Does anybody know in which ram adress the zero page from $40-$ff is shadowed?
Matthias
0
7800 zeropage handling
Started by Schmutzpuppe, Jan 18 2004 4:41 PM
6 replies to this topic
#2 OFFLINE
Schmutzpuppe
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Dragonstomper
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Posted Sun Jan 18, 2004 4:44 PM
Got it, looks like it is $2000-$2040.
#3 OFFLINE
DanBoris
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Posted Sun Jan 18, 2004 9:30 PM
Actually it's $2040-$20FF.
Dan
Dan
#4 OFFLINE
Schmutzpuppe
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Posted Mon Jan 19, 2004 12:54 AM
DanBoris said:
Actually it's $2040-$20FF.
Dan
Dan
Ups, right.
Thanks.
#5 OFFLINE
EricBall
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Posted Mon Jan 19, 2004 11:41 AM
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).
$0140 - $01FF is shadowed at $2140-$21FF and the stack grows bottom up (while ZP tends to be used top down).
#6 OFFLINE
Schmutzpuppe
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Dragonstomper
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Posted Mon Jan 19, 2004 1:55 PM
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
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
#7 OFFLINE
EricBall
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Posted Tue Jan 20, 2004 11:35 AM
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).
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).
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