<i>The I&#x2F;O address space provided another feature on Z80 (and 8080) machines, which was &quot;shadow rom&quot;. Basically you booted with some &quot;massive 8K&quot; ROM and then you could write out a byte which would swap it in memory space for a 1&#x2F;2K very simple jump table&#x2F;switcher.</i><p>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 &quot;underneath&quot; for storage of your own programs. Especially useful for programs that didn&#x27;t need the BASIC interpreter.<p><a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=fe1-VVXIEh4#t=26m41s" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=fe1-VVXIEh4#t=26m41s</a>