Sinclair ZX80 / ZX81 / Z88 Forums

From what I've read (including looking through ROM listings), it seems like I, IX, and the shadow registers all must be left alone to avoid interactions with the display interrupts. IY seems like it should be safe if restored to $4000 (which BASIC expects) before you return, but I may be missing som...

Okay, so it gets weirder... I can execute the code from the IO device at 0x3002 *if* the machine is in FAST mode, but things fall over in SLOW mode. The same code (listed below) runs correctly out of regular SRAM (in SLOW or FAST mode) Currently I'm copying the data to 0x2000 which is SRAM in this m...

So I believe trying to be cute and *execute* code from my IO device (a small stub that'd copy a larger piece out) was what really got me in trouble. The mechanism that NOP-izes most instruction fetches when A15 is high appears to work as "normal" on the zx81+38. Moving the address matching from 0xFn...

I'm kicking myself for not thinking of this first: https://twitter.com/matseng/status/1554534431523741697 https://pbs.twimg.com/media/FZLRKPSWIAEtrqV?format=jpg&name=4096x4096 Soon happy enough with the layout for my RP2040-based EPROM emulator to send for some PCBs. Managed to fit everything on the...

Excellent. Good to realize my approach is fundamentally flawed before I do another board spin! The ZX81+38 with 8K ROM / 32K RAM decodes memory this way: 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 * Instructi...

I'm building a little multi-purpose IO board to go along with the ZX81+38 I assembled... after a couple setbacks (I foolishly checked my header orientation vs the schematic, not the layout of the ZX81+38 and thus got it backwards -- addressed by inverting the header but also causing the board to fac...

The custom keycaps I ordered from WASDkeyboards arrived today and the machine is looking much more complete now:



I need to finish up the case design (rear cover, opening for expansion port).

Oh that is very slick, using the tape interface and being able to bootstrap from emulation of tape audio. My approach is definitely more involved and I figured I'd eventually end up modifying the ROM to support it transparently. Still going to finish building up these IO interfaces, since I've alrea...

Hi! I mostly grew up with a C64 that I learned BASIC and 6502 assembly on, but two years before our family got that machine, my father spent several evenings soldering together a MicroAce (unlicensed ZX80 clone) on our kitchen table and eventually hooked it up to a small B&W TV on UHF33 and we witne...

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...