IBM 5170 Technical Reference page 109

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 or
an
I/O
write operation to port hex OOFl, 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 OOFl, 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