Arty computers

Tony Duell ard at p850ug1.demon.co.uk
Sun Feb 4 14:11:35 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 

In which case simply counting M1s won't tell you how many instructions 
have bene exectued (assuming yopu could an LDIR as 1 instruciton, no 
matter how many times it repeats).

Of coruse there's another trivial correction. You want to combine M1 with 
MREQ (feed M1/ and MREQ. into an OR gate). Otherwise you'll count 
interrupt acknowledges too (M1 and IORQ asserted at the same time).

-tony




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