On 3/11/22 19:06, Francesco Messineo wrote: > Hi, > > On Fri, Mar 11, 2022 at 6:13 PM Spiro Trikaliotis > <ml-cbmhackers_at_trikaliotis.net> wrote: >> >> Hello, >> >> I am trying to understand the sources of the 1571 and 1581 drives. >> >> In the original drive sources of the 1571 and 1581 (as found on >> zimmers.net), there is a macro WDTEST: >> >> WDTEST .macro >> .ife <*!.$03 >> nop >> .endif >> .endm >> >> I am not completely familiar with the syntax of the assembler used, but >> if I understand it correctly, it issues a NOP if bit 0 and bit 1 of the >> PC are both 0 - that is, the PC address is divisble by 4. > > wild guess: > The WD1770 in the Commodore 1571 (and I guess in the 1581 too, but > I've never seen one) has its chip select line > qualified with the wrong clock. Let me explain better: when a > peripheral device must be connected to the 6502's bus, it either has a > phi2 input > or we usually qualify the select to happen during phi2 high. > Sometimes, for example with static rams, it's usually enough to > qualify the write signal to phi2 high only. > Now, the 1571 was made when all the good engineers had already left > Commodore by a long time and to save one gate, they used phi1 LOW > OR'ed with the decoded > address range to obtain the WD1770 select. Possible... But in the 1581 they use PHI2, at least according to the schematics. Also, so far I have never seen a WD1770 in a 1581, the ones I have seen used the WD1772 or VL1772. GerritReceived on 2022-03-12 12:00:03
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