As unloved as IBM’s PCjr was, with only a one-year production run, it’s hard to complain about the documentation available for it. This includes the x86 assembly listing for the BIOS, which [dbalsom] recently used this print version to create an ASM project that can be built into a byte-identical copy of the PCjr BIOS.
In order to build the BIOS image, a ZIP file has been made available that contains the requisite assembler and linker tools, all of which can be run in DOS (or DOSBox) using the provided build.bat file. This creates an executable file, which can then be converted into a BIN file using the provided exe2bin.py Python script, or of course, manually.
This image cannot be used as-is, as the PCjr has its BIOS split across two 32 kB ROMs, so splitting them is required if you intend to burn fresh ROMs. Of note is that the BIOS code is still copyrighted by IBM, so do not take this as some kind of open sourcing, unless you wish to test IBM’s legal take on 1980s BIOS code for a generally unloved system.
With an estimated 240,000 – 275,000 PCjrs sold by January 1985 and reports of hundreds of thousands of unsold PCjrs languishing in warehouses by the end. It’s hard to say how many PCjrs have survived to today, but it’s good to see that keeping this glimpse of a budget, not-quite-IBM-PC-compatible legacy alive has become a little easier again.
Heading image: IBM PCjr internals. (Credit: Binarysequence, Wikimedia)