Schmutzpuppe Posted January 18, 2004 Share Posted January 18, 2004 Does anybody know in which ram adress the zero page from $40-$ff is shadowed? Matthias Quote Link to comment Share on other sites More sharing options...
Schmutzpuppe Posted January 18, 2004 Author Share Posted January 18, 2004 Got it, looks like it is $2000-$2040. Quote Link to comment Share on other sites More sharing options...
DanBoris Posted January 19, 2004 Share Posted January 19, 2004 Actually it's $2040-$20FF. Dan Quote Link to comment Share on other sites More sharing options...
Schmutzpuppe Posted January 19, 2004 Author Share Posted January 19, 2004 Actually it's $2040-$20FF. Dan Ups, right. Thanks. Quote Link to comment Share on other sites More sharing options...
EricBall Posted January 19, 2004 Share Posted January 19, 2004 Don't forget the stack page $0140 - $01FF is shadowed at $2140-$21FF and the stack grows bottom up (while ZP tends to be used top down). Quote Link to comment Share on other sites More sharing options...
Schmutzpuppe Posted January 19, 2004 Author Share Posted January 19, 2004 Thanks for the hint. I don't find this kind of mapping very elegant the mapping should be in a row and at the beginning or end of the ram. Well, guess that have technical reasons. Matthias Quote Link to comment Share on other sites More sharing options...
EricBall Posted January 20, 2004 Share Posted January 20, 2004 Actually, I think it's kinda cool. GCC had some interesting constraints they had to work with, then build workarounds into the MARIA chip. Okay, start with the 2600, really with the 6507 & RIOT. The RIOT only has 128 bytes of RAM, and for the stack to work it has to be mapped to $0180 - $01FF. Zero page uses the same RAM, mapped to $0080 - $00FF; easy to do, just don't connect one of the address lines. This leaves $0000 - $007F for the TIA registers with easy access. Fast forward to the 7800, which has 4K of fast RAM mapped to $1800-$27FF. Built into the MARIA chip is an address decoder which (along with some external logic) has to handle 2600 mode addressing and 7800 modes (ROM & cart). For compatibility & sanity, the TIA registers are still mapped to $0000 - $001F, and the MARIA registers $0020 - $003F, leaving $0040-$00FF for RAM. For simplicity, this gets shadowed to $2040-$20FF, and the stack ($0140-$01FF) to $2140-$21FF (again, only one address line different). Quote Link to comment Share on other sites More sharing options...
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