Specifications

Theory of Operation
The local interrupt status bit (bit 0) indicates the status of the ZT 8832
interrupt request to the STD bus CPU. The STD bus interrupt status
bit (bit 1) indicates whether any of the interrupt sources sharing the
STD bus interrupt request with the ZT 8832 has an interrupt pending,
including the ZT 8832. The following steps outline an interrupt cycle
for a shared STD bus interrupt configuration:
1. The local CPU activates the interrupt request by writing to the
local control port. Note that the STD bus CPU must first remove
the interrupt mask by writing to the STD bus control port.
2. The STD bus CPU responds to the interrupt request by vectoring
to the interrupt service routine for the shared resources.
3. The interrupt service routine reads the Interrupt Status Port to
determine if the ZT 8832 generated the request.
4. If the local status bit of the Interrupt Status Port is active, then
the ZT 8832 has a request pending.
5. If the local status bit is not active and the STD bus status bit is
active, then one of the other shared resources has an interrupt
pending. If other ZT 8832s share the interrupt, the Interrupt
Status Port of each one can be read to determine if a request is
pending. If the board select option is in use, each ZT 8832 must
be selected by writing the appropriate board address to the
Board Select Port before the Interrupt Status Port becomes
accessible.
6. The interrupt service routine resets a ZT 8832 request by writing
to the STD bus control port. To prevent missing a subsequent
interrupt, it is best to reset the request as early as possible. If the
board select option is in use, the ZT 8832 must be selected
before the STD bus control port is accessible.
3-19