It was possible to add SLOW mode to the ZX80 with a minor circuit addition [1][2], and upgrade 8k ROMs to provide ZX81 capabilities were available[3][4][5]. [1

It was possible to add SLOW mode to the ZX80 with a minor circuit addition [1][2], and upgrade 8k ROMs to provide ZX81 capabilities were available[3][4][5]. [1] http://web.archive.org/web/20031006130728/http://home.micros.users.btopenworld.com/zx80/zx80nmi.html http://web.archive.org/web/20031006130728/http://home.micros... [2] https://www.myprius.co.za/ZX80.htm https://www.myprius.co.za/ZX80.htm [3] http://qrp.gr/zx80/romupgrade.htm http://qrp.gr/zx80/romupgrade.htm [4] http://www.fruitcake.plus.com/Sinclair/ZX80/ROMUpgrade/ZX80_ROMUpgrade.htm http://www.fruitcake.plus.com/Sinclair/ZX80/ROMUpgrade/ZX80_... [5] http://qrp.gr/zx80/romupgrade2.htm http://qrp.gr/zx80/romupgrade2.htm