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Understanding
emor
By Paul Lynch
Confused about computer memory? Learn how the memory
in your PC is organised & what the various terms mean.
SK ME THE years my parents
were born and I will consult
my memory and tell you. If I
have the same information in my computer, I won 't consult its memory I'll look on the hard disc. The stuff
there is called "storage ". It's not
memory.
"Memory" is one of the most misleading words in the computer field.
Understanding the difference between
memory and storage will make things
much clearer for computer users. Failing to understand it causes simple
errors which are hard to correct.
Here's a simple example: a colleague
some years ago bought a type-font
A
program which allowed for varying
sizes of cache. The faster the cache,
the faster the font operation. Reasoning that he had a 40Mb hard disc, he
allotted an 8Mb (eight megabyte)
cache. His system, of course, fell over.
According to his computer, all he really had was 2Mb - in RAM (random
access memory). He now has a modest 192Kb cache and things work well.
The original PC
Would that all memory difficulties
be sq simple to explain and understand. Of course, it isn't so, for two
main reasons. One is that, in 1992,
we're still trying to push back the
This EEMS RAM card accepts the DIP-style RAM chips. A total of 2Mb can be
installed on this full length card. (Electronic Solutions, PC Marketplace).
6
SILICON CHIP
envelope created for the original IBM
PC more than 10 years ago - a machine without hard discs, a machine
with an 8-bit bus, a machine designed
with the idea that it might help do
things like balance the family cheque
book.
The other reason is that software
developers .h ave run amok designing
applications, operation systems and
the like, with no common, comprehensive standard for memory usage.
So instead of an orderly traffic-like
flow of instructions and data through
such memory as your computer may
hold, you've got something closer to
the 9am rush on the first day of the
post-Christmas department store sales.
Several different applications in
your computer can demand the same
particular item - and demand it simultaneously. Sometimes there's a
winner. Often, everybody loses - the
systeip. hangs, or even crashes. This
may cost you important data. It certainly isn't what you want.
Almost certainly within the next
two years, you'll be able to buy PCs
which don 't have this problem - or at
least, don't have it nearly as often.
While they will look like today's PCs,
they will -have memory organised differently in what a number of US and
Most high density memory expansion boards use at least one VLSI (very large
scale integration) chip to take care of memory refreshes and memory
formatting. This 8Mb board uses SIMMs memory cards and is designed for the
286 bus. It features an EEPROM (electrically erasable programmable read only
memory) to store the memory configuration and will operate as conventional,
extended or expanded memory. (Pelham Pty Ltd).
European manufacturers have agreed
to call the Advanced Computing Environment (ACE).
ACE machines will be fast and
highly-optioned. Sadly, even before
the first ones hit the market, it's obvious that the companies which have
set up the standard have ever-soslightly different views of exactly how
it should work. So the millennium is
not quite as close as advertisers might
be suggesting later this year.
Let's have a look at the memory in
today's PC and examine why it's as it
is.
today the lineal descendants of that
first DOS are still used on all PC clones
today: today's machines are much
faster and more complex than the
original PC and DOS has had to become far more complex with them.
DOS originally addressed only 1Mb
--
2Mb
EXTENOED MEMORY
The DOS environment
All PCs are designed to operate in
an environment known as DOS. There
are many proprietary DOS packages,
Microsoft's MS-DOS being the bestknown. IBM's is called PC-DOS and
another leading brand is Digit al Research's DR DOS. All arise from a few
weeks' work by a young Bill Gates
back in the 1970s.
Gates was asked by IBM to figure
out an environment for a machine
that IBM never expected to be such a
big seller and so he designed an operating system for the size and capacity
of the first machine IBM had in mind.
It's a tribute to human ingenuity that
,-
1Mb
HIGH MEMORY USUALLY
HAS BIDS AT TOP,
VIOEO DISPLAY AT BOTTOM
640Kb
VIDEO DISPLAY
,_
AVAILABLE RAM
:=~
PERATING SYSTEM
Fig.1: how memory is configured
on most PCs with 2 megabytes of
RAM. All or part of the extended
memory shown could be configured
as expanded memory.
of memory, which was the amount in
the original PC chip, the 8088. It seemed generous, fabulously generous, at
the time and in terms of the simple
software available at that time, it was.
The 8088 chip stored its own BIOS
(basic input output system) at the
"top" of that 1Mb. IBM decided that
the video memory - the stuff that lets
you see on your monitor what's going
on in the CPU and your applications
- would start at hex A000, an "address" located 640Kb from the start of
the chip's memory. The space below
the 640Kb was allocated to CPU random access memory (which is what
most people mean when they talk
about "memory"). DOS itself needs
chip space, and grabs the lowest part
of that 640Kb of memory for itself.
The memory above the 640Kb limit
was called the Upper Memory (UM)
by IBM. No matter how much memory
you have in your PC today, the space
between 640Kb and 1Mb is still called
upper memory. It totals 384Kb.
This was sensational 10 years ago
and many thought it offered more than
mere humanity could ever desire.
Software complexity
They did not reckon on the increasing complexity of software. Here's an
example of that: one of the first DOS
packages was the word processor
WordStar which sold on one 360Kb
floppy disc. The last version I bought,
some five years ago, occupied 19. It
did a great deal more than the first
APRIL 1992
7
This 6Mb RAM card comes with drivers that support both EMS & LIM 4.0
operation. Also included is software to support a printer spooler & RAM disc.
The RAM disc can be configured to any size and will speed up any application
where frequently used files can be stored in RAM. (Rod Irving Electronics).
This 30-pin SIMMs (single-in-line memory module) card has a capacity of9 x
1Mb Fujitsu MB81C1000 surface-mount chips. Eight of these cards can be fitted
to the BOCA 8Mb card shown on the previous page. (Pacific Microelectronics).
version and it made major demands
on my hardware to do it.
But even this 1: 19 ratio doesn't give
you the full picture of how software is
pushing the hardware envelope. For
presentation-quality word processing,
I now use a Windows-based package
and its demands are such that no original PC can run it. I need a higher-level
CPU chip - 80286 or better - and I
need more memory than that original
1Mb. The new minimum is 2Mb of
RAM. And remember, word processing is one of the simplest data processing applications.
Now the 80286 chip is much faster
than the original 8086s and 8088s.
The 80386s and i80486s are faster
again. But all these younger chips are
backward compatible with the original PC chips. So they all have the
original 1Mb limit of chip memory
and the original 640Kb limit on RAM.
Expanded memory
So a new form of memory had to be
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SILICON CHIP
designed for those programs that need
more RAM than 640Kb. Leading software (yes, software) companies got
together with the chip-maker Intel and
agreed on EMS - the Expanded Memory Specification. Its first version allowed software to run above the 640Kb
barrier, in upper memory spaces unused by video, BIOS and accessory
cards. Version 4, released in 1987,
allows you to run up to 32Mb of expanded memory.
This was quite an advance on the
original 1Mb but it's only a small portion of the amount of memory you can
address on the modern PC if you have
a lot of money and the burning desire
to spend it. You can address up to
four gigabytes of memory from an
80386 chip. I can't imagine that anybody would, but then, the IBM engineers couldn't imagine that anybody
would want more than 640Kb of RAM
not so long ago.
Most PC users know of one more
type of memory - extended. This is
not a particular memory type. It's simply any and all memory in the computer above the original 1Mb that the
CPU can address. Expanded memory
is usually fitted to the computer in
expansion boards. It can be converted
by software from extended memory.
Microsoft Windows allows this on
80386 and i80486 machines and other
applications allow it as well.
Software juggling of this type speeds
up the operation of most applications
and makes others possible. If you are
working in graphic-intensive applications, you want expanded memory
and software juggling can be the
cheapest way of getting it. It can also
create clashes with other applications
or your normal mode of operation.
The memory you've software-configured as expanded cannot be used
at all on an 80286 or 80386 machine
running Windows in protected mode.
You can see that a certain amount
of confusion about memory arises from
the language designed to describe
machines now almost obsolete. If you
lash out to buy an i80486 with 16Mb
of RAM, the "upper memory" finishes
15Mb below the "top" of the actual
memory present. And by software juggling, you can have either no expanded memory in the machine, or
megabytes of expanded memory,
whenever you wish it.
You may find this linguistic eccentricity irritating and confusing, but it
arises from honourable motives. The
backward compatibility of the 86 series means that no favourite old software has to be discarded as you move
up the speed slope within the family.
Shadow RAM
There are other features of memory
that the original IBM team probably
never considered. Take shadow RAM,
for example.
Much of upper memory holds Read
Only Memory files. You can read them
but you can't change them. ROM is
slow and if you're using software that
calls on your BIOS a lot, this can slow
things down. So with some of the
newer chips, you can "shadow" (or
imitate) your ROM with part of your
much faster RAM.
This gives you faster operation, at a
price: the part of upper memory you're
using for shadow RAM is no longer
available for applications to use for
RAM. So it may be to your benefit to
use shadow RAM or it may be to your
disadvantage. Only you can tell by
trial and error.
Another piece of software juggling
is the RAM disc. You've just read how
RAM can pretend to be ROM ..It can
also pretend to be a hard disc. If you
set up a "RAM disc", you have fooled
DOS into believing it's your hard disc.
You have mock tracks and sectors and
DOS can call on it just as it does on
your hard disc. But much, much faster.
When calling out information from
the RAM disc, DOS has no need to
move read/write heads, no need to
wait while the right hard disc sector
rotates under the heads, and so on.
RAM discs are usually not large. They
measure in the kilobytes or less. If
you lose power while your RAM disc
is open, you lose everything in it.
And there's a certain amount of housekeeping. The most recent versions of
DOS offer RAM discs as part of the
package.
Disc caches
More versatile for most purposes is
the software attribute called a "disc
cache". There's a DOS disc cache and
a Windows disc cache and you can go
to the computer shop and buy a specialised utility that's nothing but a
cache.
The better ones work out what you
want your computer to do and change
around to help you do it. Key in a
print instruction, for example, and
part of the disc cache switches to enlarge your print spooler. Most hard
disc users certainly have provisions
for a disc cache in their software,
whether they use it or not.
Using it reduces wear and tear on
your disc and speeds up many of your
operations. It can cause trouble in a
number of ways - especially if you
plan to compress or unfragment your
hard disc using Norton Utilities,
PCTools or some similar package. All
such packages advise you to disable
your disc cache before disc compression. If you don't do this, you run the
risk of trashing many of the files on
your disc. The easiest way to disable
it is to put a REM notation at the start
of the line establishing the RAM disc
in your CONFIG.SYS file.
I've not addressed the issue of
memory clashes in this piece. It merits a fresh start with a clear mind.
What they are, how you recognise
them, and how you resolve them, will
be the subject of a future article. SC
*** SAVE ON OUR APRIL SP
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