Hallo David, > The ISA bus cannot run at 1mhz. A fair number o fcards might be using > this as a timing source, and it will throw things off big. I don't know of any cards using this clock-signal. > We have an 8mhz signal, its called the dotclock. THe ISA bus has a > method for dealing with slow devices. Its called wait states. Our > commodore will use them ;) If you refer to the IOCHRDY-signal, it's meant to slow down the computer, not for slowing down cards :) When addressing I/O, the PC automatically inserts enough waitstates so that the bus seems to run at 2 MHz. Until now I haven't seen any I/O-cards using this line either. This means that every I/O card is able to run on the C64 as the bus of C64 is "slower" the ISA-bus. How can one slow down a card: very simple, by extending the appropiate read/write-signal. I only can say this: I had no problems with CGA, MGP and multi-I/O cards in 1990. Dutch comment: behaalde resultaten in het verleden geven geen garantie voor de toekomst. Another matter could be: isn't the C64 not too slow for some PC-cards? IMHO I don't think so. But I cannot proove it. > For synchronization, the c64 also has waitstates. Have a look at the RDY > line. If you mean iochRDY, see remark above. If you mean the RDY-input of the processor, you won't find this at the expansionport. To access it, you really have to do some hacking. And as already mentioned, I haven't found a card yet which needs this slowdown. > When doing DMA, it doesnt matter, as ISA is plenty fast bto be able to > squeeze one byte in per c-64 cycle. I found an 1987 Intel databook with the 8237. I don't know if the contents is the same as your URL: http://support.intel.com/support/controllers/peripheral/231466.htm As said, the 8237 needs 4 cycli per byte (only two in compressed mode = 256 bytes). Running the 8237 at 4 MHz would give is a 1 MB/sec transferspeed. Please have a look at fig. 12, the memeory-to-memory transfer, at page 2-221. The MEMW cycle is only about 125 nsec. long, and that is too short. Same for compressed mode. Of course I tried to find other solutions like temporary buffers etc. but I ended up with a huge construction and still wasn't sure if it would work at all. So speaking for my self: I'll run the 8237 at 1 MHz unless one of you (including the lady) has a better idea. Groetjes, Ruud http://Ruud.C64.org/ - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail firstname.lastname@example.org.
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