Am Dienstag, 6. September 2022, 14:00:18 CEST schrieb silverdr_at_srebrnysen.com: > > On 2022-09-05, at 16:51, Jim Brain <brain_at_jbrain.com> wrote: > >>> But would that also cover the illegal Opcodes correctly? From what I > >>> understand, those are side effects of the NMOS implementation.>> > >> Were they? I always considered them to be a side-effect of the internal > >> decoder matrix which did not specifically "NOP" the unused opcodes > >> (which IIRC was done with the CMOS versions), but just bluntly decoded > >> the individual parts of the opcode, generating corresponding enable > >> signals. > >> > >> So stuff happened like e.g. loading the accumulator, but also throwing > >> the X register onto the internal data bus.> > > As I recall, a few incomplete decodes enabled two things to bump data onto > > a bus, which would then boil down to NMOS behavior with 2 outputs pushing > > electrons to the input. > Still, you don't always have to go bottom-up and reimplement NMOS physics to > get the desired behaviour. Top-down approach is OK sometimes too. You > know.. if it looks like a duck, walks like a duck and quacks like a duck.. > ;-) It is actually the easiest (and smallest!) way to implement it (except the 5 exceptions)... just have the exact same decode ROM/Table with the exact same T states, and most of those illegals magically work perfectly. The only thing you have to patch into it is the weird behaviour related to BA/DMA line, which can not be explained by a pure digital model. -- http://hitmen.eu http://ar.pokefinder.org http://vice-emu.sourceforge.net http://magicdisk.untergrund.net Mein Humor ist so elitär wie euer Gehabe!Received on 2022-09-06 15:03:17
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