IBM AT 5170 Technical Reference page 139

be cleared by an 8-bit I/O Write command to address hex FO
with DO through D7 equal to O.
The power-on self-test code in the system ROM enables IRQ 13
and sets up its vector to point to a routine in ROM. The ROM
routine clears the
I
busy
I
signal's latch and then transfers control
to the address pointed to by the NMI interrupt vector. This
allows code written for any IBM Personal Computer to work on
an IBM Personal Computer AT. The NMI interrupt handler
should read the coprocessor's status to determine if the NMI was
caused by the coprocessor.
If
the interrupt was not generated by
the coprocessor, control should be passed to the original NMI
interrupt handler.
The coprocessor has two operating modes similar to the two
modes of the microprocessor. When reset by a power-on reset,
system reset, or an I/O write operation to port hex 00F1, the
coprocessor is in the real address mode. This mode is compatible
with the 8087 Math Coprocessor used in other IBM Personal
Computers. The coprocessor can be placed in the protected mode
by executing the SETPM ESC instruction.
It
can be placed back
in the real mode by an I/O write operation to port hex 00F1, with
D7 through DO equal to O.
The coprocessor instruction extensions to the microprocessor can
be found in Section 6 of this manual.
Detailed information for the internal functions of the Intel 80287
Coprocessor can be found in books listed in the bibliography.
Coprocessor 2-5