MBASIC 5.21 Source Reconstruction

[aaw...@gmail.com]

For CP/M, the widely used and distributed version of Microsoft BASIC is 5.21. Unfortunately, the original source code for this version has been lost to time.

This project takes the nearest available 5.2x sources and reconstructs them to compile byte-for-byte identical to the original MBASIC 5.21 binary.

Background

It has been a constant request on retro computing forums to get the MBASIC 5.21 sources. Some folks just want to learn how the interpreter works. Others want to add extensions for specific hardware or implement new features.

A set of sources labeled "MBASIC 5.2" exists, but while they are in the same family, they are clearly not the sources for 5.21. These original 5.2 sources are preserved in the mbasic_52/ directory.

Approach

1. A custom disassembler (ud80) was used to disassemble the original mbasic.com 5.21 binary
2. The disassembly was analyzed to identify differences from the 5.2 sources
3. Minimal changes were made to the 5.2 sources to generate a byte-for-byte match with 5.21
4. The reconstructed sources are in mbasic_521/mbasic_src/

Project Structure

mbasic2025/ ├── mbasic_52/ # Original MBASIC 5.2 sources (reference) ├── mbasic_521/ │ ├── mbasic_src/ # Reconstructed 5.21 sources │ ├── com/ # Reference mbasic.com 5.21 binary │ ├── disasm/ # Disassembly of mbasic.com │ ├── build.sh # Build script │ └── utils/ # Utility scripts └── README.md

Building

Prerequisites

You need um80, a MACRO-80 compatible assembler toolchain for Linux.

Install from PyPI:

pip install um80

Or install from source:

git clone https://github.com/avwohl/um80_and_friends.git cd um80_and_friends pip install -e .

Build Process

cd mbasic_521 ./build.sh

This will:

1. Assemble all source modules (.mac files) into relocatable object files (.rel)
2. Link them together to produce out/mbasic_go.com
3. Generate a symbol file out/mbasic_go.sym

um80 Toolchain

The um80 toolchain includes:

um80 - MACRO-80 compatible assembler

um80 source.mac -o output.rel [-l listing.prn]

ul80 - LINK-80 compatible linker

ul80 -o output.com [-s] file1.rel file2.rel ...

• -s generates a .sym symbol file
• -x outputs Intel HEX format
• -p ORIGIN sets program origin (default: 0x100)

Source Modules

The interpreter is built from these modules (in link order):

Module
Description
bintrp.mac
Main interpreter core
f4.mac
Floating point math
biptrg.mac
Program storage/management
biedit.mac
Line editor
biprtu.mac
Print utilities
bio.mac
I/O routines
bimisc.mac
Miscellaneous functions
bistrs.mac
String handling
binlin.mac
Line input
fiveo.mac
File I/O
dskcom.mac
Disk commands
dcpm.mac
CP/M interface
fivdsk.mac
Disk file handling
init.mac
Initialization

License

The original MBASIC sources are copyright Microsoft. This reconstruction is provided for educational and historical preservation purposes.

Links

• Project repository: https://github.com/avwohl/mbasic2025
• um80 assembler: https://github.com/avwohl/um80_and_friends