIR, CHDIR, SHELL, RESET
Date/time	DATE$, TIME$, TIMER (read/write; DATE$/TIME$ assignment accepted)
Environment	ENVIRON, ENVIRON$
Screen	LOCATE, COLOR, WIDTH, SCREEN, KEY ON/OFF/LIST, KEY n,"string"
Graphics	PSET, PRESET, LINE, CIRCLE, DRAW, PAINT, GET, PUT, VIEW, WINDOW, PALETTE, PMAP
Sound	SOUND, BEEP, PLAY (MML parser, PulseAudio backend)
Memory	DEF SEG, PEEK, POKE, BSAVE, BLOAD
Hardware I/O	OUT, INP, WAIT, MOTOR, STICK, STRIG
Device stubs	ERDEV, ERDEV$, IOCTL, IOCTL$, COM, LCOPY
Misc	KEY, TRON/TROFF, OPTION BASE, MID$ assignment
System	SYSTEM

Program I/O (SAVE / LOAD)

SAVE writes the current program to a file. The default format is tokenized binary (compact, fast to load), matching the original GW-BASIC behavior:

SAVE "myprog.bas"       ' tokenized binary (default)
SAVE "myprog.bas",A     ' ASCII text (human-readable, editable)

LOAD reads a program file, auto-detecting the format from the first byte:

LOAD "myprog.bas"       ' auto-detects binary or ASCII
LOAD "myprog.bas",R     ' load and run immediately

Binary files use the standard GW-BASIC 0xFF header format. Command-line loading (./gwbasic file.bas) also auto-detects format.

MERGE loads an ASCII file without clearing the current program, overlaying lines by number. CHAIN loads and runs a new program, optionally preserving variables listed by COMMON.

INKEY$ Extended Keys

INKEY$ returns a zero-length string when no key is available, a one-byte string for regular ASCII keys, or a two-byte string for extended keys:

K$ = INKEY$
IF LEN(K$) = 2 THEN scan = ASC(MID$(K$, 2, 1))

Extended keys return CHR$(0) as the first byte and the IBM PC scan code as the second. Common scan codes:

Key	Scan	Key	Scan
F1-F10	59-68	Home	71
Up	72	PgUp	73
Left	75	Right	77
End	79	Down	80
PgDn	81	Ins	82
Del	83

Printer Output (LPRINT / LLIST)

LPRINT works identically to PRINT but sends output to the printer:

• Default: output is appended to LPT1.TXT in the current directory
• --lpt /dev/lp0 (Linux) or --lpt LPT1 (FreeDOS): send to real hardware
• --lpt report.txt: redirect to a custom file

LLIST lists the program to the printer, with optional line number ranges (LLIST, LLIST 10-50, LLIST -100).

Both support PRINT USING, semicolons, commas, TAB(), and SPC().

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

Sprite Capture and Blit (GET / PUT)

• GET (x1,y1)-(x2,y2), array — capture a screen rectangle into an integer array
• PUT (x,y), array [, action] — blit a captured sprite back to the screen

The action parameter controls how the sprite combines with the existing screen:

Action	Effect
XOR (default)	Exclusive OR — drawing twice erases the sprite
PSET	Overwrite screen with sprite pixels
PRESET	Overwrite screen with inverted sprite pixels
AND	Bitwise AND of screen and sprite
OR	Bitwise OR of screen and sprite

Sprite data is stored in CGA-compatible packed format: word 0 is the width in bits, word 1 is the height, and remaining words contain packed pixel data matching the original GW-BASIC representation.

VIEW / WINDOW / PALETTE

• VIEW [[SCREEN] (x1,y1)-(x2,y2) [,[fill][,border]]] — define a graphics viewport. Without SCREEN, drawing coordinates are relative to the viewport origin. With SCREEN, coordinates remain absolute. Without arguments, resets to full screen.
• WINDOW [[SCREEN] (x1,y1)-(x2,y2)] — map logical coordinates onto the viewport. Without SCREEN, Y increases upward (Cartesian); with SCREEN, Y increases downward. Without arguments, resets to physical coordinates.
• PALETTE [attribute, color] — remap a color attribute to a different physical color (0–15). Without arguments, resets to the default CGA palette.
• PMAP(coordinate, function) — convert between logical and physical coordinates. Function 0/1 = logical→physical X/Y; 2/3 = physical→logical X/Y.

Example

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

DEF SEG / PEEK / POKE

DEF SEG, PEEK, and POKE provide access to a virtual 8086 address space that emulates the memory layout programs expected on a real IBM PC:

DEF SEG = &HB800       ' select CGA video buffer segment
POKE 0, 65             ' write 'A' to top-left screen cell
PRINT PEEK(1)          ' read the color attribute
DEF SEG                ' reset to default segment

Emulated Memory Regions

Segment	Address Range	Description
0040	BIOS data area	Video mode (0049), column count (004A), cursor position (0050-0051), timer ticks (006C-006F), screen rows (0084), keyboard shift flags (0017: bit 7 = insert mode)
B800	CGA text buffer	Character/attribute pairs in text mode (even byte = char, odd byte = attr, 80 columns × 25 rows = 4000 bytes)
B800	CGA graphics buffer	In SCREEN 1/2: CGA interlaced layout (even rows at offset 0, odd rows at 0x2000, 80 bytes/row)

All other segments read as 0 and writes are silently discarded. The timer tick counter at 0040:006C tracks real time at the original 18.2 Hz rate.

BSAVE / BLOAD

BSAVE saves a block of virtual memory to a binary file with a 7-byte header. BLOAD loads it back. These operate on whichever segment was last set by DEF SEG.

DEF SEG = &HB800
BSAVE "screen.bin", 0, 4000    ' save the CGA text buffer
CLS
BLOAD "screen.bin"              ' restore it
BLOAD "screen.bin", 100         ' load to a different offset

The file format is compatible with the original GW-BASIC: byte 0 = 0xFD, bytes 1-2 = segment (LE), bytes 3-4 = offset (LE), bytes 5-6 = length (LE), followed by the raw data bytes.

String Space and Garbage Collection

All string data lives in a contiguous pool (default 32,768 bytes), matching the original GW-BASIC's string space architecture. Allocation is a bump pointer; dead strings are reclaimed by a compacting garbage collector that runs automatically at statement boundaries when the pool is running low.

CLEAR ,8192             ' set string space to 8KB and clear variables
PRINT FRE("")           ' trigger GC and print free bytes

CLEAR n (or CLEAR ,n) resizes the string space to n bytes and clears all variables. FRE("") forces a garbage collection pass and returns the free bytes; FRE(0) returns the current free count without collecting.

Hardware I/O (OUT / INP / WAIT)

OUT, INP, and WAIT provide access to emulated IBM PC I/O ports, enabling classic programs that drive hardware directly — speaker tones via the 8253 PIT, CGA palette tricks, and serial port polling.

OUT &H43, &HB6              ' PIT channel 2: lobyte/hibyte, mode 3
OUT &H42, &HD3 : OUT &H42, &H04   ' set frequency divisor (1235 → 966 Hz)
OUT &H61, INP(&H61) OR 3    ' speaker on
OUT &H61, INP(&H61) AND &HFC ' speaker off

Emulated Ports

Port	Device	Behavior
&H42–&H43	8253 PIT channel 2	Speaker frequency divisor (1193180 / divisor Hz)
&H61	PPI Port B	Bits 0–1 control speaker; both set = tone on, either clear = off
&H3D8	CGA mode control	Mode register; writes with changed mode bits trigger SCREEN changes
&H3D9	CGA color select	Background color (bits 0–3) and palette select (bit 5)
&H201	Game port	Returns &HF0 (no joystick connected)
&H3F8–&H3FE	COM1 serial	Minimal stub; LSR (&H3FD) returns &H60 (transmitter ready)
Default	—	Reads return &HFF (floating bus), writes silently discarded

Related Functions

• INP(port) — read a byte from an I/O port
• STICK(n) — joystick axis position (returns 128 = center, n = 0–3)
• STRIG(n) — joystick button state (returns 0 = not pressed)
• WAIT port, mask [, xor_mask] — busy-wait until (INP(port) XOR xor_mask) AND mask is nonzero; Ctrl+C breaks out
• MOTOR [n] — accepted and silently ignored (cassette motor control)

When the PPI speaker bits are set, the PIT frequency divisor is used to generate a continuous tone via PulseAudio (same backend as SOUND / PLAY).

Environment Variables

• ENVIRON "var=value" — set an environment variable (uses the C putenv() call)
• ENVIRON$("var") — read an environment variable (returns "" if not set)

ENVIRON "GREETING=Hello"
PRINT ENVIRON$("GREETING")   ' prints: Hello
PRINT ENVIRON$("PATH")       ' prints the system PATH

Date/Time Assignment

DATE$ and TIME$ can be assigned to set the date and time:

DATE$ = "01-15-2026"
TIME$ = "14:30:00"

These assignments are accepted for compatibility but do not modify the system clock. Reading DATE$ and TIME$ always returns the current system date and time.

Device Stubs

The following device-related statements and functions are accepted for compatibility with programs that reference them, but return stub values since there is no real device hardware:

• ERDEV — device error code (always 0)
• ERDEV$ — device error name (always "")
• IOCTL [#n,] string — device control string output (accepted, ignored)
• IOCTL$(n) — device control string input (always "")
• COM ON / COM OFF / COM STOP — serial port event trapping (accepted, ignored)
• LCOPY [n] — screen dump to printer (accepted, ignored)
• CALL / CALLS — machine code execution (raises Illegal function call)
• RESET — close all open files (equivalent to CLOSE)

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.

Full-Screen Editor (TUI)

When running interactively, GW-BASIC 2026 presents the authentic full-screen editor:

• 25×80 screen buffer by default, or full terminal size with --full
• Press Enter on any screen line to re-enter it as BASIC input
• Insert/Overwrite toggle (Insert key)
• Home/End/Delete/Backspace/Escape for line editing
• Ctrl+C interrupts running programs
• Uses the ANSI alternate screen buffer for clean terminal restore on exit

Function Keys

Default F1-F10 bindings match the original GW-BASIC:

Key	Default	Key	Default
F1	LIST	F6	,"LPT1:" + Enter
F2	RUN + Enter	F7	TRON + Enter
F3	LOAD"	F8	TROFF + Enter
F4	SAVE"	F9	KEY
F5	CONT + Enter	F10	SCREEN 0,0,0 + Enter

• KEY ON — show the function key bar on line 25
• KEY OFF — hide the bar
• KEY LIST — display all definitions
• KEY n, "string" — redefine a function key

Piped Mode

When stdin is not a terminal (piped input), the TUI is not activated. The interpreter reads lines from stdin and writes output directly to stdout, suitable for scripting and test harnesses.

Program Editing

• EDIT [linenum] — display a program line for editing in the TUI; press Enter to re-store it
• AUTO [start][,increment] — automatic line numbering mode
• RENUM [new][,[old][,increment]] — renumber program lines (patches all GOTO/GOSUB references)
• DELETE range — delete program lines (DELETE 10-50, DELETE -100, DELETE 200-)

Event Trapping

GW-BASIC supports event-driven programming through trap handlers that fire between statements during program execution.

Timer Events

ON TIMER(n) GOSUB line    ' register handler (fires every n seconds)
TIMER ON                  ' enable timer trapping
TIMER STOP                ' suspend trapping (events are queued)
TIMER OFF                 ' disable trapping (events are discarded)

Function Key Events

ON KEY(n) GOSUB line      ' register handler for F-key n (1-10)
KEY(n) ON                 ' enable trapping for key n
KEY(n) STOP               ' suspend trapping (events are queued)
KEY(n) OFF                ' disable trapping

Event handlers execute as implicit GOSUBs. The RETURN statement returns to the interrupted code and clears the handler's in-progress flag. Events do not fire inside their own handler (re-entrant protection).

TIMER STOP / KEY(n) STOP queue events while stopped; switching to TIMER ON / KEY(n) ON fires the pending event immediately.

References

• Microsoft Corporation. Microsoft GW-BASIC: User's Guide and Reference. Microsoft Press, 1989. ISBN 978-1-55615-260-3.
• Inman, Don and Bob Albrecht. The GW-BASIC Reference. Osborne McGraw-Hill, 1990. ISBN 978-0-07-881644-4.
• Ahl, David H. BASIC Computer Games: Microcomputer Edition. Workman, 1978. ISBN 978-0-89480-052-8.
• Microsoft Corporation. GW-BASIC User's Manual. Microsoft, 1987. (OEM bundled; no ISBN.)
• Microsoft Corporation. GW-BASIC Source Code. Released 2020.