Numeric Data Processor (8087)
The 8087’s busy signal informs the V20 that the 8087 is executing an
instruction. It is connected to the V20 POLL/[8088 TEST] pin to
provide synchronization via the V20 WAIT instruction in the case
where the V20 must wait for an 8087 result before the V20 continues
with subsequent instructions.
MEMORY ADDRESSING
The 8087 has seven different memory operand formats. Six of them
are longer than one 16-bit word. All are an even number of bytes in
length and are addressed by the host at the lowest address word.
When the V20 executes a memory reference escape instruction,
intended to cause a read operation by the 8087, the V20 always reads
the low-order word of any 8087 memory operand. The 8087 saves the
address and data read. To read any subsequent words of the operand,
the 8087 must become a local bus master by requesting the bus from
the V20. This transfer of local bus control occurs in a manner similar
to the bus control exchange method used by DMA devices. DMA
transfer requests for bus control take a higher priority than 8087
requests for bus control. Upon gaining control of the local bus, the
8087 increments the 20-bit physical address that it had saved to
address the remaining words of the operand.
When the escape instruction is intended to cause a write operation by
the 8087, the 8087 saves the address but ignores the data read.
Eventually, it gains control of the local bus, then performs successive
write and increment address operations writing the entire data value.
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