Protected Mode; Hardware Interface - HP Vectra Reference Manual

Protected Mode
Setting the 80286 microprocessor into protected mode does not
automaticaly set the coprocessor into protected mode. The
coprocessor can be placed in the protected mode by executing
the appropriate software instruction. Refer to
HP Vectra Technical
Reference Manual Volume
2:
System BIOS
for 80287
programming information. Placing the 80287 into protected
mode provides memory management, protection mechanisms,
and associated instructions. Protected mode allows HP Vectra PC
to use other operating systems and to use multi-user applications.
Hardware Interface
The 80287 operates on the same 16MHz clock as the 80286. It
internally divides this clock by three.
The 80286 sends OP codes and operands to the 80287 through
the bidirectional
110
port addresses OF8H, OFAH, OFCH. It also
receives results through these same ports.
The coprocessor asserts the BUSY signal when processing an
arithmetic, logarithmic, or trigonometric calculation. The WAIT
instruction forces the 80286 to wait until the coprocessor is
finished executing the calculation before it can retrieve the data.
(Many numeric instructions have an implicit WAIT.)
There are six different error conditions that the 80287 can detect.
If the corrsponding exception mask is not set, the 80287 will set
its ERROR signal. In turn, this will generate a hardware interrupt
(INT 13), and will latch the BUSY signal. The offending calculation
remains in the 80287, so the 80286 can read the status and
handle the error. The latched BUSY can be cleared by an I/O write
to port OFOH with DO through D7 equal to O. The 80287 can also
be set to handle errors internally (using predefined results).
100 Numeric Coprocessor