Specifications

Numeric Data Processor (8087)
Read and Write Operations
If a read operation is required, the NDP latches the address and
operand as it appears. If the operand is more than one word long, the
NDP is granted access to the address, data, and control buses for
subsequent read operations. This means that although the NDP does
not have the capability of determining the operand starting address, it
can latch the starting address and increment it for subsequent
operations. If the instruction includes a write to memory operation,
the NDP latches the address and, like the V40, ignores the data during
the dummy read. The NDP is then granted control of the buses to
complete the write operation.
The NDP must get control of the V40 address, data, and control buses
to write numerical operands to memory or to read numerical operands
longer than one word from memory. A request/grant/release protocol
is then used between the V40 and the NDP to carry out this transfer of
control:
1. The NDP generates a request to the V40 to gain control of the
bus resources.
2. The V40 responds by releasing the bus and granting control to
the NDP.
3. The NDP completes the memory operation and releases control
back to the V40.
This entire operation is transparent to the programmer since it is done
with hardware.
The V40 can continue to fetch and execute instructions while the
NDP is performing numerical computations. The V40 WAIT
instruction can be used to suspend V40 operation until the NDP has
completed the numerical computation.
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