Re: identifying an unknown eprom content

From: groepaz_at_gmx.net
Date: Thu, 27 Aug 2020 13:44:58 +0200
Message-ID: <1911491.Jadu78ljVU_at_rakete>
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


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Received on 2020-08-27 14:04:00

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