NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/sys/mac/hardware/21420

Text File | 1992-11-12 | 3.4 KB | 62 lines

Newsgroups: comp.sys.mac.hardware
Path: sparky!uunet!nwnexus!kanefsky
From: [e-mail] (Steve Kanefsky)
Subject: Re: Summary of IIsi 20->25 MHz upgrade log
Message-ID: <[e-mail]>
Sender: [e-mail] (System Security Officer)
Organization: The 23:00 News and Mail Service
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Thu, 12 Nov 1992 16:35:57 GMT
Lines: 50

In article <[e-mail]> [e-mail] writes:
>The fpu on most cards is in fact clocked off the ][si's clock signal, but not
>because it has to be. The 68030 co-processor interface is entirely asynchronous
>and the cpu and any co-processor can be hooked up to whatever clock they want
>to be. The hardware of the co-processor interface takes care of the necessary
>details of the communication. It is perfectly reasonable to think of hooking up
>a 50 MHz fpu to a 50 MHz clock on a card and have it work just fine (and 2 1/2
>times as fast as the fpu normally works with a IIsi.) The main reason that you
>never see 40 or 50 MHz (or anything besides 20 MHz for that matter..) fpu's
>for the IIsi is that they would cost more. Companies are out there to make a
>buck. The amount of people they tgink would buy a card with a faster fpu is
>fairly small, so they don't want to have to go through the trouble of making
>them and having them in stock. The 68882 chips that are rated at 50 MHz are
>much more expensive than 20 MHz chips, so that and the (relatively small) cost
>of adding a clock to the card with the fpu, along with the space that it would
>take up, are the reasons why you just don't see it. I would personall like to
>see someone who has some pc-board experience copy a IIsi pds card with fpu,
>but instead of having the fpu clock-in pin hooked to the IIsi clock signal line
>put a 50 MHz clock signal chip or whatever is needed. I personally don't have
>any of the resources available for doing such a thing, so I can only dream
>about it.

Actually you do see adds for fast FPUs.  Look in the back of any
MacUser or MacWorld magazine.  I've seen them for the LC/LCII up to
50mhz, and I _think_ I've also seen them for the IIsi.

The ability to run the FPU at a different speed than the CPU seems
most advantageous when it's the CPU that runs faster than the FPU
(instead of the other way around, as you described).  Although I'm
sure some people need the extra number-crunching speed of a faster
FPU, I would think most people would prefer to buy an accelerator
card with a faster CPU, without having to also pay for a faster FPU
and while still taking advantage of the slower FPU on the motherboard
(for those Macs that have one).  The DayStar 68030 PowerCaches can do
that.  There was even talk recently of using both the internal FPU in
their new Turbo '040 accelerator *and* a 68882 FPU on the motherboard.
Only the functions which aren't implemented in the '040's internal
FPU (e.g. trig functions) would be routed to the 68882.
Unfortunately, I think they've decided against doing that for now.

Incidentally, since we were talking about the IIsi here, when I say
"FPU on the motherboard", that also applies to an FPU on an adaptor
card or on another PDS card in the IIsi.  There are a couple of
different ways you can use a PowerCache without FPU in a IIsi, and
still make use of a slower FPU on another board.


--
Steve Kanefsky