See all articles from Syntax/Syntax ZX80 v3 n7
Step-by-step guide for building a piggyback ROM switch to run both 4K integer BASIC and 8K ROMs in the ZX80 without swapping chips. Uses a single-pole switch mounted inside the ZX80 case, with both ROMs soldered together except at pin 24 (Vcc), which is switched to activate one ROM at a time.