Also not all of the addresses used for the on board ROM or the on board RAM are fully decoded. Indeed, the Z80 address line A15 is not used at all by the rest of the ZX81 circuitry. Ī standard ZX81 has minimal address decoding. Yes, I took the picture and then altered the program but forgot to take another picture before switching off. Please correct me if I'm wrong! Memory mapped I/O then avoids the need for a tiny bit of machine code, just to be able to read/write those I/O ports. I'm getting rusty, but it seems ZX81 Basic doesn't have commands to directly read or write to I/O ports? Kinda weird for such a low-level, hacking friendly machine. Memory mapped I/O is pretty clever, though. About as easy to design hardware-wise, much more flexible in terms of use. That provides you with 24 I/O's, software programmable as input or output, and even special commands to toggle single I/O pins. Personally I'd have stuck a 82C55A (or something like that) on those addresses.

Of course one could use that for 1001 purposes, but that's all there is to it, really. Or you read from some memory address, and value comes from the 8 TTL-level inputs on the other connector. So you write to some memory address, and the 8-bit value shows up as 8 TTL-level signals on a connector. Afaict it's a simple latch (for writing) and bus buffer (for reading), addressed via memory mapped I/O.

Zx81 projects