 The unexpanded ZX81 's tiny memory presented a major challenge to programmers . Simply displaying a full screen takes up to 793 bytes , the system variables take up another 125 bytes , and the program , input buffer and stacks need more memory on top of that . Nonetheless , ingenious programmers were able to achieve a surprising amount with just 1 KB . One notable example was 1K ZX Chess by David Horne , which managed to include most of the rules of chess into only 672 bytes . The ZX81 conserved its memory to a certain extent by representing entire BASIC commands as one @-@ byte tokens , stored as individual " characters " in the upper reaches of the machine 's unique ( non @-@ ASCII ) character set . 
