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23
DIA
ACS, ACTH
DIBC ACD, ACTR
;READ DATA WORD INTO ACS
;READ CONTROL BITS INTO ACD
;CLEAR ACTR DONE FLAG
Once the ACTR has completed the reception of a word (20
bits), it sets Done to 1.
The program can determine this con-
dition in one of. two ways.
By using the I/O SKP instruction,
the program can test the condition of the Done flag.
SKPDN ACTR
;IS DONE l?
JMP • -1
;NO ••• CONTINUE TO WAIT
;YES ••• MESSAGE RECEIVED
Another way is to utilize the interrupt system that is standard
on the NOVA {ECLIPSE) computer.
The interrupt system is made
up of an interrupt request line to which each I/O device is
con~
nected, an Interrupt On flag in the CPU, and a 16-bit interrupt
priority mask.
The Interrupt On flag controls the status of the
interrupt system.
If the flag is set to 1, the CPU will respond
to and process interrupts.
If the flag is set to ¢, the CPU
will not respond to any interrupts.
An interrupt request is
initiated by an I/O device when it completes its operation.
When
the ACTR completes a message reception, as well as setting Done,
it also places an interrupt request on the interrupt request line,
provided that the bit in the interrupt priority mask {bit 8)
which corresponds to the priority level of the device is
the mask bit is 1, the ACTR can set its Done
~o
p.
If
l, but does not
place an interrupt request on the interrupt request line.
If the Interrupt On .flag is 1 at the time. the processor
completes execution of any instruction, the processor honours
any requests on the interrupt request line.
If the Interrupt