    AST RESEARCH, INC.         TECHNICAL BULLETIN #0744A                 03-04-92

                  AST 486 BASED SYSTEMS WITH ADAPTEC 1540 AND 1542B

    PROBLEM:       AST 486 based systems using an Adaptec 1540 and 1542B SCSI HBA
                   shows a significant loss of performance by Checkit and
                   Sprinrite.  Data transfer rates are reduced by 33 percent when
                   compared to a Premium/386-33 or a Premium 386SX-16 system.

    CAUSE:         Adaptec's controller request for the bus are being held off
                   while the 486 completes code fetch sequences from Adaptec's
                   byte-wide BIOS ROM.  This results in 16 accesses to the ROM
                   over the bus, because the 486 processor fetches 128 bits at a
                   time then releases the bus.  The 386 processor fetches 32 bits
                   then releases the bus.  Since the 486 processor holds Adaptec
                   controller off the bus longer during this access, the overall
                   performance is significantly reduced as compared to the 386
                   based system.

    AFFECTED
    SYSTEMS:       AST 486 based systems

    SOLUTION:      It has been determined that this problem will not be fixed.
                   Customers experiencing this problem should use QEMM-386
                   version 5.1 or equivalent as a workaround.  This application
                   will allow shadowing of the Adaptec BIOS which will resolve
                   the problem.