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   º         386 UNIX/XENIX VERSIONS AND INTEL MATH COPROCESSORS             º
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     386 XENIX/UNIX AND THE 387DX (A.K.A. ERRATA 21)

         When 386(TM)-based versions of Xenix or other Unix implementations
         are run on a computer containing the Intel 387(TM) DX, the
         computer will lock up under certain conditions.  The problem won't
         occur in the DOS environment.  For the lock up to occur, all of
         the following must happen simultaneously.

         þ   demand page virtual memory must be active
         þ   a 387DX must be installed and in use
         þ   DMA (Direct Memory Access) must occur
         þ   the 80386 must be in a wait state

         When all these conditions are true at the same instant, the 386DX
         ends up waiting for the 387DX and vice versa.  Both processors
         continue waiting for each other indefinitely.  The problem lies
         within certain versions the 386DX, not with the 387DX.

         Intel published Errata 21 immediately after the problem was
         discovered to inform its OEM customers.  At that point, it became
         the responsibility of each manufacturer to implement a fix in
         their hardware or software product.  Some manufacturers, like
         Compaq and IBM, responded accordingly by modifying their system
         boards to prevent these lock ups from occurring.

         The Errata 21 problem is present only in the B Stepping version of
         the 386DX and not the later D Stepping version.  You can identify
         the D Stepping version of the 386DX by the letters DX in the part
         number,  for example 386DX-20.
         A hardware solution is available from Bell Technologies Inc. of
         Fremont, CA.  The $195 Math Adaptor is a small board that  plugs
         into the 80386 socket on the system board.

         If you need more information, contact your computer or software
         manufacturer to see how they have chosen to deal with this
         problem.  Also, an article on page 5 of the April 25, 1988 issue
         of Info World contains a technical description of the problem.

         Because of this problem, SCO Xenix 386 Release 2.2.2 does not
         support the 387DX.













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   February 25, 1991         Intel FaxBack # 3054                       Page 1