Problem with running SHREB programs on ZX81+38 clone
Posted: Mon Jul 25, 2022 9:54 pm
I've been trying to get various SHREB based stuff to run on a ZX81+38 (rev1.7) and have not had much luck. WRX16K style hi res graphics seem to work fine on this machine. I suspect some subtle incompatibility in the memory layout or decode logic might be causing problems. The machine has 32KB static RAM onboard, mapped at 0x0000 and 0x8000, but obscured by 8KB of ROM at 0x0000-0x1FFF and that same ROM at 0x8000-0x9FFF (but only during nM1 / instruction fetch).
My guess is either there's something wrong with how the access to the 0x2000-0x3FFF region is setup, or the mirrored regions above 0x8000 are causing some confusion, but I'm not quite sure how to chase the issue down.
ZX81+38 info:
https://sinclairzxworld.com/viewtopic.php?f=7&t=3357
https://github.com/mahjongg2/ZX81plus38
ZX81+38 chip select circuitry:
Moderator note: moved from SHREB Quickies topic. Mark
Code: Select all
0000-1FFF ROM-A
2000-3FFF RAM-1
4000-5FFF RAM-2
6000-7FFF RAM-3
8000-9FFF RAM-0/ROM-A*
A000-BFFF RAM-1
C000-DFFF RAM-2
E000-FFFF RAM-3
* Instruction fetches from ROM, other r/w to RAM
ZX81+38 info:
https://sinclairzxworld.com/viewtopic.php?f=7&t=3357
https://github.com/mahjongg2/ZX81plus38
ZX81+38 chip select circuitry:
Moderator note: moved from SHREB Quickies topic. Mark