Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!portal!cup.portal.com!ts
From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.sys.mac.programmer
Subject: Re: The 80486 is braindead (was Re: Xerox sues Apple!!!)
Message-ID: <25470@cup.portal.com>
Date: 29 Dec 89 08:16:34 GMT
References: <14960@boulder.Colorado.EDU> <213@amix.commodore.com>
  <25347@ubvax.UB.Com> <9378@hoptoad.uucp>
Organization: The Portal System (TM)
Lines: 61

Tim Maroney, Mac Software Consultant, sun!hoptoad!tim, tim@toad.com says:
> In article <25347@ubvax.UB.Com> fred@ubvax.ub.com.UUCP (Fredrik Noon) writes:
> >A more annoying deficiency (as it will be with us for a while) is that
> >Apple castrated the 68k test-and-set instruction after the Mac+.  It
> >expects fancier software (e.g., System 7) to run on the strength of MHz
> >and MMUs alone?  Not all Macs have the latter, but I know of no good
> >reason why all Macs can't execute a TAS anymore (can somebody else?).
> 
> Huh?  I've used TAS instructions within the last two years and they've
> worked, though not lately.  I don't see how Apple could break part of
> the hardware instruction set; I'm pretty sure they use standard
> off-the-shelf 68K processors with no custom microcode.

It should be possible to "break" TAS on a standard 68000 if the
/DTACK generation logic determines when to deassert /DTACK by
looking at /AS.

A 68000 read bus cycle goes like this:

	68000			Other thing
	-----			-----------
	put address on bus

	assert /AS
	assert /UDS and/or /LDS

				put data on bus

				assert /DTACK

	deassert /AS
	deassert /UDS and/or /LDS

				stop putting data on bus
				deassert /DTACK

The logic that generates /DTACK wants to know when the 68000 has the
data so that /DTACK can be unasserted.  The simplest way to do this
is to look at /AS.  The more complicated way is to look at both
/UDS and /LDS ( an extra AND gate!  Gotta watch those pennies...:-) ).

A read-modify-write cycle is similar, except that after the 68000 has
gotten the data, it deasserts /UDS and /LDS, but does *NOT* deassert
/AS.  It then waits for /DTACK to be deasserted.  The data is then
placed on the bus and /UDS and/or /LDS are asserted.  When it gets
/DTACK it will deassert /AS, /UDS, and /LDS.

If the logic that generates /DTACK is watching for /AS to be deasserted
to know when to deassert /DTACK, a read-modify-write cylce will hang
it up, since /AS will not be deasserted!

Anyway, that's how to "break" TAS on a 68000.  The 68020 and 68030
work quite a bit differently.  I don't think there is an easy way
to "break" TAS on these in the sense of the above ( i.e., hang the
system ).

What I don't understand is why you need to use a TAS instruction on
a Mac.  Do you have another bus master on your motherboard?


						Tim Smith