The Sinclair ZX80, launched in 1980 as a low-cost home computer, operates primarily by drawing the TV screen using clever hardware and running mostly NOP instructions rather than focusing on code execution. It achieves video output without a dedicated video chip by using the Z80 CPU, minimal RAM and ROM, and TTL logic chips to multiplex the address and data buses, intercepting reads in the high address mirror area to generate video signals through character ROM lookups and timing the CPU clock precisely to match TV scan lines, enabling the screen to be refreshed 50 times per second while handling input and memory access with minimal hardware complexity.
http://blog.tynemouthsoftware.co.uk/2019/10/how-the-zx80-works.html