Display Memory
With displaying a screen consuming 793 bytes of precious, precious memory
Actually the ZX81 was a bit cleverer than that. If it detected it only had 1K of RAM, it used a slightly different way to story the screen: It stored each line up to its newline. That way, short lines of text used less memory than long lines. I recall seeing a game that only used the top-left corner of the screen to save memory.
Biting the hand that feeds IT
