GW-BASIC 2026

A portable C reimplementation of Microsoft GW-BASIC, using the original 8088 assembly source (released by Microsoft in 2020) as the authoritative reference.

This is not a transpilation — it reimplements the algorithms in clean C11 with modern data structures while targeting bug-compatible behavior.

Building

mkdir -p build && cd build
cmake .. && make

Requires a C11 compiler and CMake 3.10+. PulseAudio (libpulse-simple) is optional — detected at build time for SOUND/BEEP/PLAY support.

Usage

Interactive mode launches the authentic GW-BASIC full-screen editor:

$ ./gwbasic
GW-BASIC 2026 0.6.0
(C) Eremey Valetov 2026. MIT License.
Based on Microsoft GW-BASIC assembly source.
Ok
PRINT 2+2
 4
Ok
FOR I=1 TO 5:PRINT I;:NEXT
 1  2  3  4  5
Ok

Run a program file:

./gwbasic tests/programs/prime_sieve.bas

Pipe input:

echo '10 FOR I=1 TO 10:PRINT I*I;:NEXT' | ./gwbasic

What Works

Data types: INTEGER (%), SINGLE (!), DOUBLE (#), STRING ($)

Operators: + - * / ^ \ 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$, INSTR, INPUT$

Statements:

Category	Statements
Output	PRINT, LPRINT, PRINT USING, WRITE, CLS
Variables	LET, DIM, ERASE, SWAP, DEFINT/SNG/DBL/STR
Control flow	GOTO, GOSUB/RETURN, FOR/NEXT, IF/THEN/ELSE, WHILE/WEND, ON...GOTO/GOSUB
Input	INPUT, LINE INPUT, DATA/READ/RESTORE, INKEY$
Program	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, ERROR, ERR, ERL
User functions	DEF FN, RANDOMIZE
File management	KILL, NAME, FILES, MKDIR, RMDIR, CHDIR, SHELL
Date/time	DATE$, TIME$, TIMER
Screen	LOCATE, COLOR, WIDTH, SCREEN, KEY ON/OFF/LIST
Graphics	PSET, PRESET, LINE, CIRCLE, DRAW, PAINT
Sound	SOUND, BEEP, PLAY (MML)

Full-Screen Editor (TUI)

When running interactively, GW-BASIC 2026 presents the authentic full-screen editor that people remember from the 1980s:

• 25×80 screen buffer with free cursor movement (arrow keys)
• Enter on any screen line re-enters it as BASIC input
• Insert/Overwrite toggle (Insert key, cursor shape changes)
• Function key bar on line 25 (KEY ON/KEY OFF/KEY LIST)
• Default F1-F10 bindings (F1=LIST, F2=RUN, F3=LOAD", etc.)
• Ctrl+C interrupts running programs
• Piped input bypasses the TUI entirely — scripts and test harnesses work unchanged

Graphics

Graphics mode is activated with SCREEN 1 (320×200, 4 colors) or SCREEN 2 (640×200, monochrome). Drawing commands render to a virtual framebuffer and output via Sixel graphics, which works in terminals like xterm, mlterm, foot, and WezTerm.

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

Architecture

The interpreter follows the original GW-BASIC's internal structure:

Source text → Tokenizer (CRUNCH) → Token stream
                                      ↓
                              Expression evaluator (FRMEVL)
                                      ↓
                              Statement dispatcher (NEWSTT)
                                      ↓
                              TUI screen buffer (interactive)
                                      ↓
                              HAL (platform I/O)

Module	File	Original
Tokenizer	tokenizer.c	GWMAIN.ASM
Evaluator	eval.c	GWEVAL.ASM
Interpreter	interp.c	BINTRP.ASM
TUI editor	tui.c	—
Graphics	graphics.c	—
Tokens	tokens.c	IBMRES.ASM
Errors	error.c	GWDATA.ASM
Math	math_int.c, math_float.c, math_transcend.c	MATH1/2.ASM
Strings	strings.c	BISTRS.ASM
File I/O	fileio.c	BIPTRG.ASM
PRINT USING	print_using.c	BIPRTU.ASM
Sound	sound.c	—
Platform	hal_posix.c	OEM*.ASM

Key design differences from the original:

• IEEE 754 floating point (MBF conversion only for CVI/CVS/CVD compatibility)
• Dynamic memory allocation instead of 64KB segment layout
• malloc'd strings instead of compacting garbage collector
• setjmp/longjmp for error recovery

Tests

52 test programs in tests/programs/, with CI via GitHub Actions:

bash tests/run_tests.sh

Compatibility testing against real GWBASIC.EXE under DOSBox-X:

bash tests/run_compat.sh --generate   # generate .expected from GWBASIC.EXE
bash tests/run_compat.sh              # compare gwbasic output against .expected

License

MIT License. See LICENSE.