Arty computers

Chuck Guzis cclist at
Sat Feb 3 22:26:37 CST 2007

On 4 Feb 2007 at 1:00, Tony Duell wrote:

> What happenms with the 'repeat' instructions, LDIR, etc? I seem to 
> remember they keep on re-fetching the opcode, do they assert M1 each time?

Yes indeed it does.  The repeated operations run the CPU through RNI 
and decode each time.  Which is why the Z80 gained most of its speed 
over the 8085 by having one fewer clock in an instruction cycle, not 
by the added instructions.  The same sort of behavior exhibits itself 
on some of the x86 CPUs--where:

    mov	al,ds:[si]
    dec	cx
    mov	es:[di],al
    jnz		l1

runs about as fast as rep movsb (and, in cases, somewhat faster).  On 
those same CPUs, replacing the dec cx/jnz with a loop instruction 
will actually slow things down.

That doesn't mean that an analogue meter wouldn't indicate 
variations.  On a Z80, the instruction length can vary from 1 to 5 
(IIRC)  bytes.  Given additional cycles for memory and I/O space 
references, the instruction issue rate can vary quite a bit.

Which may have absolutely nothing to do with how much useful work the 
CPU's actually doing.


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