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
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.
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