RE: Commodore 8296GD

Hello,

> I wrote a simple machine language program that pokes various values to
> $E888 and tries to read from $9000, $A000, $B000 and $E800. It turns
> out that some values hang the computer even when interrupts are
> switched off, and the computer will not reset after that (it even
> doesn't beep), so most probably they map out the $E888 register from
> writing so it cannot be restored.
> 
> I did not see any significance of bits 3-6. Here are the results of
> the read test (please note that I also tried reading from $F000 but it
> always returns ROM):
> 
> 0, 2, 8, 10:
> $9000 - RAM, $A000 - RAM, $B000 - RAM, $E800 - RAM
> 
> 1, 3, 9, 11:
> $9000 - RAM, $A000 - RAM, $B000 - RAM, $E800 - I/O
> 
> 4, 6, 12, 14:
> $9000 - ROM, $A000 - ROM, $B000 - ROM, $E800 - RAM
> 
> 5, 7, 13, 15:
> $9000 - ROM, $A000 - ROM, $B000 - ROM, $E800 - I/O
> 
> 128, 129, 136, 137:
> hang
> 
> 130, 138:
> $9000 - RAM, $A000 - RAM, $B000 - RAM, $E800 - I/O
> 
> 131, 139:
> $9000 - RAM, $A000 - RAM, $B000 - RAM, $E800 - RAM
> 
> 132, 140:
> $9000 - RAM, $A000 - RAM, $B000 - ROM, $E800 - I/O
> 
> 133, 141:
> $9000 - ROM, $A000 - RAM, $B000 - ROM, $E800 - RAM
> 
> 134, 142:
> $9000 - RAM, $A000 - ROM, $B000 - ROM, $E800 - I/O
> 
> 135, 143:
> $9000 - ROM, $A000 - ROM, $B000 - ROM, $E800 - RAM

At a look to the pcb photos a try to make a schematic. It's not complete, but the 74LS174 is an six bit register. On Bit is the select for the mux 74LS258. Four bits go to the input (b) of the four mux. The other input (a) come from the four jumper. I don't know the correct output, but this must be the cable to the jumper JU1 ... JU7 on the 8296 board. So i think this register are software controlled jumper JU1 ... JU7 (or better a subset).

> One more thing - the register is not cleared by a reset. So if you map
> out the BASIC ROM, then press the reset switch, the computer will not
> restart because the BASIC ROM is still mapped out. Only switching it
> off and on will work. Quite sloppy design, because resetting the
> computer while performing some graphics operation may render it
> unusable unless physically turned off (losing all RAM contents).

The register 74LS174 has a /CLR input at pin 1. I see a track near this pin. On the other end of this track is pin 2 (/RES) of 6545. So the question is, isn't that connected?

Martin


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