Which CPU Should You Purchase
by Lloyd Borrett
Technical Cornucopia, February 1990
In recent months many conflicting stories have been
circulating about which Intel microprocessor you should have
in your next PC. IBM and Microsoft made an announcement at
Comdex in November that their "Platform for the 90s" was to
be based on the capabilities of the Intel 80386SX, 80386,
80486 and future Intel family microprocessors. Intel
themselves have been running an advertising campaign in the
USA which stresses you should always choose an 80386SX over
an 80286 or earlier microprocessor.
All of this has left the average PC purchaser bewildered.
So what are the differences between the various Intel
microprocessors, and why are so many major players in the
industry now suggesting the Intel 80286 and earlier
microprocessors have had their day? Please read on...
In the beginning...
When IBM released the IBM PC back in August 1981 it was
the first mass-produced 16-bit personal computer. It was a
big break away from the existing 8-bit Zilog Z-80, Motorola
6800 and Intel 8080 based systems then dominating the
market. And yet at the same time the IBM PC was also an
Instead of choosing the Intel 8086 chip, IBM chose the
Intel 8088 chip. The difference was that although both
internally were full 16-bit processors with the same
instruction set and able to address 1 MByte of memory, the
8088 talked to the outside world via an 8-bit data bus,
while the 8086 used a 16-bit data bus. The result was a 50%
reduction in I/O performance. But more significantly, it
meant a relatively low cost system could be built using the
cheaper 8-bit peripheral chips, and that this new system
could run a new generation of software.
Enter the Intel 80286
In August 1984, IBM announced the IBM PC/AT. The AT
contains an Intel 80286 microprocessor which is able to
address up to 16 MBytes of memory. This 16-bit
microprocessor operates in either of two incompatible
modes — real (compatible) mode and protected mode.
Real mode is designed to run programs in exactly the same
manner as they run on the 8088/8086 microprocessor. Thus
when running DOS, real mode is used. The 80286 runs as
nothing but a "fast 8088/8086".
However, Intel's designers of the 80286 wanted it to be
much more than that. They wanted to add such features as
memory management, memory protection, and the ring
protection mechanism, which allows an operating system to
protect one application from another. But they weren't able
to do this while remaining fully compatible with the
8088/8086 microprocessor, so they added the protected mode.
When the Intel 80286 is running in protected mode it
provides these important new features, but it will not run
programs written for the 8088/8086. Unfortunately , the
designers of the 80286 didn't appreciate the importance of
compatibility in the PC marketplace. They designed the 80286
so that it can run in either mode but can't switch back and
forth at will. Thus the extended addressing range and
protected mode architecture of the Intel 80286 sit unused
when running DOS.
32 bit PCs arrive
In October 1985 Intel introduced the 80386, a 32-bit
microprocessor. The Intel 80386 addresses 4 gigabytes of
memory, compared with 1 megabyte possible on the 8088/8086.
With virtual memory, the 80386 addresses 64 tetrabytes, or
70 billion bytes of memory. The magnitude of this is
impossible to visualise easily. It is 100 million times the
memory available under DOS.
To the operating system designer the 80386 is a magical
device. It's capable of running many operating systems
simultaneously. It can run 386 software, 286 software and
DOS software at the same time. With its Virtual Monitor 8086
mode, the 80386 can run multiple 8086 applications as if
each had its own 8086.
The Intel 80386 has a segmented architecture similar to
the 80286. But 80286 segments are limited to 64 KBytes while
on the 80386 segments can be as large as 4 gigabytes.
Writing programs that use large arrays and other data
structures is easier using large segments, and because
compilers have a hard time generating optimal segmented
code, converting large 8086/80286 programs to the 80386 can
produce dramatic increases in performance.
During 1986 we saw Compaq and others introduce PCs using
the 80386 microprocessor. But without an operating system to
exploit the features of the 80386 these PCs were nothing but
"super fast 8088/8086" systems.
Something was missing
Software and hardware developers were very aware that
there was a lot of power going to waste inside every 80286,
and much more in every 80386. What was needed was a new
operating system that would let us exploit the power of
these microprocessors, while maintaining a degree of
compatibility with DOS.
The technical hurdles to developing a multitasking
operating system that is at least nominally compatible with
earlier versions of DOS are formidable. In fact Microsoft
had started working on a multitasking, real mode only
version of DOS back in January 1983. Eventually it was
released as a special OEM product called MS-DOS version 4.0
(not to be confused with today's PC/MS-DOS 4.00). This
project continued and in 1985 IBM and Microsoft signed a
Joint Development Agreement that provided for the design and
development of what was to become Operating System/2 (OS/2).
In April 1987, along with the introduction of its new
PS/2 of systems, IBM announced that together with Microsoft
they were developing this new operating system called OS/2,
and that they expected to make it available before the end
Meanwhile, back in the real world...
During late 1987 and through into 1988 we saw a rapid
change in the PC marketplace. Most large purchases of
personal computers were of 80286 based systems. Very few
corporate and government accounts were now purchasing
8088/8086 based systems.
Intel had licensed other chip manufacturers to produce
80286 microprocessors. These second source manufacturers
pushed along increases in the clock speeds of 80286 based
systems. 80286 based systems were delivering about 5-10
times the performance of, and cost about half as much as,
early 8088/8086 systems. As IBM and Microsoft had promised
that OS/2 would run on 80286 based systems, the purchase of
such an 80286 based system implied a longer effective
working life for the system.
80386 based systems were significantly more expensive and
typically only purchased for demanding applications such as
LAN file servers, CAD/CAM and desktop publishing.
Enter the Intel 80386SX
In 1988 we saw the first PCs using the Intel 80386SX, a
half-way house between the 80286 and 80386 microprocessors.
The Intel 80386SX is to the 80386 as the original 8088 is to
the 8086. Internally the 80386SX is a full 32-bit 80386
processor with the same instruction set as the real 80386.
But as far as the outside world is concerned, it's a 16-bit
80286 like chip. It was created for two reasons.
Firstly, Intel having licensed other chip makers to
manufacture the 80286 chip found these other manufacturers
had not only been gradually increasing the clock speed of
the 80286 but they had also been lowering prices. Intel's
share of the 80286 market had been gradually declining. By
introducing a baby 386 chip that allowed an 80386 work-alike
to be produced at 80286 system prices, Intel could increase
sales and profitability.
Secondly, the 80286 is actually a flawed design. It's
okay for use as a faster 8088/8086 but it doesn't support
multi-user multi-tasking software as well as it could. The
80386 is much, much better at this.
80386 /80386SX software
A few software developers realised that it was possible
to exploit some of the features of the Intel 80386
instruction set in programs still running under DOS. Thus we
saw the introduction of many 80386 specific programs during
1988 and 1989.
Some of these programs such as Borland's Paradox 386
database delivered significant performance improvements over
the non-80386 versions. Other programs such as Quartdeck's
QEMM 386 and Qualitas' 386Max Professional used features of
the 80386 to extend the limits of DOS.
Microsoft even produced an 80386 specific version of
their Windows graphical user interface, and announced plans
to make Windows more closely resemble the Presentation
Manager graphical user interface of OS/2.
OS/2 has a lot of code built into it to handle the
multi-user multi-tasking and memory management instructions
that are built into the 80386 but missing from the 80286.
That code means increased overhead. Increased overhead means
The OS/2 developers, and many application software
developers, were already making noises about how much better
a version of OS/2 that allowed them to fully exploit the
potential of the 80386 would be. It soon became common
knowledge that such a product was in the pipeline.
Already some of the more far sighted system purchasers
realised the significance of the Intel 80386SX and proceeded
to purchase 80386SX based systems in preference to 80286
based systems. During 1989 we saw the sales of 80386SX and
80386 increase dramatically as more and more people realised
the benefits of this more powerful platform.
And now the Intel 80486
Towards the end of 1989 we saw the first Intel 80486
based system shipped. The Intel 80486 microprocessor is
essentially a new, very high-performance version of the
The 80486 typically provides two to three times the
performance of an 80386. Frequent instructions execute in
one cycle which results in performance levels similar to
those found in RISC chips.
The 80486 chip includes integrated memory management with
paging, floating point and cache memory units along with a
high performance integer unit. Previously these functions
resided on other support chips such as the 80387 maths
coprocessors. Now one chips does it all, and it's much
faster because of the reduced inter-chip communication
Intel also added improved multiprocessor support to the
80486. New instructions speed manipulation of memory based
semaphores. On-chip hardware ensures cache consistency and
provides hooks for multilevel caches. Thus we can expect to
see more systems similar to the Compaq SystemPro that fully
exploit the multiprocessor capabilities of the 80486.
Software developers are also looking at ways to exploit
these multiprocessor capabilities. We are now seeing the
power of typical minicomputers and mainframes being put into
a desktop personal computer.
As the gap between entry level and high end personal
computers becomes wider so it becomes even more difficult to
make the right choice. But there are simple guidelines that
can be applied.
- If you need power and performance then you
will be looking to 80486 and 80386 based systems at the top
end of the range.
- If you know you will be migrating to the
OS/2 operating system in time then you should only be
considering 80386SX, 80386 and 80486 based systems.
- If you are already networking your systems
together, or planning to, then it's fairly certain you will
benefit greatly from the use of 80386SX workstations in
preference to 80286 based workstations.
- If you are running a medium size
application that today demands an 80286, then the chances
are that as you grow you will exceed the limits of the
80286. You are less likely to exceed the limits of an
80386SX, and if a 386 version of the software becomes
available you will greatly benefit.
- If you only want to run simple application
stand-alone, but would like to be able to try simple OS/2
applications when they become more common, then get an 80286
based system. But if you can afford the extra money, get an
80386SX anyway. You'll regret not doing so when you realise
that like everyone else you too underestimated the power and
performance you'd really need.
Why the bias to the 386 family
When the IBM PC was introduced in August 1981 it allowed
for 64 KBytes on the motherboard. When the PC/XT was
released it allowed for 256 KBytes on the motherboard. Today
almost all 8088/8086 based systems are shipped with 640
KBytes and most 80286 based systems ship with 1 MByte of
Very few of today's typical applications will run in a
system with less than 640 KBytes of memory. Many software
manufacturers are having great difficulty making their
programs fit into the 640 KByte limit of DOS and are looking
to the features of a new DOS that exploits some of the
features of the 386 family.
Some software developers have all but given up on the
hope that they can make their applications run under DOS and
are only working on OS/2 based applications. Once OS/2
applications that truly exploit the benefits of a
multitasking operating system start shipping, you're going
to want to run them. And to run these OS/2 applications you
really will need the features of an 80386SX, 80386 or 80486.
These 386 family microprocessors will become the basis of
the next generation of software applications.
More still to come...
As we saw the 80286 replace the 8088/8086, which in turn
replaced the early 8-bit microprocessors, so shall we see
the 386 family replace the 80286. As the nature of our
applications change and demand more resources, and system
prices lower, even the 80486 system will become commonplace.
And as to the future. Well we know IBM and Microsoft are
working on OS/2 for the Intel i860 RISC chip. Thus we should
be able to expect such a product accompanied by a new range
of systems based on single and multiple Intel i860s sometime
IBM recently showed off a 16 MBit 50 nanosecond memory
chip produced on existing production lines. In 1992 we can
expect to see personal computers using these chips.
Applications that demand such resources will be developed
and will become commonplace. But few, if any, of these
applications can be expected to run in the DOS/Windows
environment <196> they'll almost certainly use OS/2. At
least by purchasing a 386 family computer today, you'll be
able to run some of these applications in two years time,
thus greatly extending the life of your investment.
Saturday, 15 October 2011