I think this is the cause of the Challenger ROM issuing an occasional Bad Track Format error on b2. It initiates a seek with data register=255, and cancels it pretty much straight away (within a few tens of microseconds) - sounds like DDOS might do the same thing. The problem bit of code is LB9F3 in Greg Cook's disassembly: http://regregex.bbcmicro.net/chal220.asm.txt
b2 was doing an emulated disk head movement straight away, so as an initial fix/workaround, I've shuffled things about a bit so that there's a step rate delay before the first seek. That's probably more like how it works in real life anyway. So there's always at least 3 ms before the actual emulated drive head movement takes place, plenty of time for DDOS to cancel the seek.
There's still a lot of work to be done on b2's 1770 emulation (see also the comment in the disassembly...), but maybe I can now put it off for yet another day...
--Tom
b2 was doing an emulated disk head movement straight away, so as an initial fix/workaround, I've shuffled things about a bit so that there's a step rate delay before the first seek. That's probably more like how it works in real life anyway. So there's always at least 3 ms before the actual emulated drive head movement takes place, plenty of time for DDOS to cancel the seek.
There's still a lot of work to be done on b2's 1770 emulation (see also the comment in the disassembly...), but maybe I can now put it off for yet another day...
--Tom
Statistics: Posted by tom_seddon — Sun Jun 08, 2025 8:34 pm