Re: identifying an unknown eprom content

From: didier_at_aida.org
Date: Thu, 27 Aug 2020 14:12:44 +0200
Message-ID: <a5ec6e41-3841-cd11-cf5f-50d28b45a453_at_aida.org>
some code for the madison z-ram ?


On 27/08/2020 14:02, groepaz_at_gmx.net wrote:
> Am Donnerstag, 27. August 2020, 13:53:43 CEST schrieb Hans Liss:
>> On 2020-08-27 13:44, groepaz_at_gmx.net wrote:
>>> Am Donnerstag, 27. August 2020, 13:41:46 CEST schrieb groepaz_at_gmx.net:
>>>> Am Donnerstag, 27. August 2020, 13:32:56 CEST schrieb Hans Liss:
>>>>> Odd code. It seems to have some magic I/O at $8000/$8001, with $8001
>>>>> maybe producing side effects on read.
>>>>>
>>>>> Also, the sequence starting at $e7a0 is kinda epic. It laboriously
>>>>> stores code, instruction by instruction, at $0400 - $041b, and then
>>>>> jumps to $0400. I wonder why.
>>>> that would be the typical thing to do if you eg want to bank out the ROM
>>>> you are running from, to call code from another ROM (bank), or from RAM
>>>>
>>>> $400x seems to be I/O to me as well... there really must be more to this,
>>>> ie without the schematics or knowledge of the hardware it doesnt make a
>>>> lot of sense
>>> this is the code at 0400
>>>
>>> .C:0400  A2 FF       LDX #$FF
>>> .C:0402  A0 FF       LDY #$FF
>>> .C:0404  A9 5C       LDA #$5C
>>> .C:0406  8D 00 80    STA $8000
>>> .C:0409  CA          DEX
>>> .C:040a  D0 F8       BNE $0404
>>> .C:040c  88          DEY
>>> .C:040d  D0 F5       BNE $0404
>>> .C:040f  A9 A2       LDA #$A2
>>> .C:0411  CD 00 04    CMP $0400
>>> .C:0414  D0 03       BNE $0419
>>> .C:0416  4C 32 E8    JMP $E832
>>> .C:0419  4C 2D E8    JMP $E82D
>> You were faster! :)
>>
>> It's a bit strange. It's a clear sign of self-modifying code, checking
>> the value at $0400 like that. But I can't see anything in the ROM code
>> that might change that value.
> perhaps there were two CPUs in the system? and this is the "locking" between
> them? that kind of thing wasnt terribly uncommon in the PETs
>
> --
>
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Received on 2020-08-27 15:01:18

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