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8

Directly to answer your question: yes, if the RAM copy is substituted after the machine has started up; otherwise, not reliably, as it's likely the first five bytes have been altered. With reference to The complete Timex TS1000/Sinclair ZX81 ROM disassembly notice the 'skip constants' routine starting at the bottom of page 87 of the PDF, page 52 of the ...


43

The ZX80 and ZX81 use the Z80’s opcode fetch bus cycle for graphics collection — the opcode fetch is appropriated to obtain a character index and the refresh cycle is used to obtain a row of pixels. Of course the actual refresh address isn’t actually likely to be correct for the pixel fetch so the ULA replaces the low nine bits with a combination of the ...


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