Numeric Data Processor (8087)
No 8087 interrupts
All errors on the 8087 are always masked. Numeric interrupts are not
possible. Leave the 8087 INT signal unconnected.
Single interrupt system
The 8087 is the only interrupt in the system. Connect the 8087 INT
signal directly to the interrupt request 0 (IR0) at the 8259A
Programmable
Interrupt
Controller
(PIC)
via
jumper
W5A.
Alternatively, tie the INT signal to the Non-Maskable Interrupt (NMI)
request via jumper W51. Note that STD DOS uses IR0 for the Timer
0 interrupt; therefore, the 8087 may not drive IR0 in ZT 8809A STD
DOS systems.
High priority 8087 interrupt
The 8087 interrupt is a stop everything event. Choose a high priority
interrupt input to the PIC that terminates all numerics related activity.
This is a special case since the interrupt handler may never return to
the point of interruption (that is, reset the system and restart rather
than attempt to continue operation). IR0 is the highest level interrupt
available at the 8259A PIC, although NMI takes higher priority over
the maskable interrupts such as IR0.
Low priority 8087 interrupt
Numeric exceptions or numeric programming errors are expected. All
interrupt handlers should use the 8087 only with all errors masked.
This is to prevent the possibility of an 8087 interrupt service routine
being interrupted by a higher priority service routine, which may try
to use the 8087 and cause another error interrupt. This prevents proper
handling of the first error-caused interrupt.
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