I had a ZX-81 and it was not poorly documented. Also, the Z80 assembly language was quite advanced compared to others from the same era (fe 6502). It had 16 some bit registers and 2 register sets (with at that time plenty of registers) that could be swapped.
The machine had a Basic interpreter that used bytecodes (to save memory), so programmers regularly mixed adopted a mixed style : basic + asm .
Anyway, to write a chess program for that machine is however rather impressive.
