6502.org

I posted an updated version of the disassembled Atom OS ROM.

This version can be assembled with ca65 to match the binary exactly. I've updated the source to use labels instead of hard coded absolute addresses, so it's possible to make modifications to the code, and then re-assemble it to produce a working binary.

Dear Arlet,

I must say I don't think I've ever seen a ROM OS listing as well commented as this one. Bravo, Sir!

Cheerful regards, Mike

The commenting was already done by someone else, but it was not in a format that could be reassembled.

...and while the standard was of using BBC BASIC on the Atom was via a plug-in board that used a BASIC 2 ROM and a small MOS ROM that provided access to the tape system, there is actually a development version of BBC BASIC targetted to the Atom itself, which gives you access to all the normal Atom MOS functions.

Hmm... I found a place where there's a jump table that only contains the LSBs. For the MSB, a fixed value of $FD is used. Obviously this poses potential problems for relocation.

I've modified the source code to use symbol labels instead of hard coded numeric values, so now it is possible to relocate the code a little bit (as long as it stays in the $FDxx page).