Path: utzoo!utgpu!water!watmath!clyde!rutgers!gatech!hubcap!ncrcae!ncr-sd!hp-sdd!hplabs!hpda!hpesoc1!hpcllla!hpcllcm!daryl
From: daryl@hpcllcm.HP.COM (Daryl Odnert)
Newsgroups: comp.arch
Subject: Re: conditional branches
Message-ID: <960001@hpcllcm.HP.COM>
Date: 12 Feb 88 18:16:05 GMT
References: <191@telesoft.UUCP>
Organization: HP ITG/ISD Computer Language Lab
Lines: 35

The Hewlett-Packard Precision Architecture (a.k.a. Spectrum) includes
some conditional branch instructions that may be of interest.

There are instructions that allow you to
   -- compare two registers and conditionally branch based on the
      result of the comparison

   -- compare a register vs. an immediate and conditionally branch
      based on the result of the comparison

   -- add two registers and conditionally branch based on certain
      relationships between the operands

   -- add an immediate to a register and conditionally branch based
      on certain relationships between the operands

Perhaps more directly related to Roger's suggestion are the following
two instructions:
   
   BB -- branch on bit: tests a bit at a fixed position in a register
      and branch based if the bit is on or off (depending on a bit set
      in the instruction word.)

   BVB -- branch on variable bit: conditionally branches based on a bit
       at a position in a register specified by the value in another register.

These last two instructions allow an optimizing compiler to perform
the transformation suggested.  I'm not sure whether the test is
done early enough in the pipeline for the hardware to treat the
instruction as a no-op or an unconditional branch.  The answer may be
different depending on which hardware implementation you look at.

Daryl Odnert
Hewlett-Packard Computer Language Lab
{outside world}!hplabs!hpcllcm!daryl