[Someone]

https://retrocomputing.stackexchange.com/a/11456:

“Space Invaders uses a simple display format where bytes are read from memory in order via an address counter, and shifted out via a shift register. Timing is controlled by discrete hardware.

The video hardware has priority over the CPU. When it needs to read a byte it asserts the 8080's READY signal, giving it exclusive access to the memory bus. This can be seen on the schematic (https://www.robotron-2084.co.uk/taito/documents/space-invade...) at 5F.

This technique has the advantage of the 8080 not having to be synchronized with the display hardware at all.”