Should I strive for 100% compatibility with original ZX81, or is it okay to make some improvements that you wouldn't notice with regular ZX81 software? For example:
- Right now I have a 32KB SRAM wired up to occupy 16KB in 4000-7FFFh area (and AFAIK mirrored at C000-FFFFh), with the other half unused. This to minimize required logic (entire mod is just a few track cuts and <10 cm. wiring in total!). But I'd like to use that entire 32K, for example in 2000-9FFFh area.
- ZX81 system ROM uses just a few I/O ports, but hardware responds to many mirrors of those. I'd like to cut that down, so you could use other I/O ports freely without affecting NMI enable/Vsync circuitry.
FWIW: I don't care much for the hardware side of things, if I need an extra signal to connect to my 'ULA', that's OK. I'm interested solely in the software side of things.
On a related note: do you know some programs/demo's, that are good for testing specific ZX81 hardware features? I'm thinking about programs that access such non-standard I/O ports, mirrored ROM/RAM area's, play with hsync/Vsync timing, etc. In short: the kind of programs that let you quickly determine whether your ZX81 behaves like the real thing. Preferably short proggies...