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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.micro.68k,net.micro.16k
Subject: Re: 68020 vs 16k - is the 020 worth the wait?
Message-ID: <3493@utzoo.UUCP>
Date: Mon, 30-Jan-84 17:31:10 EST
Article-I.D.: utzoo.3493
Posted: Mon Jan 30 17:31:10 1984
Date-Received: Mon, 30-Jan-84 17:31:10 EST
References: <179@tropix.UUCP>
Organization: U of Toronto Zoology
Lines: 56

Some skeptical comments on Robert Moore's observations:

	The 68020 will contain a cache for text refrerences, with
		only 2 clock cycles per instruction running from the
		cache.  The 16k doesnt have a cache.

I hope this "cache" is going to be bigger than the two-word one on
the 68010, or else it's useful only for "loop mode".

	The 68020 provides ...
		... memory resident pointer addressing modes.

What's that?  And what good is it?  Have you checked out the 16k's
"external" addressing mode?

	The 68020 fpu, due out 3 months after the 020, provides ...
		[lotsa good stuff]

This does sound good, assuming it runs at decent speed.  National's
FPU is not bad but is not a world-beater.  But "3 months"???  Come
off it.  Especially since it's relative to the arrival of the 68020,
which is itself off in the never-never.  (Especially if one wants a
bug-free version.)

	The 68020 will be provided with clocks from 12 to 25 Mhz.  The
		design center is around 20 Mhz.  The 16k is hard to get
		above 6 Mhz.  They are hopeing for 10 Mhz eventually.

They claim to have 10 MHz right now.  I haven't actually bought the
parts to check how well they work, but this sounds better than a claim
that the "design center" is around 20 MHz.  When do we actually get to
see parts that *run* at 20 MHz?  If it's at the same blinding speed that
we got to see 10 MHz 68000's, I'm not holding my breath.  Note that the
folks at National have been heard mumbling about higher clock speeds for
the 16k, once they have the 10 MHz stuff rock solid.  Any bets on who
gets there first?

	The 68020 paged mmu will allow variable page size and number of
		levels.  This is fixed in the 16k's mmu.

If the 68020 "paged mmu" is going to be a clone of the existing 68000
mmu, I don't want to even see a spec sheet.  I no longer have any faith
in Motorola's ability to get things right in an mmu.  I see no great
use for variable number of levels, and "variable page size" is dead easy
to get on the 16k, assuming you're willing to make your page sizes
multiples of 512.  I trust Motorola will insist that they be powers of
2, as before?
	
	These and other considerations make the 68020 worth waiting for.

Not to me.  Ok, so I'm biased.  I still think that Motorola is making
an awful lot of promises about the 68020 and may take quite a while to
make good on them.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry