Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!ll-xn!husc6!bbn!rochester!crowl
From: crowl@cs.rochester.edu (Lawrence Crowl)
Newsgroups: comp.arch
Subject: The VAX Always Uses Fewer Instructions
Message-ID: <10595@sol.ARPA>
Date: 15 Jun 88 15:07:45 GMT
References: <6921@cit-vax.Caltech.Edu> <28200161@urbsdc>
Reply-To: crowl@cs.rochester.edu (Lawrence Crowl)
Organization: U of Rochester, CS Dept, Rochester, NY
Lines: 48

In article <28200161@urbsdc> aglew@urbsdc.Urbana.Gould.COM writes:
>In article <491@daver.UUCP> daver@daver.UUCP (Dave Rand) writes:
>>I am confused. How can a risc machine have a higher "vax mips" than native
>>mips? MORE (not less) risc instructions are required to do the same task,
>>when compared to a vax.
>
>Not always. Consider A=B+C, all in registers:
>    VAX:
>	mov rB,rA
>	add rC,rA
>    3 address RISC:
>	add rA,rB,rC
>
>So, we have an existence proof. What characteristics of the machine actually
>let this happen?

This is incorrect.  The VAX has three address arithmetic instructions.  So the
above example for a VAX (destinations are always on the right side) is:
 
        addl3 rB, rC, rA
 
It also takes four bytes to encode this instruction, the same as most RISC
machines.  The VAX instruction set wins (on number of instructions executed)
when using complex data structures because of the extensive addressing modes.
For example, the loop to add two vectors into a third on the VAX is:
 
   top: addl3 (rA)+, (rB)+, (rC)+
        sobgeq rD, top

which takes seven bytes for two instructions.  Most RISCs I now would have
something like the following loop (again destinations on the right):

   top: load  rA, rM
        load  rB, rN
        add   rM, rN, rO
        store rO, rC
        add   rA, 1, rA
        add   rB, 1, rB
        add   rC, 1, rC
        add   rD, -1, rD
        bgeq  top

which takes something on the order of thirty-six bytes and nine instructions.
I cannot think of any general computing task (such as the loop above) in which
the VAX will not execute fewer instructions.  Anyone?
-- 
  Lawrence Crowl		716-275-9499	University of Rochester
		      crowl@cs.rochester.edu	Computer Science Department
...!{allegra,decvax,rutgers}!rochester!crowl	Rochester, New York,  14627