See all articles from Syntax/Syntax ZX80 v3 n5
Sinclair made the ZX81 RAM easy to decode up to 16K, but you may want to add memory beyond this point. To add more than 16K memory to your ZX80/81, you must do more than just: /A13 AND /A14 AND /A15 = /ROM CS. You’ll also need to account for the behavior of the display file.