Hallo allemaal, I need some advise (or a vote). The 65xx is able to access I/O or memory like a 80x86 does; this CPU is able to tell wether it is an I/O- or Memory-operation. My first solution to solve this problem is to create a bit which tells the ISA-bus what kind of operation it is. Due to the design of the total system, I need a way to tell the logic what the source and destination of each DMA-transfer is: the C64 or a ISA-card. Again I used two bits. The above system caused one problem: I'm not able to perform a read from I/O and write it to memory on the ISA-bus without the need to flip the I/O-bit between every read and write. I do have an alternative and that is to reserve two 64 KB blocks for telling the system what type of transfer is meant: - $FFxxxx: I/O - $FExxxx: C64 - other address: ISA-bus memory transfer. FYI: I have two 8-bit latches for addressing A16..23. One is enabled only during write operations, the other during read operations. This enables me to perform transfers beyond the 64K-border. Using again the above sample, the "read-latch" is filled with $FF, the "write-latch" with (for example) $B8 (videocard) and now the C64 is able to read data from the RS232-port and to write it directly to the memory of the videocard. The only disadvantage I see for the moment is that we loose 128 KB on a total of 16 MB. Some feedback would be appreciated. Thanks. Groetjes, Ruud http://Ruud.C64.org - This message was sent through the cbm-hackers mailing list. To unsubscribe: echo unsubscribe | mail email@example.com.
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