Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!hplabs!pyramid!prls!mips!hansen
From: hansen@mips.COM (Craig Hansen)
Newsgroups: comp.arch
Subject: Re: 16 & 32 bit vs 32 bit only instructions for RISC.
Message-ID: <1757@mips.mips.COM>
Date: 3 Mar 88 00:20:55 GMT
References: <2574@im4u.UUCP> <9740@steinmetz.steinmetz.UUCP> <7538@apple.Apple.Com>
Lines: 44

In article <9740@steinmetz.steinmetz.UUCP> sungoddess!oconnor@steinmetz.UUCP writes:
>There is NO intrinsic reason 16-bit instructions would decode slower than
>32-bit instructions. In fact, they can ultimately decode FASTER :
>the fewer bits your decoder has to look at, the faster it can be.
>Barring other complications of course.

>Dedicating 64 pins purely to instruction fetch (assuming a Harvard
>architecture) is quite a lot of a rather scarce resource. Sure
>you wanna do this on a micro ?

I guess I've been kicking around the RISC world too long, because I've
heard these arguments all before, when people were saying RISC
machines weren't going to be any faster than CISC machines, and they
were just as wrong then as they are now. Instruction bandwidth is
important, but not so important that you should go back to compacted
instructions. 32-bit instructions aren't much larger than 16-bit
instructions, particularly when a register-allocating compiler is
used, and the benefit to permitting parallel decoding of instructions
with register fetching is a tremendous win.

Generally, optimized MIPS code about 10% to 50% larger than "optimized"
VAX code, as generated by 4.3 UNIX, and is often equal or smaller in
size than optimized 68k code, as generated by Sun compilers.

Here's some data for text size for these machines, noting that the
same source code is used for all machines. However, since the libraries
may be different, and the MIPS machine has a large page size,
the data on small benchmarks is a litle distorted:

		mips	sun3	vax
	awk	 61k	106k	 34k
	ccom	193k	156k	120k
	compress 29k	 25k	 14k
	diff	 45k	 33k	 25k
	hspice	860k	729k	635k
	wolf	266k	303k	212k

...those big 32-bit instructions don't look so bad next to
the machines design for compact encodings...

-- 
Craig Hansen
Manager, Architecture Development
MIPS Computer Systems, Inc.
...{ames,decwrl,prls}!mips!hansen or hansen@mips.com   408-991-0234