Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!crowl
From: crowl@cs.rochester.edu (Lawrence Crowl)
Newsgroups: comp.arch
Subject: Condition Codes in General Registers
Message-ID: <6834@sol.ARPA>
Date: 16 Feb 88 03:23:22 GMT
References: <191@telesoft.UUCP> <1556@gumby.mips.COM> <208@telesoft.UUCP> <7405@apple.UUCP>
Reply-To: crowl@cs.rochester.edu (Lawrence Crowl)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 17

Several authors have been arguing over conditional branching using condition
codes versus using booleans in general-purpose registers.  Why not store the
condition codes in a general-purpose register?  This would result in a
scheduable two-instruction sequence with multi-way branching at the cost of
using a general purpose register and slightly lower code density.  For example,

    compare R1 with R2 place condition code in R3
    branch to ADDRESS if condition code in R3 is GREATER
    branch to ADDRESS if condition code in R3 is EQUAL
    branch to ADDRESS if condition code in R3 is LESS

Will this approach work?  Will the condition code comparison in the branch cost
too much?  (It is an instruction constant.)  Comments please!
-- 
  Lawrence Crowl		716-275-9499	University of Rochester
		      crowl@cs.rochester.edu	Computer Science Department
...!{allegra,decvax,rutgers}!rochester!crowl	Rochester, New York,  14627