gwbasic-c

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+. No other dependencies.

Usage

Interactive mode:

$ ./gwbasic
GW-BASIC 0.4.0
(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 examples/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

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
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
User functions	DEF FN, RANDOMIZE
File management	KILL, NAME
Screen	LOCATE, COLOR, WIDTH

Graphics statements (SCREEN, PSET, LINE, CIRCLE, DRAW, PAINT, etc.) are parsed but produce no output — programs that use them won't crash.

Architecture

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

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

Module	File	Original
Tokenizer	tokenizer.c	GWMAIN.ASM
Evaluator	eval.c	GWEVAL.ASM
Interpreter	interp.c	BINTRP.ASM
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
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

27 test programs in tests/programs/:

for f in tests/programs/*.bas; do
    echo "$(basename $f):"
    timeout 5 ./build/gwbasic "$f"
done

License

MIT License. See LICENSE.