HP 3000 III Series Manual page 274

I/O System
7-24. I/O Priorities
There are two types of priority in the I/O system; interrupt
priority and service priority. The ability of a device to inter-
rupt the CPU is based on a priority structure that is separate
and distinct from the priority structure that handles service
requests.
The interrupt poll determines the priorities of all I/O inter-
rupts. The interrupt poll originates in the lOP (figure 7-11)
and is wired in series through every Device Controller in the
system. The proximity to the lOP on this line determines the
interrupt priority of each controller. The desired wiring se-
quence is dependent on system configuration. Physically, the
interrupt poll is a twisted-wire pair (signal and ground) con-
nected into and out of each unit at INT POLL IN and INT POLL OUT
connector pins. Functionally, the interrupt poll is an lOP
response to a received Interrupt Request (INT REQ line in the lOP
Bus). The poll propagates through each non-requesting unit and
stops at the first requesting unit. The unit then returns INT
ACK (Interrupt Acknowledge) and its device number to the lOP.
The lOP then generates an interrupt signal to the CPU. When the
CPU is ready to process the interrupt, it uses the device number
saved in the lOP (Interrupt DEVNO Register) to refer to the
device.
Service priority, unlike the series-linked structure of interrupt
priority, is determined in two levels. For Multiplexer Channel
devices, the first level determines the priority among two or
more Multiplexer Channels. The second level determines the pri-
ority of each Device Controller associated with that Multiplexer
Channel. Figure 7-11 shows only the first-level determination of
priority among Multiplexer Channels by means of a data poll; the
remaining priority determination is by logic not shown. The data
poll operates very much like the interrupt poll. That is, when
the lOP receives a Service Request, it sends out a data poll.
The first requesting Mul tiplexer Channel encountered by the poll
stops propagation of the poll and proceeds to specify the kind of
service required. Therefore, since priority is determined by
proximity to the lOP, the poll is wired through each Multiplexer
Channel in the des ired pr ior i ty sequence. The second -level pr i-
ority determination for Multiplexer Channel devices is by a ser-
vice request number. Since each Multiplexer Channel can handle
16 Device Controllers, there are 16 service request numbers (0
through 15). Each Device Controller associated with a given Mul-
tiplexer Channel is uniquely wired by a jumper to connect to one
of these 16 numbers to give the Device Controller a specific pri-
ority level. (Service request number 0 is the highest priority
and 15 is the lowest priority.)
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