The I/O address space provided another feature on Z80 (and 8080) machines, which was "shadow rom". Basically you booted with some "massive 8K" ROM and then you

The I/O address space provided another feature on Z80 (and 8080) machines, which was "shadow rom". Basically you booted with some "massive 8K" ROM and then you could write out a byte which would swap it in memory space for a 1/2K very simple jump table/switcher. The 6502 had a similar feature, that let you turn off the ROM mapping into the 64k of address space so you could access the RAM "underneath" for storage of your own programs. Especially useful for programs that didn't need the BASIC interpreter. https://www.youtube.com/watch?v=fe1-VVXIEh4#t=26m41s https://www.youtube.com/watch?v=fe1-VVXIEh4#t=26m41s