Z80 instruction fetch mechanism

Chuck Guzis cclist at sydex.com
Sun Jan 4 12:33:42 CST 2009


On 4 Jan 2009 at 11:32, Jules Richardson wrote:

> It does seem logical (to both of us, anyway) that it's just a set of 
> flip-flops set by the prefix bytes to indicate different states - but so far 
> it seems that this isn't actually documented anywhere (presumably because it 
> doesn't *need* to be as far as anyone using the CPU is concerned)

>From an operational standpoint concerning DD and FD, it makes sense.  
All of those "undocumented" instructions involving half of IX or IY 
are just normal DE and HL-involved instructions with a prefix byte.

But the CB and ED opcodes are radically different from their single-
byte cousins and I suspect that a different mechanism is at play 
here.  Perhaps they switch to a different section of the decoding 
PLA.

Cheers,
Chuck




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