Re: ROM Dump of Amiga Keyboard controller

From: Jim Brain <>
Date: Fri, 18 Jul 2014 19:38:05 -0500
Message-ID: <>
On 7/18/2014 5:17 PM, Segher Boessenkool wrote:
> Or you read from f800..ffff and you don't need any compare ;-)
Yep, I agree.  My 6502 ASM-fu is not that strong.
> It's the same as any other BRK, except the stack writes are turned into reads.
> Seven cycles total; first three are accessing the stack, next two are
> accessing the reset vector, last two are fetching the next instruction.
> Depending on how you count and on how your phases are aligned etc. you'll
> need to add some cycles at the start, e.g. for fetching the 2nd opcode byte.
> There is also a cycle or so delay from taking the reset pin high to the BRK
> being forced.  And the 6500 perhaps adds extra delay from its special reset
> circuitry, who knows.  But I doubt it.
> So sim it on visual6502 and find out ;-)  It looks like the four cycles
> are exactly what it takes to get to the vector reads.
But, I use 4 cycles from RESET to code execution.  I figured 7 cycles, 
but the system responds all the way down to 4.

Maybe if there was a code sequence I could use to see when it truly 
responds to code.  The initial code is:


But, if it didn't recognize the first 3 cycles, it would do:


That seems like it would fail.


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Received on 2014-07-19 01:02:42

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