Capability Extensions - HP Vectra Reference Manual

At power-up the BIOS detects the 80287 and enables hardware
interrupt 13. The interrupt handler clears the latched BUSY signal,
and transfers control to the NMI handler. The NMI handler should
read the status of the coprocessor to determine if the NMI was
caused by the 80287. If it was not, control should be passed to
the original NMI interrupt handler.
Capability Extensions
By programming with the instruction set of the 80287
coprocessor the data types, registers, and instructions to the
microprocessor are extended. The data types are expanded
because the 80287 deals with these directly rather than
employing a program sub-routine. The register space of the
coprocessor is equivalent to the register space of the
microprocessor, but it is organized as eight 80-bit registers. This
register space allows constants and temporary results to be held in
registers during calculations, thus reducing memory accesses and
improving speed as well as system bus availability. The register
space can be used as a stack or as a fixed register set. When used
as a stack, operations are only on the top two stack elements.
Numeric Coprocessor 101