Comment by kens
13 years ago
Thanks for the interesting information. I'm skeptical that the Z-80 designers were planning ahead for 16 bits, though. Simpler BCD math is a possibility - I'll look into this as I examine the Z-80 more. The 6502 wins, though, for crazy but efficient decimal arithmetic - it has a complex patented circuit that detects decimal carry in parallel with the addition/subtraction, and another circuit to add the correction factor to the result without going through the ALU again. So you don't need a separate DAA instruction or additional cycles for decimal correction.
General question: what things about the Z-80 would you guys like me to write about? Any particular features of the chip? Register-level architecture, gates, or the silicon? Analyzing instructions do cycle by cycle? Gate counts by category? Comparison with other microprocessors?
Would love any and all analysis, but most interesting to me would be instruction details and especially the undocumented side effects. I'd also like to see comparison with the 8080 and how Zilog improved/changed the design.
...what things about the Z-80 would you guys like me to write about?
Undocumented instructions! The MOS 6502 had plenty of these and I understand the Z-80 did too.