DATIP from clr always? (obscure unibus & pdp-11 question)

John A. Dundas III dundas at caltech.edu
Thu Aug 31 10:29:28 CDT 2006


At 2:55 PM -0700 8/30/06, Don North wrote:
>Brad Parker wrote:
>>An obscure unibus & pdp-11 question:
>>
>>Does "clr" always do a DATIP (i.e. read-modify-write) on all 11's?
>>
>
>According to the PDP-11 CPU handbooks, the J-11 based designs did 
>the extra optimization so a CLR does not do the read, only the write 
>cycle (see item #36 in later handbooks).
>

For those that don't have this handy and to fully document the 
discussion, here is the text of item 36 from the 1983-84 Architecture 
Handbook:

CLR and SXT do just a DATO sequence for the last bus cycle: 23/24, J-11.

CLR and SXT do DATIP-DATO sequence for the last bus cycle: all others.

John



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