Re: Build your own REU
Date: 2001-04-15 13:14:03

Hallo Bogax,

> sync on ADDSTB and insert S3s ?

I own you all a big OOPS: I don't know how it happened but I simply missed 
the READY-input. This input plus 4 Mhz should give us an almost 1 MB/sec 
transfer. "Almost" because we loose a cycle every 256 bytes to update 
(Something worth to filosophize about: C128's 2 MHz-mode ???)

> synchronizing the 8237s cycles with the C64s timing

The problem is that a synchronized 4 MHz signal does not mean that the 
produced read- and write-cycli are sunchronized as well. So I'll use two 
573's to create a temporary buffer in the databus between the C64 and 
ISA-bus. Although actually not needed then, it worked fine in my first 

> Also, perhaps you could manage a memory to memory transfer at something
> like 1.5 PHI2 cycles per transfer ie 3 cycles of PHI2 to move a byte

I'm afraid that M2M transfer will stick at one byte for every two cycles. 
Unless we use two 8237's. I'm still playing with this idea because of the 
speed we gain but also the fact that I haven't worked out the scheme for 
telling the 8237 what byte goes/comes to/from to which part of the circuit 
during this type of the transfer. I'll explain:

One bit should be enough to tell the circuit that an I/O-DMA goes to the 
C64 or to the ISA-bus it self. But with an M2M-transfer we have more 
1) C64 -> ISA
2) ISA -> C64
3) C64 -> C64
4) ISA -> ISA
I didn't thought about the two last until now. Hmmm, writing down things 
sure help to clarify them as well, I am beginning to believe that a 
"simple" 74159 could do the trick :) And then we don't need a second 8237 
(unless speed with M2M prevails). 

Groetjes, Ruud

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