RE: Weird behavior of C64 IEEE interfaces
Date: 2006-08-01 12:36:11

Hallo Anders,

> The first few bytes are vectors to $B009 (start) and $B076

Ah, thanks! Hadn't seen that yet.

> Maybe someone finds it interesting to compare the VIC-1112
> ROM dump....

In fact I started disassemble it to find out how it works, this because of the weird behavior of the C64 IEEE interfaces. (This evening I want to test the VIC-1112's with CBM-HD as well.) But I already found out (but not for 100% sure yet) that the used protocol doesn't look exactly what I expected: it doesn't handle both NRFD and NDAC as it should according the official IEEE timing diagrams. If this indeed is the case, then it seems that CBM-HD indeed is to blame because it is not flexible enough.

Regarding the C64 IEEE intefaces: as engineer I would rather use the VIC-1112 as base then one of the original IEEE drives. A simplified protocol would explain my troubles. Only funny that the drives themself except it as well. 

> ... and see if they have anything in common.

I found pieces of code with no reference at all. Not one or two, but about twenty! I first ruled out the obvious things like JMP and JSR. Next I checked the JMP (xxxx) construction:
  lda #$xy
  sta RAM
  lda #$Bz
  sta RAM+1
In this case I looked for #$B in my sources and of course I found one or two but not enough to cover the twenty I'm talking about. 

Another weird thing: the piece starting at $B273 is also interesting: it stores data at $9C01, the IO3 area. Another piece can be found at $B0DD. This piece calls a routine at $B59D which stores into and reads data from $9C01. $9C00 is used as well.

My impression is that someone used an already existing cartridge as base to create this one. A very sloppy job IMHO. OK, but as already said, I haven't finished the disassembling yet so we stil can run into some suprises. 

    / __|__
   / /  |_/     Groetjes, Ruud
   \ \__|_\
    \___|       URL:


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