Path: utzoo!attcan!uunet!mcvax!ukc!dcl-cs!aber-cs!pcg
From: pcg@aber-cs.UUCP (Piercarlo Grandi)
Newsgroups: comp.arch
Subject: Re: 68020 vs. 68030 speed (was Re: 80486 vs. 68040 code size)
Message-ID: <954@aber-cs.UUCP>
Date: 15 May 89 23:18:21 GMT
Reply-To: pcg@cs.aber.ac.uk (Piercarlo Grandi)
Distribution: eunet,world
Organization: Dept of CS, UCW Aberystwyth
	(Disclaimer: my statements are purely personal)
Lines: 35

In article <4175@ficc.uu.net> peter@ficc.uu.net (Peter da Silva) writes:

    One thing you have to keep in mind in the 80386 versus 68030 debate is
    that, yes, there are fewer 68030 systems out there then 80386 boxes...
    but the vast majority of the 80386 systems are crippled by the poor
    performance of the AT bus. I have been benchmarking a 25 MHz 386 against
    our 16 Mhz 80286es on multibus. The speed advantage of the 80386 is
    almost entirely lost in the low performance of the bus. In fact many
    of our development tools run faster on the 286 or on a 386 in 286 mode
    on the multibus.

Well, the problem is not the AT bus, because virtually all 386 ATs use
caching and a private 32 bit bus for CPU-memory accesses. CPU bound programs
on such 386s as I have seen don't have a problem with the AT bus, because
they do not use it. On the other hand, the stock AT bus peripheral
controllers suck (euphemism), in particular the disc controller, that is
single threaded and has a transfer rate of about 160kbytes/sec.  If you get
a good disc controller (e.g. a 1:1 RLL/ESDI, or even better a multithreading
fast SCSI one), things improve A LOT. The limit of the AT bus for
peripherals is about 1MByte second, e.g. about like the Unibus, and there
are few peripherals that can get up to that, in machines of that class.

    
    And once you leave behind AT-bus compatibility and cheap AT peripherals
    there's no reason to stick to the 80386. Either RISC, or if you're
    conservative the motorola chips, are far better. At least motorola
    doesn't have a history of abandoning their entire architecture every
    time they come out with a new chip.

I cannot resist saying that given the premises, this really should be a plug
for the NS 32000 family, not the MC 68000 one. :-) :-).
-- 
Piercarlo "Peter" Grandi           | ARPA: pcg%cs.aber.ac.uk@nsfnet-relay.ac.uk
Dept of CS, UCW Aberystwyth        | UUCP: ...!mcvax!ukc!aber-cs!pcg
Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk