Date: 2005-10-06 20:09:52
Seems that one short session on the real hardware, where I was able to promptly check everything, cleared the remaining doubts immediately... On 2005-10-05, at 12:09, Spiro Trikaliotis wrote: >> >> That's another interesting thing. You mean the status of the port >> bits on $1801 is a kind of "latched" one and the same bits are real- >> time accessible at $180f? >> > > Yes, exactly. It does not have to be latched, you can enable or > disable > it. > > I only noticed (was bitten) this once when I had a problem with the DC > ($1C01) where the first byte was always wrong. I found out that LDA > $1C01; LDA $1C01 immediately one after the other fixed that. This is a very important hint! [...] > > >>> While the floppy perform the BVC *, >>> the C64 has enough time to put the next value into the port which is >>> read by the 1541 via the EOR $1801 again. >>> >> >> The weak point for me here is the LDA $1801. It happens immediately >> after sending the handshake, for which the 64 waits. What is then >> being read? Since 1541 is faster than the 64, chances are that the >> port is being read before 64 puts the next value on the port, am I >> right? Race condition comes to mind... >> > > Well, the C64 cannot react that fast. This is impossible, as both > 6502/6510 run at almost the same speed, and the C64 is waiting in a > loop. Thus, you read the old value. Seems correct - and important. > > Why did the programmer add the LDA? I don't know. Perhaps, he wanted a > delay of 3 cycles? Perhaps, he wanted to confuse others? It seems for clearing the "latch". When I replaced EOR:LDA with LDA:LDA (on both sides) everything worked as properly. But when I removed the LDA immediately after the handshake, leaving only one as I would normally do - it no longer worked reliably. My current best guess is that it has to do with what has once bitten you. > To be honest, I don't know why the programmer chose to use EOR. > These take > exactly the same time and space as LDAs. Once I got the above working, I rechecked the whole flow again and found out that he does a primitive checksumming with those EORs and then "verifies" the checksums if that option is enabled, which I didn't use earlier. Now I set it as default for further actions. Thanks to all of you guys, Spiro being the first in line as without those few hints I would certainly have spent more time trying to figure those out. -- Arise, Sir Plenty of Bugs, Sir Mega of Lomaniac, Sir Screen of Blue, Sir Embrace of Extend, Sir 640 of K.... Message was sent through the cbm-hackers mailing list
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