Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!amdahl!amdcad!phil
From: phil@amdcad.AMD.COM (Phil Ngai)
Newsgroups: comp.sys.ibm.pc
Subject: Re: 286-based accelerator boards -- followup
Message-ID: <22465@amdcad.AMD.COM>
Date: 27 Jul 88 03:04:48 GMT
References: <14742@shemp.CS.UCLA.EDU>
Reply-To: phil@amdcad.UUCP (Phil Ngai)
Organization: Advanced Micro Devices
Lines: 68

In article <14742@shemp.CS.UCLA.EDU> wales@CS.UCLA.EDU (Rich Wales) writes:
about the choices for upgrading his turbo XT clone.

I'd like to throw in a few commments. These are just opinions.

First, I would avoid a 286-based accelerator boards for XTs.
You'd be stuck with the 8-bit memory bus and I think you'd notice
the performance difference very easily.

Buying an AT clone motherboard seems more attractive. If you do so,
watch out for card slot spacing and the location of the keyboard
connector. Either make sure it fits your case or buy a new case to go
with the new motherboard. 

Expect prices of around $90 for an AT class power supply and $90 for
an AT clone case. You can pay less if you shop hard.

You probably won't have any problem with your existing video board.
In any case the MIT Herc clones we have use only two RAM chips and
replacing them shouldn't be too expensive, if you need to, which
I doubt.

I have heard that most of the disk speed up between XT and AT is due
to faster seek times on the AT disk itself. There are those who claim
the 16-bit bus does not make disk IO much faster, although it does
make a difference for memory accesses. If you figure the disk can
deliver either 5 or 10 million bits per second and the memory can
deliver 16 or 32 million bits per second (either a byte every 500 ns
or two bytes every 500 ns) then you wouldn't expect an 8-bit disk
controller to be too much of a bottleneck. 

If you do decide to choose an AT clone motherboard, I would look for
something that allowed you to split 1 meg on the motherboard into 640K
for DOS and 384K for LIM expanded memory. Boards built around the new
CHIPS NEAT chip set are in this category. 

Some MBs only allow you to put 640K on (16 256K and 16 64K chips).
Others allow 1 meg split as 640K DOS and 384K extended.

Access to extended memory (above the 1 meg that DOS understands) is
slower because you have to go into protected mode and leaving it
is usually slow. (But NEAT speeds this up too.)

Access to LIM expanded memory goes at normal memory speeds since you
never have to leave real mode. You will find this useful for putting
RAM disks or disk caches in.

You should also look for the ability to clock the bus at a different
speed from the CPU. Various MBs can clock the bus at a constant 8 MHz,
eliminating your worries about slow video, etc, while running the CPU
at 10, 12.5, or beyond. 

WD has an interesting motherboard which only has three slots. They
can do this because almost everything you can think of is integrated.
The only thing you need to add is a networking card. Memory, video,
disk, serial, mouse, printer, they have it all. Unfortunately, I
don't know the price yet.

You should be able to get a new motherboard for around $400. Add
another $180 for case and power supply and $300 for a disk. Now you're
up to nearly $900. You may need a new keyboard too. Is this still
cheaper than a complete new clone. At least you're reusing your
monitor, video card, serial, floppy, etc. 

We haven't even begun to discuss the 386...
-- 

I speak for myself, not the company.
Phil Ngai, {ucbvax,decwrl,allegra}!amdcad!phil or phil@amd.com