MBASIC 5.50 — BASIC-86 for MS-DOS and CP/M-86

A text-mode Microsoft BASIC interpreter rebuilt from the 1983 GW-BASIC sources specifically for FCB DOS and CP/M-86 family: graphics and PC hardware removed, tokens and behaviour aligned on MBASIC 5.28/ MBASIC-86 5.22. one source tree producing an MS-DOS .COM and a CP/M-86 .CMD.

A similar but far more advanced approach can also be found at gw-basic-2026. If you intent to work on *nix and more recent ms-dos environment, have a look at it.

1. Why and what

Why	Fun, nostalgia and history: get a working, buildable Microsoft BASIC-86 back from published source
Source	microsoft/GW-BASIC (1983, MIT)
Result	MBASIC 5.50: everything MBASIC 5.28 does, nothing PC specific, Works on both DOS and CP/M
Rule	May do more than 5.28 / 5.22, never less
Manual	docs/basic-5.0.pdf: BASIC-80 5.0 reference manual, also valid for this version (5.50 follows the 5.x language)

The family, as far as the sources tell

Product	Year	CPU / OS	Relation
Altair BASIC	1975	8080	Origin of BINTRP ("Bill Gates and Paul Allen", Monte Davidoff math)
BASIC Rev. 4.51 [CP/M Version]	1977	8080 / CP/M-80	Earlier CP/M release (banner: "Copyright 1977 (C) by Microsoft"), before the 5.x line and MBASIC.COM naming; not part of these sources, binary kept in ref/
BASIC-80 (MBASIC) 5.21	~1981	8080 / CP/M-80	Disk BASIC 5.x, the language MBASIC users know
BASIC-86 5.22	1982	8086 / CP/M-86	5.x mechanically translated from 8080 to 8086 (headers: "translation created … by Version 4.3")
MBASIC 5.28	1983	8086 / MS-DOS	Same 5.x line on MS-DOS; adds BLOAD/BSAVE, CALLS, DATE$, TIME$
GW-BASIC	1983	8086 / MS-DOS	Same translated core + generalized I/O (GIO), screen editor, graphics, sound, events
MBASIC 5.50 (this)	2026	8086 / MS-DOS, CP/M-86	GW-BASIC core minus PC specifics, 5.28 tokens and behaviour, teletype console

2. Runs on

Quick start-up checks (banner, FILES, program entry) on PCE/ibmpc; not an extensive test.

<sub>PC-DOS 1.10 (.EXE)  ·  CP/M-86 1.1 (.CMD)

Concurrent DOS 4.1 (.CMD)  ·  DOS Plus 1.2 (.CMD)</sub>

Platform	Status	Notes
MS-DOS / PC-DOS 1.10+	✔ checked on PC-DOS 1.10	Only DOS 1.x calls (00h–2Eh), FCB files, 8.3 names, no paths
MS-DOS 2.x–6.x	✔ checked on MS-DOS 6.22	Same binary
CP/M-86 1.1 (BDOS 2.2)	✔ checked	DATE$/TIME$ → "Illegal function call" (no clock)
CP/M-86 Plus, Concurrent CP/M / DOS, DOS Plus (BDOS 3.x, 4.1)	✔ checked on Concurrent DOS 4.1 and DOS Plus 1.2 (.CMD)	DATE$/TIME$ via BDOS 155/104
Any 8086/8088 machine	✔	No BIOS, video or port access by the interpreter
emu2-cpm86 (DOS and CP/M-86)	✔ fully tested	All tests/ run here

Binary	Size	Memory
build/dos/mbasic86.com	~34 KB	64 KB data segment
build/cpm/mbasic86.cmd	~35 KB	code group + data group up to 64 KB

3. Command line

mbasic86 [file] [options]: the optional program file is loaded and run, options start with / and may come before or after the file name (only one file name allowed). Anything else gives "Error detected in command line".

Option	Effect
/F:n	Number of files: accepted for compatibility, ignored (files are dynamic)
/S:n	Maximum record size: accepted for compatibility, ignored (records are dynamic)
/M:n	Limit the data segment to n bytes (decimal or &H hex), e.g. /M:&HC000 leaves 46922 bytes free instead of 63290; more than is available is refused
/NOB[ANNER]	No banner and no "Bytes free" message (5.50 extension)
/NOR[UN]	Load the program but do not run it, stay at Ok (5.50 extension)

Option names are case-insensitive and may be abbreviated down to three letters (/NOB, /NOR). Examples: mbasic86 pspdump, mbasic86 /nob /nor pspdump, mbasic86 prog /m:&HC000.

4. Feature parity

Area	5.22 CP/M-86	5.28 DOS	GW-BASIC	5.50
Tokens (tokenised programs interchangeable)	131 keywords	136 keywords	own numbering	5.28 numbering, 136
Core language, math, strings, PRINT USING, arrays, DEF FN	✔	✔	✔	✔
WHILE/WEND, CHAIN/COMMON/MERGE, ON ERROR/RESUME	✔	✔	✔	✔
Sequential/random files, FIELD/LSET, FILES/KILL/NAME	✔	✔	✔	✔
OPEN "A" (append)	✘	✔	✔	✔
BLOAD/BSAVE, CALLS	✘	✔	✔	✔
DATE$, TIME$	✘ (plain variables)	✔	✔	✔ (CP/M: BDOS 3.x)
PEEK/POKE/DEF SEG/USR/CALL/INP/OUT/WAIT	✔	✔	✔	✔ (WAIT breakable with ^C)
USR "undefined" check (slot = FFFF)	✘ byte compare CMP DL,0FFh: DEF USR address &HxxFF gives "Illegal function call"	✔ word compare	✔ word compare	✔ word compare, both targets
SAVE ,A	✔	✔	✔	✔
SAVE ,P (protected: FE header, XOR-encoded; LIST/EDIT/PEEK/POKE/binary SAVE blocked)	✔	✔	✔ own tokens	✔ files interchangeable with 5.28 both ways
EDIT line editor (D C S K I X H L A E Q)	✔	✔	full screen	✔ MBASIC style
AUTO, RENUM, DELETE, TRON/TROFF, NULL	✔	✔	✔ (no NULL)	✔
Input keys ^U ^X ^R ^O, rubout \x\, ^C/^S	✔	✔	screen keys	✔
Device names KYBD: SCRN: LPT1:	✘	partial	✔	✔
Division by zero trappable by ON ERROR	✘	✔	✔	✔
ERR 57 message	"Disk I/O error"	"Device I/O Error"	"Device I/O Error"	CP/M-86: "Disk I/O error", DOS: "Device I/O Error"
ERR 68 message	Unprintable error	Unprintable error	"Device Unavailable"	"Device Unavailable" (device names)
Command line /F: /S: /M:	✔	✔	✔	✔ (/F: /S: accepted, ignored: files and records are dynamic)
Command line /NOB[ANNER] (no banner, no "Bytes free"), /NOR[UN] (load the program, don't run it)	✘	✘	✘	✔ before or after the file name, e.g. mbasic86 /nob pspdump (5.28 accepts options only after the file name)
Graphics, sound, CLS/LOCATE/COLOR/SCREEN, KEY	✘	✘	✔	✘ removed
COM:, light pen, joystick, cassette, events (ON KEY…)	✘	✘	✔	✘ removed
CHDIR/MKDIR/SHELL/ENVIRON, VARPTR$, Kanji	✘	✘	✔	✘ removed

Verification: identical to 5.28 (DOS) and 5.22 (CP/M-86) on all tests/ scripts except the reviewed extras in tests/accept/dos and tests/accept/cpm; both builds identical to each other except ERR 57's wording and an emulator limit (tests/accept/parity).

5. Dependencies

What	Where	Used for
cpm86-crossdev	https://github.com/tsupplis/cpm86-crossdev	pcdev_masm (MASM 5.10), pcdev_link (LINK 3.65a), cmdinfo
emu2-cpm86	https://github.com/johnsonjh/emu2-cpm86	runs the DOS tools, both builds and the reference BASICs
GNU make, python3, unix2dos	host	build scripts, test driver, CMD packer
Reference binaries	ref/	test oracles, see below

Reference binary	Version	Target	Used by
ref/mbasic86.com	MBASIC 5.28	MS-DOS	make test (DOS)
ref/mbasic86.cmd	BASIC-86 5.22	CP/M-86	make TARGET=cpm test
ref/mbas521.com	MBASIC 5.21	CP/M-80	reference only (not used by tests)
ref/obas451.com	BASIC Rev. 4.51	CP/M-80	reference only (not used by tests)

6. Build and test

Command	Does
make	both targets
make dos / make cpm	one target (build/dos/mbasic86.com, build/cpm/mbasic86.cmd)
make run / make TARGET=cpm run	start the interpreter under emu2
make test / make TARGET=cpm test	every tests/*.txt on reference and build, diff
make parity	every test on both builds, diff
make testall	the three suites (dos, cpm, parity) in parallel, logs in build/test-*.log, PASS/FAIL summary
make DEBUG=1 …	build with error tracing
make dist	flat build/mbasic.zip: mbasic86.com, mbasic86.cmd, reference mbas528.com (5.28), mbas522.cmd (5.22), mbas521.com (5.21 CP/M-80), obas451.com (4.51 CP/M-80), LICENSE.md
make clean	remove build/

Layout
*.asm, *.inc	interpreter (MASM 5.10), oem.inc switches, cfg.inc generated per target
giotty.asm, oemio.asm	teletype console, line editor, OS primitives
tools/	runbas.py test driver, mkcom.py EXE→COM, mkcmd.py EXE→CMD, normout.sh
tests/	scripts (\k ^C, \e ESC, \^X control keys, first line #args: = command line, NAME.bas companion file), accept/ reviewed differences
examples/	ivtdump.bas: 8086 interrupt vector table (DEF SEG=0, PEEK); pspdump.bas: MS-DOS PSP / CP/M-86 base page at BASIC's DS:0 (DEF SEG, PEEK); beep.bas: PC speaker beep (OUT, INP, PC hardware only). emu2 shows placeholder vectors; real values on real hardware

7. Possible evolution

Idea	Note
Extensive testing on real DOS 1.10 / CP/M-86 1.1 / Concurrent DOS	start-up checked, full tests/ not yet run there
LINK 5.10b	currently rejects the object set; LINK 3.65a used
Exact file lengths on CP/M 3 (LRBC)	binary files end on 128-byte records today
8087 support, more devices	only if it stays OS-generic

8. Fun facts and quirks (found in the 1983 code)

Finding	Where
Machine-translated 8080 code: every module says "translation created … by Version 4.3", and 8080 tricks survive, e.g. DB 271O (the 8080 LXI B opcode) to jump over the next two bytes	gwmain.asm SNERR…TMERR
SAVE ,P "encryption" is two XOR passes keyed on the floating-point coefficient tables of EXP and LOG ($EXPCN, $LOGP, comments say ATNCON/SINCON)	giodsk.asm PENCOD/PDECOD
It was once "GW-CPM BASIC": the switch file is titled "Common file to produce 2-segment 8086 GW-CPM BASIC", and half-finished CP/M-86 date code (BDOS 155) was left in	oem.h, gwsts.asm
OEM ghosts: switches for MELCO, ZENITH, SIRIUS, TSHIBA, PC8A (NEC)…, and a CLD ;Because of Melco BIOS Bug	gwmain.asm, gwsts.asm
Exit to DOS by pushing segment:offset and doing a far RET, because the "translator can't handle JMPI ,adr yet"; many instructions are hand-assembled in octal with the INS86 macro	giodsk.asm SYSTME, oem.h
Credits in the header: "originally written on the PDP-10 from February 9 to April 9 1975 — Bill Gates wrote a lot of stuff, Paul Allen wrote a lot of other stuff and fast code, Monte Davidoff wrote the math package"	gwmain.asm
Generated switch files: "Pascal program HFILE searches for the following line - DO NOT MODIFY"	oem.h
Honest comments: RET ;YOU WOULDN'T BELIEVE IT IF I TOLD YOU, RET ;THIS IS A KLUDGE TO MAKE CHRGET WORK	gwmain.asm, gwdata.asm
GW-BASIC still thinks it is MBASIC: "FIX … SO THAT IF ^C DURING MBASIC FOO, WONT EXIT TO SYSTEM"	dskcom.asm
Typing fossils: literal backspace characters in comments (EXPECT USUALL^H NUMBERS), and files padded with NULs / ^Z to 128-byte CP/M records	gwmain.asm, bimisc.asm, all .h files
The DOS critical-error handler (INT 24h) never returns with IRET: it discards 20 bytes of DOS stack and jumps straight into BASIC's ERROR	gio86.asm DSKERR
"8086 versions force stack entries to be an even length": every FOR entry carries an extra padding byte on the stack	gwmain.asm
The USR "undefined slot" check is hand-assembled in octal (INS86 203,372 + DB 377O = CMP DX,-1); BASIC-86 5.22 got it wrong as a byte compare (80 FA FF)	gweval.asm USRFN
NULL n still sends n NUL characters after each line — padding for mechanical teletypes that need time to return the carriage	bimisc.asm NULL

Companion projects

Project	Description
cpm86-kernel	CP/M-86 1.1 distribution rebuilt from patched and reconstituted sources
ccpm86-y2k	CCP/M-86 3.1 distribution rebuilt from patched and reconstituted sources
cpm86-crossdev	Unix CP/M-86 cross development project (compilers, emulation and tools)
cpm86-hacking	CP/M-86 miscellaneous tools and PCE emulator helpers
cpm86-cmdtools	CP/M-86 .cmd file manipulation tools
cpm86-ports	CP/M-86 application ports in C and assembler
cpm86-vi	STevie vi port for CP/M-86 and PC-DOS 1.1
cpm86-msbasic	A recreaction of msbasic-86 for CP/M-86 and PC-DOS 1.1 from gwbasic sources
cpm86-menu	A submit replacement and a menu system for submit orchestration
pcdos11-hacking	PC-DOS 1.1 distribution, tools and notes

License

MIT (see license). Original source © Microsoft Corporation.