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              DIFFERENCE BETWEEN THE 80386SX AND THE 80386
              --------------------------------------------

The following are the differences between the 80386SX and the 80386:

1. The 80386SX generates byte selects on BHE# and BLE# (like the 8086 and
   80286) to distinguish the uppoer and lower bytes on its 16-bit data
   bus. The 80386 uses four byte selects, BEO# - BE3#, to distinguish
   between the different bytes on its 32-bit bus.

2. The 880386SX has no bus sizing option.  The 80386 can select between
   either a 32-bit bus or a 16-bit bus by use of the BS16# input.  The
   80386SX has a 16-bit bus size.

3. The NA# pin operation in the 80386SX is identical to that of the NA#
   pin on the 80386 with one exception: the 80386's NA# pin cannot be
   activated on 16-bit bus cycles (where BS16# is LOW in the 80386 case),
   whereas NA# can be activated on any 80386SX bus cycle.

4. The contents of all 80386SX registers at reset are identical to the
   contents of the 80386 registers at reset, except the DX register.  The
   DX register contains a component-stepping identifier at reset, i.e.

   in 80386, after reset     DH = 3 indicates 80386
                             DL = revision number;

   in 80386SX, after reset   DH = 23H indicates 80386SX
                             DL = revision number

5. The 80386 uses A31 and M/IO# as selects for the numerics coprocessor.
   The 80386SX uses A23 and M/IO# as select.

6. The 80386 prefetch unit fetches code in four-byte units.  The 80386SX
   prefetch unit reads two bytes as one unit (like the 80286).  In BS16
   mode, the 80386 takes two consecutive bus cycles to complete a prefetch
   request. If there is a data read or write request after the prefetch
   starts, the 880386 will fetch all four bytes before addressing the new
   request.

7. Both 80386 and 80386SX have the same logical address space.  The only
   difference is that the 80386 has a 32-bit physical address space and
   the 80386SX has a 24-bit physical address space.  The 803886SX has a
   physical memory address space of up to 16 megabytes instead of the 4
   gigabytes available to the 80386.  Therefore, in 80386SX systems, the
   operating system must be aware of this physical memory limit and should
   allocate memory for applications programs within this limit.  If an
   80386 system uses only the lower 16 megabytes of physical address, then
   there will be no extra effort required to migrate 80386 software to the
   803886SX.  Any application which uses more than 16 megabytes of memory
   can run on the 80386SX if the operating system utilitzes the 80386SX's
   paging mechanism. In spite of this difference in physical address
   space, the 80386SX and 80386 CPUs can run the same operating systems
   and applications within their respective physical memory constraints.


file: /Techref/intel/DXSXDIF.HTM, 2KB, , updated: 1999/2/20 10:27, local time: 2023/2/5 22:44,
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