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 PC/AT Compatibility
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     CA-Clipper/ExoSpace works on all IBM PC/AT-compatible computers.  There
     are, however, some machines that claim AT compatibility, but fail in
     certain important respects--an 80286 CPU does not make a machine
     compatible.  The BIOS code must also be compatible.

     Because the 80286 does not have a direct way to switch from protected
     mode to real mode, CA-Clipper/ExoSpace forces the switch by requesting a
     reset (much like power on or rebooting).  When it receives the reset,
     your ROM BIOS startup code must be able to distinguish a restart from a
     power up, in order to return to the CA-Clipper/ExoSpace kernel.

     The 80386 and 80486 processors have a direct way to switch from
     protected mode to real mode.  If you have an 80386 or 80486, you will
     not encounter this problem.

     CA-Clipper/ExoSpace will run on any PC that:

     1. Has an 80286, 80386, or 80486 CPU.

     2. Can run MS-DOS or compatible operating system, Version 3.0 or later.

     3. Supports IBM-PC/AT BIOS Interrupt 15h function 88h (get extended
        memory size).

     4. Is compatible with the IBM PC/AT in these respects:

        .  CMOS RAM (for saving time/date, configuration data, and the
           restart code) accessible via I/O ports 70h and 71h

        .  Intel 8259-compatible Programmable Interrupt Controller,
           accessible via I/O ports 20h and 21h (additional controllers may
           be cascaded to the first)

        .  Intel 8042 auxiliary processor, with control port at I/O
           60h, and status port at I/O address 64h

        .  "AT-compatible" ROM BIOS code that supports some form of
           restarting