This worked with my modded 16K ZX80 and my brother's Timex 1000, but does not work (yet) with the z81 series emulators. So what happens? Would tracing the USR call and how the screen is displayed reduce or add to the magic?
rand usr 975
rand usr 975
First, belated hellos from me to everyone as well! It is really nice to read about everyone's projects. Here is just something funny my brother discovered in the early 80s. Usually RAND USR n where n is a random address in the ROM crashes the ZX81. But for n=975, something interesting happens; screen information outside the normal part is activated while rebooting:
This worked with my modded 16K ZX80 and my brother's Timex 1000, but does not work (yet) with the z81 series emulators. So what happens? Would tracing the USR call and how the screen is displayed reduce or add to the magic?
This worked with my modded 16K ZX80 and my brother's Timex 1000, but does not work (yet) with the z81 series emulators. So what happens? Would tracing the USR call and how the screen is displayed reduce or add to the magic?
Re: rand usr 975
After the RAND USR 975, the stack pointer has bit 15 set, so probably the magic occurs because of POPs and PUSHes within the vertical and horizontal margins. But this would mean other memory operations (besides execution of NOPs for the normal display) with the ZX80 (and perhaps ZX81) hardware with addresses above 32K could be visible too - can anyone confirm this? [Note that EightyOne crashes in SLOW mode because NMIs occur just when the collapsed screen is being built; and only generates pixels with opcode execution.]