Building an IBM PCjr BIOS From Source: A Rare Legacy Reverse-Engineering Win

Misryoum reports on a project that reconstructs the IBM PCjr BIOS from printed assembly listings, producing a buildable, byte-identical image for emulator work and ROM experiments.

IBM’s PCjr never quite won over the crowd, but Misryoum readers now have a fascinating new way to study what made it tick.

A recent build effort takes something most people never get: the original printed x86 assembly listing for the PCjr BIOS.. Using that source material. the project turns the BIOS into an ASM project that can be assembled into a byte-identical copy of the firmware—an unusually precise target for retrocomputing work where “close enough” is common.

The practical workflow is intentionally old-school.. The project provides a ZIP package with the assembler and linker tools required to build the BIOS. and it’s designed to run under DOS or DOSBox.. A simple build.bat kicks off the process, producing an executable output that then gets converted into a BIN image.. For that conversion step. a Python helper script (exe2bin.py) is included. though there’s room for manual conversion if you prefer.

Here’s the part that matters if you’re thinking beyond emulation.. The PCjr doesn’t store its BIOS as a single contiguous ROM.. Instead, its firmware is split across two 32 kB ROM chips.. So even if you successfully generate a BIOS image in BIN form. you can’t usually burn it directly into modern “one image fits all” workflows.. You have to split the output to match the PCjr’s two-ROM layout—otherwise. the hardware won’t see what it expects at each chip select.

This is also where the legal and archival reality of firmware projects shows up.. Even though the code is derived from IBM’s printed listing, the BIOS code remains copyrighted.. Misryoum stresses that “build it from source” doesn’t automatically become “free to ship or redistribute. ” especially if you’re planning to distribute binaries or ROM images.. For most hobbyists. the safest path is to use the result for personal testing—emulators. diagnostics. and preservation—rather than treating it like open source.

The larger significance is cultural as much as technical.. IBM’s PCjr is often remembered for being “unloved. ” with a short. limited production run and a reputation that never quite matched the brand.. Estimates from the mid-1980s suggest hundreds of thousands were sold by early 1985. and there were also reports of large numbers sitting unsold in warehouses as the market moved on.. Exactly how many units remain today is hard to pin down. but the trend in retro computing is clear: preservation efforts tend to lag until someone makes reproduction and study simpler.

That’s what makes this kind of build-from-printed-source project stand out.. It’s not just about getting a working ROM; it’s about validating that the reconstruction is faithful enough to be byte-identical.. For historians of computing and engineers who like to see how early firmware really behaved, that precision reduces guesswork.. It also creates a foundation for deeper experiments—like comparing emulator behavior to expected firmware logic—without turning the project into an endless cycle of “interpretation drift.”

For Misryoum readers who already tinker with vintage systems. the take-away is straightforward: if you want to understand the PCjr at the firmware level. having a repeatable build pipeline built around authentic printed assembly is a rare advantage.. And if you’re building your own ROM set. the two-chip split requirement is the checklist item that can save hours of frustration.

As preservation moves from collecting to reconstructing. projects like this hint at where retro work is heading: less “mystery firmware archaeology” and more careful. reproducible builds that help keep fragile legacy platforms alive—at least long enough for the next generation of tinkers to learn from them.