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.
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