Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!apple!agate!garnet.berkeley.edu!deadman
From: deadman@garnet.berkeley.edu (Ben Haller)
Newsgroups: comp.sys.mac.programmer
Subject: Re: m68k timing tables & Mac
Message-ID: <1991May4.220019.7528@agate.berkeley.edu>
Date: 4 May 91 22:00:19 GMT
References: <1991Apr29.123112.4331@vax5.cit.cornell.edu>
Sender: root@agate.berkeley.edu (Charlie Root)
Distribution: comp
Organization: Stick Software
Lines: 24

In article <1991Apr29.123112.4331@vax5.cit.cornell.edu> tcd@vax5.cit.cornell.edu writes:
>I'm trying to do some assembly language programming on a Mac and was 
>wondering how to interpret the timing tables in e.g., the MC68020 User's
>Guide relative to a Mac.  I'm not trying to get real precise, but just 
> [stuff omitted]
>Clearly, I'm a novice so please don't be too technical.

  Well, if you're a novice the timing information for the 68000 (versus the
68020) is much simpler, and should be sufficient for what you're trying to
do.  The timings for the 68020 in practice tend to *approximately* mirror
the 68000 instructions, but they're much more complex.  Actually, the best
way to determine the fastest code on a 68020 is to write all the
alternatives and clock them.  Of course, this isn't always easy/possible.
  There are a few important differences between chips in the 68000 line,
however, that you should be aware of if you care.  For example, register
shifts on the 68030 (not sure about the 020) run in a constant time that
doesn't depend on the shift count, while on the 68000 it's 8+2n or 6+2n, I
don't remember which, clocks where n is the shift count.  These kinds of
differences can, if fact, mean that you might want to write different code
and switch which code to run depending on which processor you're running
on.  Who knows.

-Ben Haller (deadman@garnet.berkeley.edu)
"Pop - ffffff" - Andrei