The Intel 80386 SX
by Lloyd Borrett
Technical Cornucopia, January–February 1989
Much has been said recently about the introduction of the
Intel 80386SX or P9, but as usual little has been said about
the real benefits of the 80386SX, and where it fits into the
new generation of personal computers and PC based solutions
that are coming our way.
What is it?
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 Z 80, 6800 and 6502
based systems that then dominated the market. And yet at the
same time the IBM PC was also an 8 bit machine.
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, 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 performance. But more
significantly, it meant a relatively low cost new machine
that could run a new generation of software.
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.
Why was it created?
Firstly, Intel have licensed other chip makers to
manufacture the 80286 chip. These other manufacturers have
been gradually increasing the clock speed of the 80286 and
lowering prices. Intel's share of the 80286 market has been
gradually declining. By introducing a baby 386 chip Intel
can 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.
Operating System/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 chip. That code
means increased overhead. Increased overhead means reduced
Many software developers are already recognising this and
producing 386 versions of their software to make use of the
full potential and performance of the 386. It's quite likely
we'll see a 386 version of OS/2 by the end of 1989.
The benefits and problems
The significance of producing a 386 with a 16 bit data
bus is that it simplifies the chip. This means increased
production yields. It also means simpler system design to
produce a working motherboard.
With a 16 bit data bus there is no need for manufacturers
to come up with yet another 32 bit bus design. They can use
the cheaper 16 bit peripheral chips and circuitry. The lower
number of pins on the 80386SX chip makes board design and
manufacture a lot easier.
But there are some disadvantages. The 16 bit data bus
means that full 32 bit memory accesses take at least twice
as long. The high 16 MHz clock speed of the 80386SX means
that many 80386 like speed up tricks have to be incorporated
into the design. Static column RAM, interleaving and RAM
caching are just some of the speed up tricks needed to allow
the 16 bit peripheral chips to cope.
However, the performance difference between the 80386 and
the 80386SX is not as great as that between the 8086 and the
8088. An 80386SX should only run about 10 percent slower
than an 80386 running at the same clock speed. (This is
because of the improved memory access design inherent in the
386 design, along with fewer wait states.)
The promise of the 80386SX
What the 80386SX promises is 32 bit processing at a speed
not much below that of the 80386, but at a much lower price.
All of the special features of the 386 over the 286
remain. Manufacturers, software houses, and users can all
upgrade more cheaply to 386 32 bit processing.
Saturday, 15 October 2011