
                          Processors
                          by Jim Reeb
                     From 02/93 MACE TIMES

  What is the difference between 386SX, 386DX, 486SX, and 486DX
  processors?

  All 386 and 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 up to 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 addressability is only 4 Gigabytes. They can address
  considerably more than that, up to 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 upgraded 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 vx 32 bit for 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 for memory
  access.

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

  In the case of the 386, the SX and SL (an upgraded SX for use in
  low power notebook computers) versions have a smaller external
  data path; 16 bit vs 32 bit for 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 any
  thing different when writing software for a 386SX that he
  wouldn't do with a 386DX. Naturally, this can mean that and386SX
  will run somewhat slower than a 386DX, given the same clock
  frequency. The 386SX also has had some of its address lines
  removed. This results in a lower physical memory addressability,
  but its virtual memory addressability remains at 64 Terabytes.
  The Intel and clone 386SXs can address up to 16 Megabytes, the
  Intel 386SL can address 32 Megabytes, and the Intel and clone
  386DX can address 4 Gigabytes physically.

  IBM has a specialized version of the Intel 386SL called the
  386SLC, which has added an internal 8K cache. It is otherwise pin
  compatible with the regular Inter 386SL.

  AMD and C&T Corp. both have clones of the Intel 386SX and 386DX.
  They are generally very much the same as their Intel counterparts
  and are direct replacements of each other, both in terms of pin
  compatibility, and software compatibility.

  In the case of the 486, there are different variations within the
  SX theme. Intel, Cyrix, and IBM all have made different
  modifications to the 486DX design to come up with their economy
  486.

  The 486DX is distinguishable from all 386 and economy 486
  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 die.

  Intel's variation of the economy 486 is called the 486SX. The
  486SX has had its internal math coprocessor cut, but all data
  paths and address lines remain intact. The Intel 486SX and 486DX
  are not pin compatible with each other, so one cannot simply
  remove a 486SX and place a 486DX in its place or vice-versa.

  Cyrix has two levels of economy 486s; the 486SLC and the 486DLC.
  For the 486SLC, Cyrix has cut the math coprocessor too, but it
  has also reduced the size of the internal cache from 8K to 1K,
  and repackaged the CPU to be pin compatible with a 386SX CPU.
  Since it is pin compatible with an Intel 386SX, the 486SLC can
  only address 16 Megs of memory, and it has a 16 bit external data
  path. Cyrix is positioning its offering as a direct replacement
  of the 386SX. For the 486DLC, all the same modifications that
  Cyrix made for the 486SLC apply, except that it is made pin
  compatible with an Intel 386DX rather than 386SX. Since the
  486DLC is pin compatible with the Intel 386DX, it will be able to
  address 4 Gigabytes, and it has a 32 bit external data path. The
  Intel 486SX and the Cyrix 486xLC are not pin compatible with each
  other, but they are software compatible.

  IBM has come out with its own variation of the 486, confusingly
  called the 486SLC. IBM's 486SLC is not pin compatible with the
  Cyrix 486SLC. It is pin compatible with the Intel 386SL/IBM
  386SLC, however. The IBM 486SLC does not have a math coprocessor
  built-in, but it has a larger internal RAM cache than all other
  486s, 16K. Because of its 386SL/386SLC heritage, the 486SLC
  incorporates the same power-saving features, and it can address
  up to 32 Megs, 16 bits at a time.

  If you think that all of this is confusing, wait until next year!
