You're linking to information on a ZX Spectrum which is not a ZX81. The ZX81 had a monochrome character mapped display with no pixel access (well - there were tricks that could sort of give you pixel access but they came much later than the time 1k chess was written)
Sorry about that... i didn't mention colour depth originally, someone else brought it up (trying to look smart).
6502 was my platform, so I missed the exact Z* platform...
I was sharing general info on assembly for 8bit, not specific to one platform (ZX81). This seems to have incensed some people who took a narrow view on what sort of comment was permitted.
