HIMEM Space
It took me about 30 years to realise that "1K" mode 7 isn't 1K at all - it's a thousand bytes, so there should be 24 bytes left above the 1,000 bytes needed from the screen which start at HIMEM.
HIMEM in non-shadow mode 7 is 31744, or &7C00, and 1000 bytes above that is &7FE8. This is below the top of user memory at &8000, where the paged ROMs live, so we should be able to use this area for storage.
A convenient way of filling up 24 bytes is to use the 24-byte string, TIME$. Here I have poked it into &7FE8, and it reads back. It doesn't corrupt the screen either, which shows the area is outside the mode 7 screen memory.
