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: graphics and PC hardware removed, tokens and behaviour aligned on MBASIC 5.28, one source tree producing an MS-DOS .EXE and a CP/M-86 .CMD.

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.

_{PC-DOS 1.10 (.EXE) · CP/M-86 1.1 (.CMD)}

_{Concurrent DOS 4.1 (.CMD) · DOS Plus 1.2 (.CMD)}

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.exe`	~35 KB	64 KB data segment
`build/cpm/mbasic86.cmd`	~35 KB	code group + data group up to 64 KB

3. 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)	✔
Correct file case handling (legacy of CP/M implementation)	✘	✔	✔	✔
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)
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).

4. 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)

5. Build and test

Command	Does
`make`	both targets
`make dos` / `make cpm`	one target (`build/dos/mbasic86.exe`, `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 DEBUG=1 …`	build with error tracing
`make dist`	flat `build/mbasic.zip`: `mbasic86.exe`, `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, `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

6. 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
`.COM` build for DOS	like 5.28 (31,744 B `.COM`); saves the 512 B `.EXE` header, but the start-up segment setup must change
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

7. 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
pcdos11-hacking	PC-DOS 1.1 distribution, tools and notes

Name		Name	Last commit message	Last commit date
docs		docs
images		images
ref		ref
tests		tests
tools		tools
.gitignore		.gitignore
LICENSE.md		LICENSE.md
Makefile		Makefile
README.md		README.md
biboot.asm		biboot.asm
bimisc.asm		bimisc.asm
bintrp.inc		bintrp.inc
biprtu.asm		biprtu.asm
biptrg.asm		biptrg.asm
bistrs.asm		bistrs.asm
call86.asm		call86.asm
dskcom.asm		dskcom.asm
fiveo.asm		fiveo.asm
gio86.asm		gio86.asm
gio86u.inc		gio86u.inc
giodsk.asm		giodsk.asm
giolpt.asm		giolpt.asm
giotbl.asm		giotbl.asm
giotty.asm		giotty.asm
gwdata.asm		gwdata.asm
gweval.asm		gweval.asm
gwinit.asm		gwinit.asm
gwlist.asm		gwlist.asm
gwmain.asm		gwmain.asm
gwram.asm		gwram.asm
gwsts.asm		gwsts.asm
ibmres.asm		ibmres.asm
ibmres.inc		ibmres.inc
itsa86.asm		itsa86.asm
math1.asm		math1.asm
msdosu.inc		msdosu.inc
next86.asm		next86.asm
oem.inc		oem.inc
oemio.asm		oemio.asm

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

1. Why and what

The family, as far as the sources tell

2. Runs on

3. Feature parity

4. Dependencies

5. Build and test

6. Possible evolution

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

Companion projects

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

1. Why and what

The family, as far as the sources tell

2. Runs on

3. Feature parity

4. Dependencies

5. Build and test

6. Possible evolution

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

Companion projects

License

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