Fri 17 Jul 92  6:57
By: Yousuf Khan
To: All
Re: FAQ: 386, 486, SX, and DX?
St:
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Q1) What is the difference between 386SX, 386DX, 486SX, and 486DX
processors?

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A1) All 386 & 486 processor models are "32-bit" CPUs. What this
means is that the processors' registers are large enough to load
and store data that is upto 32-bits in length. It does not matter
if the processor model is designated SX or DX, it is still 32 bit.

One misconception about these processors is that their maximum
memory addressibility is only 4 Gigabytes. They can all address
considerably more than that, upto 64 Terabytes. Most of it is
virtual memory -- memory that is not real, but simulated from disk
or some other storage media.

Now for the differences. From a software developer's point of view
a 486 does not look very different from a 386, and it isn't. A 486
is an uprated version of the 386, with considerably re-engineered
internal circuitry, which generally allows it to execute
equivalent instructions faster. So given a 386 and 486 rated at
the same frequency, the 486 will generally be faster. The extra
speed of the 486 is due to its extra wide internal data pathways
(128 bit vs. 32 bit for the 386), which allows data to flow back
and forth concurrently, instead of serially. There is also an
internal ram cache, which is a buffer between the 486 and the slow
external ram which could cause the 486 to wait too long for memory
access.

The difference between SX and DX depends entirely on whether you
are talking about 386 or 486. In general, an SX is an economy
version of the full DX processor, usually with some feature
removed from it.

In the case of the 386, the SX and SL (an uprated SX) versions
have a smaller external data path: 16-bit vs. 32-bit for the DX.
This smaller data path does not mean that the 386SX's 32-bit
registers are disabled, it simply means that the SX has to take
two passes to fill the same register. This is done automatically
by the processor's own circuitry, and the software developer does
not have to do anything different when writing software for 386SX
that one wouldn't do with a 386DX. Naturally this can mean that an
386SX will run somewhat slower than a 386DX, given the same
frequency. The 386SX also has had some of its address lines
removed. This results in a lower physical memory addressibility,
but its virtual memory addressibility remains at 64 Terabytes. The
386SX can address upto 16 Megabytes, the 386SL can address 32
Megabytes, and the DX can address 4 Gigabytes physically.

In the case of the 486, there are different variations within the
SX theme. Intel's variation of the 486SX has had its internal math
corprocessor cut, but all data paths and address lines remain
intact. Cyrix's variation of the 486SX has cut the math copro too,
but it has also removed the address lines and external data path,
and repackaged the CPU to be pin-compatible with a 386SX CPU.
Cyrix is positioning its offering as a direct replacement of the
386SX. The Intel 486SX and the Cyrix 486SX are not pin-compatible
with each other, but they are software-compatible.

The 486DX is distinguishable from all 386 and 486SX offerings
because it has an internal math coprocessor. The math coprocessor
used to be a separate add-on extension to a processor, because all
its extra functions required more silicon than was available on
the CPU. With increased miniaturization techniques, the 486DX was
able to incorporate the circuitry directly into its own silicon
circuitry.