    AST RESEARCH, INC.         TECHNICAL BULLETIN #0629                  06-27-90

                        AUTOSENSING OPERATION OF THE VGA PLUS

    When the AST VGA Plus (part number 202262-003) is configured to operate in 16
    bit mode (switch position "two" = ON) the entire memory address range from
    C0000h to DFFFFh is in 16 bit mode for all system memory accesses.  This is
    due to the design of the IBM AT standard I/O bus.  This means that an 8 bit
    adapter which is configured to reside in this region will not be recognized
    by the system.

    Version 3.0 of the VGA Plus BIOS (part numbers 107000-682 and 107000-683) has
    the ability to detect the standard IBM ROM signature "AA 55".  If, when the
    system is being initialized, the VGA BIOS detects this signature anywhere in
    the 128K range where it resides, the VGA Plus will switch back to eight bit
    operation to avoid conflicts.

    The BIOS cannot detect the presence of RAM (e.g., shared memory) on an
    adapter, however.  In order for adapters which have this type of memory
    (e.g., IBM Token Ring II adapters) to be configured to operate in this
    address range, the VGA Plus has to be manually switched back to 8 bit
    operation (switch position "two" = OFF) to avoid conflicts.