This is what the documentation says... no 00 unfortunately:

Error Codes ----------- The BIOS may give a "System Error" code at bootup. This error code is a combination of the following cod […]
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Error Codes
-----------
The BIOS may give a "System Error" code at bootup. This error code is a
combination of the following codes:

01h - Bad system BIOS checksum
02h - Bad RAM in main system memory
04h - Bad RAM in video card
10h - Bad RAM in vector area (this also in main system memory)
20h - Bad expansion ROM checksum

Note that the codes are in hexadecimal. The "System Error" code given by the
BIOS is a sum of the above codes. For example, if the code 26 is given, the
individual errors are 02h, 04h, and 20h.

Sorry for the live debugging. I've inspected the RAM where the error code is generated from (found by looking through the BIOS source) and it appears to be 04 "Bad RAM in video card." Now I not only need to figure that out, but also why 04 is displaying as 00.

Turns out the video memory check error was caused by a mistake confusing how REPZ should work with STOSW. Now I am looking into why all the text stays on the first line.

The "staying on the same line" bug ended up being another CPU bug, in some of the first instructions I implemented (MOV, A0-A3) . I caught this by finally implementing automated CPU tests. Just goes to show how important CPU tests are when implementing an emulator...

Anyway, if this thread is at all helpful to other people, perhaps it will be with the message "implement CPU tests early."

Good work! Have fun, writing a PC emulator from scratch is a process full of blood, sweat and tears. Lots and lots of tears. But once everything comes together, it feels awesome!

Thanks! Now to implement the floppy disk controller... I think after that I'm ready to boot DOS.