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.
ROM routines is a jump into known addresses... so no extra impact on the 1K. Parameters are read from registers.
Color of 4bit? only a slight increase. Often acheivable in 1 instruction. Limit of 2 colors in 8x8 block on ZX81[1]
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[1] http://www.giantbomb.com/zx-spectrum/60-16/