In the pursuit of the ever more turbocharged PC I have recently
modified an IBM AT to function with no wait state memory. This RAM deviates from
IBM's standard of one wait state per ram access cycle. This was done because of
the relatively slow ram chips available at time of the AT's release. Now however
256k ram chips with access times down to 70 ms are available. These new chips do
not need to have the cpu WAIT for them to cough up data, hence NO WAIT STATE ram
is one third faster (two clock cycle vs. three of the IBM standard).
So what? If you have to ask don't continue with this, but if you
want a faster buggy read on. The improvements can be seen using Peter Norton's
SI (sysinfo) program. A Standard AT at 6mhz 1WS(one wait state) rates as a 5.7,
( 1.0 being a standard IBM PC 8088 4.77 mhz 1WS), an 8mhz AT 1WS comes in at 7.1
an 8 mhz AT zero WS yields an 8.5. Simple math shows it to be a 17% increase.
Where's the other 13%...SI is trying to measure on cpu speed, its designed to be
non ram intensive and does not show the full benefit.
The Cheetah Memory Card from Cheetah Intl is a 2.5 mb card
holding five banks of 18 100 nanosec 256k ram chips. Its architecture differs
from most other popular board in that banks are 512k in size not 256. This
reduces the flexibility of memory assignment tricks (like supporting Intel/Lotus)
The cheetah board (CB) does not supportIntel/Lotus EMS standard at the present.
I'm playing around with it though and have my hopes more on this later. However
these 512K blocks of ram can be placed anywhere in the AT's 16 mb address space.
Why the big 512k blocks....SPEED. All memory boards work at 6mhz ok and dolotus intel ems tricks but they fall flat at high mhz,especially when you cram
more than one card in a machine. Spurious memory errors crop up
continually.
Case in point. The Talltrees JRAM-AT is a 2 meg card for the AT, on try to
boost the cpu speed to 8mhz brought on a rash of parity errors if the beast
would even boot up. Multiple calls to talltrees and switching chips and cards
and endless pain produced naught. They finally admitted that only a single
JRAM card can reliably be placed in even a 6mhz machine. Cheetah does away with
this. Not only will the board perform at higher mhz where others fail, its
no wait state memory boosts its value even more. Hopefully the appearance
of dos 4.0 or 5.0 will reduce the intel/lotus standard to the gibberish that
it is on an 80286 machine since it reproduces the native talents of the machine.<<^M>>
<<^J>>
You can run no wait state two ways with the cheetah, the
simplest is to set up the motherboard with 256k (pull chips from banks 2
and 3 (leaving 0 and 1 intact) and readjust jumper J18 to jump pins two and
three ). Your AT now rightly thinks it has 256k on the motherboard, the
remaining dos workspace ( 257 to 640k ) will NWS memory on the Cheetah
Board. Cheetah has a utility called force which can block off the first
256k of memory forcing programs into the 257-640 memory space. I prefer to
load a ram disk cache program ( lightning ) setting its size to 256k which
achieves the same purpose as force.com but allows constructive use of the
motherboard memory.This is the easiest way, but very limited, your left
with a 400 odd k workspace and if your hitting your head against the 640k
limit its too small.
!!!!!!!!! Only for the stout hearted !!!!!!!!!!!
The final solution is to junk all the ram in your AT, i
choose this path because all the chips on my motherboard were crudo 200 ns
access chips. i scrapped them and then performed minor brain surgery on the
motherboard. Find chip U72. See the diagrams. To orient yourself pin 20 is
the closest pin towards the front of the machine closest to the card cage.
Pin 11 is on the card cage side of the chip closest to the back of the
unit. Clip pin 11 close to the chip and solder a jumper from the
motherboard side of the cut over to pin 20. see diagrams. This causes your
AT to look to the buss for its base memory. With a standard cheetah board
filled with five banks of 18 256k chips, you'll be able to assign only one
512k bank to lower memory. hence will have a 512k nws machine. you can then
do one of two things. replace a bank of 256k chips with 64k chips (be sure
to use fast chips with access times at or below 100). you can then assign
this 128k block to base memory as well to achieve at 640k nws machine (with
a 1.98meg nws ram disk). I am using instead my old cmi memory 128k memory
expansion board (what a piece work 128k ram on a full slot board thanks
ibm) (this is the board you get if you ordered a 640k stock AT. ) This
allows you to run fast in 512k and slow down only when you push up into the
top 128k.
Caveats............
To run nws at 8mhz requires changing jumpers j2 and j3 on
the cheetah board to east and west respectively.
The nws option is jumper controlled on trhe cheetah board, a
small micro switch can be installed to allow external flip flopping between
the ibm standard 1ws and the 30% faster nws.
What no intel/lotus! I don't think this is a long term
concern dos 4.0 or 5.0 is sure to allow use of the full 16mb workspace and
the other magic of the memory managing unit in the 80286. intel lotus
divides memory into 16k pages, it takes time to be assign addresses to all
these pages throughout the megabytes of extended memory you may want to add
to your AT. this is were other boards fail when operating at higher mhz or
with more than one board in the system.
This modification worked well for me, if you do exactly as i
did there are no guarantees that it will work since production tolerances
have improved in recent vintage AT's, so if yours is old it may not have.
the get up and go to tolerate the 8mhz no wait state combo. But if your
too new you may have the Altered AT ROM BIOS chips which won't let you go
faster than 6mhz in the first place.
If you want to see what a 8mhz no wait state AT is like call my bbs the
board operates on one. NYUMC BBS 212 889 7022 300/1200/2400 N 8 1
