Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!waikato.ac.nz!ldo
From: ldo@waikato.ac.nz (Lawrence D'Oliveiro, Waikato University)
Newsgroups: comp.arch
Subject: Re: Loop instructions
Message-ID: <1991Apr24.174259.3541@waikato.ac.nz>
Date: 24 Apr 91 17:42:59 +1200
References: <1991Apr16.152438.3445@waikato.ac.nz> <12739@pt.cs.cmu.edu> <1991Apr21.210031.16749@leland.Stanford.EDU>
Organization: University of Waikato, Hamilton, New Zealand
Lines: 29

In article <1991Apr21.210031.16749@leland.Stanford.EDU>,
dhinds@elaine18.Stanford.EDU (David Hinds) asks:

"I'm not familiar with the 68000 instruction set, but couldn't [DBRA]
be adapted to 32-bit counts by just splitting the count into upper and
lower half-words and using a nested pair of 16-bit loops?  This would
have minimal overhead - a few instructions in loop setup - but would
have essentially the same performance as the 16 bit count for long loops."

Yup, and here's how you do it:

	; loop initialization, including loading 32-bit
	; unsigned count into d0, comes here
	bra.s	@7		; enter loop at bottom, as usual
@1	swap	d0
@2	; ... body of loop
@7	dbra	d0, @2
	swap	d0
	dbra	d0, @1

My instruction timings indicate that DBRA is preferable to a simple SUBQ/BNE
on the 68000, 68010, 68020 and 68030. This is because DBRA is faster when
the branch is taken.

Lawrence D'Oliveiro                       fone: +64-71-562-889
Computer Services Dept                     fax: +64-71-384-066
University of Waikato            electric mail: ldo@waikato.ac.nz
Hamilton, New Zealand    37^ 47' 26" S, 175^ 19' 7" E, GMT+12:00
MADAME MACLUHAN, fortune-telling and back rubs. "The medium is the massage."