From: rsrodger@wam.umd.edu (Yamanari)
Subject: Re: If things had been different... (was: FCC etc)
Organization: University of Maryland, College Park
Date: Tue, 5 Jan 1993 15:46:12 GMT

In article <1993Jan4.190822.1001@pphbau.atr.bso.nl> paul@pphbau.atr.bso.nl (PPH Bauwens) writes:
>: >The 68030 grew more gracefully than the x86 did, though.  I mean, the
>: >386 contains *two* MMUs and *three* incompatible instruction sets...
>
>I must contradict here. The upwards compatability of the 68K line at the
>operating system (kernel) level is almost nonexistent.


		68010   Introduced changes to supervisor mode that
			broke a lot of code.

>Example: 	68020	no MMU, so everyone build their own junk


			68020 designs could use anexternal MMU from Motorola.
			Mac IIs had sockets for it.

>		68030	has an MMU, so people use it.
>		68040	has an MMU that is _incompatible_ with the one
>			in the 68030


			Not true.  It is simply a *subset* of the 68030's
			MMU.  (Fewer page sizes available).  Most SW
			ran just fine. (Only the OS should care, anyway)


>			has a partial floating point coprocessor that is
>			_incompatible_ with the 68881/68882 chip


			Again, not true.  The FPU had a subset of the
			68882 instruction set.  The rest was done in 
			firmware.  I don't think *any* SW broke on this one.



>		68050	renamed into 68060 in order not to lose the
>			number game against Intel (if and when it
>			finally arrives).


			Exactly. Marketroids.


>The upward compatability on both the operating system _and_ application
>level that we currently enjoy (?) with the PC architecture is totally
>impossible with the 68K line.


	Utterly, completely false.  Talk to Mac or Amiga users, or
	evenusers of old M68k based SUN systems.  There were essentially
	3 compatibility problems in the upgrades, all of which could
	have been avoided and most of which affected few, if any, programs.

	These were:

	1.  Programmers used the extra 8 bits at the top of the 68000 
	address register as an extra general-use register.  The 68000
	only used 24 bits of this register.

	All code that practiced this broke under the 68010.

	2.  Many SW programs used an incompatible means of accessing
	the SR (might be wrong--I haven't thought about this in 
	years) registers through supervisor mode.  These programs broke
	in the transition from 68000 to 68010.

	3.  Some OS's had problems adapting to the differences between
	the 68030 MMU and the 68040 MMU.  This was an OS-only problem.



>(I can succesfully run a 286 protected mode UNIX on a 386 and I would 
>expect it also to run on my 486. Try that with 68K).


	99% of the compatibility pc users are so proud of comes 
	from the fact that we're forced to use out 486, 386 and 286 machines
	as glorified, high-speed 8 bit 8088's.

	BTW:  Any SW that ran on the 68010 will probably run on systems
	today, excusing problems introduced by the OS.  (I.e. System 7
	on the mac broke a lot of SW, AmigaDos2.0 onthe Amiga did the
	same) 



>Motorola did a very bad job on the compatibility of the 68K line.


	Wrong.  The real problem (which is for the most par 
	nonexistant--are you trying to actually tell people that 68k
	based macs, 68020 based mac, '030 and 040 based macs are all forced
	to run different versions of the same SW??) was extremely poor
	programming habits that programmers used on the 68k.  There
	were several things they could have done to prevent these problems
	(i.e. follow Motorola's guidelines for compatibility).  The 24/32
	bit register problem was caused by utter stupidity on the part ofthe
	folks who did so.

	You are far overstatingthe problem.  Most of the compatibility
	issues on this angle were out of the picture 5 or 6 years ago.  
	All new compatibility problems are caused by OS changes.  PC users
	don't experiance these because they, forthe most part, use a 
	glorified 8 bit version of CP/M.

	I challenge you to show me a problem that came into play after
	the release of the 68010 that could be blamed on Motorola.


>This is in my opinion te most important reason they failed.
>Too little, too late.


	Nope.  Motorola's 88k and 68k lines are being abandoned because
	they failed to get the 2nd generation of bug-fixed 88k's out the
	door--they were years late--anf a new 68k is just becomeing 
	unproductive.  Eventually, the '060 will come along, but by that
	time--who knows?  that chip that can emulate a 25mhz 68040 or
	33mhz '486 will probably be catching up about then, and from
	what I've readit's a fairly simple chip next to the real McCoys.




-- 
      Blaming "society" for your problems is like blaming clouds for rain.
 ---  boycott == coercion == censorship == closed mindedness == cowardice  ---
	      "Psalms 23:1  The LORD is my shepherd; I shall not want."  
		Shepherds are for sheep.   -----      Pain teaches.