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From: daryl@hpcldko.HP.COM (Daryl Odnert)
Newsgroups: comp.arch
Subject: Re: Branch Delay Annullment
Message-ID: <650002@hpcldko.HP.COM>
Date: 15 Jun 88 21:27:03 GMT
References: <22065@amdcad.AMD.COM>
Organization: HP NSG/ISD Computer Language Lab
Lines: 22

The Hewlett Packard Precision Architecture uses the following approach:
When a branch instruction is executed, the delay slot instruction executed
before control flow is transferred to the branch target.  A bit is provided
in each branch instruction, however, that can cause the nullification of
the delay slot instruction.  Exactly what happens depends on the direction
of the branch and whether or not it is a conditional branch.

       When nullification is selected in a branch instruction...

   Branch type         Branch direction    Branch taken?   Is the delay slot
							   instruction
							   executed?
   conditional         forward             yes                    no
   conditional         forward             no                     yes
   conditional         backward            yes                    yes
   conditional         backward            no                     no
   unconditional       forward             always                 no
   unconditional       backward            always                 no


Daryl Odnert
HP Computer Language Lab
hplabs!hpcllcm!daryl