Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: Commodore, Amiga, Apple, and MAC
Message-ID: <10435@cbmvax.commodore.com>
Date: 29 Mar 90 20:28:29 GMT
References: <15003@snow-white.udel.EDU> <10363@cbmvax.commodore.com> <1917@awdprime.UUCP>
Reply-To: daveh@cbmvax (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 37

In article <1917@awdprime.UUCP> robin@reed.UUCP (Robin D. Wilson/1000000) writes:
>>In article <15003@snow-white.udel.EDU> BARRETT%FOREST.ECIL.IASTATE.EDU@cunyvm.cuny.edu (Marc Barrett) writes:
>>>Unlike the Amiga 3000, however, all of these custom chips run at the full speed 
>>>of the microprocessor.

>I thought that on the A1000/500/2000 line the custom chips ran 2x as fast as
>the main processor.  

Essentially true.  The Amiga chips run a memory cycle in two 7.16 MHz clock
cycles, the 68000 in those systems runs a memory cycle in four 7.16 MHz clock
cycles (for NTSC; the PAL machines run 7.09 MHz clocks).

Since the A2500/30, the CPU subsystem speed was fully decoupled from the
Video subsystem speed.  When the CPU needs to access anything in the video
subsystem, it synchronizes to video speeds.  You can expect all future
Amigas to work basically this way, regardless of the video and CPU clock
speeds in any given machine.

All Mac II's except the Mac IIci have a similar split.  The video subsystem
is on the NuBus, and the CPU must sync up and take wait states to communicate
over the NuBus, which runs a minimum memory cycle of two 10MHz clocks (though
most NuBus devices require at least three 10MHz clocks).  Older Mac IIs (the
Mac II and Mac IIx) use a 15.6 MHz CPU clock, twice that of the Mac 512/Mac 
Plus 7.8 MHz clock, probably because they use a number of the same gate arrays, 
all of which were designed to run at 7.8MHz.  The Mac IIci runs a CPU clock
of 25MHz, and slows down to talk to some of it's I/O chips.  The Mac IIfx
runs a 40MHz CPU and cache clock, and slows down for reads of main memory any
other access of I/O, whether on the motherboard or NuBus.  I haven't read
enough on it to know just how much of a slowdown there is for various
motherboard resources.  That's no big surprise, though; DRAM isn't as fast
as SRAM, but it's a heck of a lot faster than most things CPUs communicate
with, like I/O chips and ROM.

-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
                    Too much of everything is just enough