PDP11 PeripheralsHbk 1972 - Trailing-Edge

1.2.2 Interrupt Structure
If the appropriate interrupt enable bit is set, in the control and status register of a
device, transition from 0 to 1 of the READY or ERROR bit causes an interrupt
request to be issued to the processor. Also if READY or ERROR is a 1 when the interrupt
enable is turned on, an interrupt request is made. If the device makes the
request at a priority greater than that at which the processor is running and no
other conflicts exist, the request is granted and the interrupt sequence takes
place:
a. the current program counter (PC) and processor status CPS) are pushed onto
the processor stack;
b. the new PC and PS are loaded from a pair of locations (the interrupt vector) in
addressed memory, unique to the interrupting device.
Since each device has a unique interrupt vector which dispatches control to the
appropriate interrupt handling routine immediately, no device polling is required.
Furthermore, since the PS contains the processor priority, the priority at which an
interrupt request is serviced can be set under program control and is independent
of the priority of the interrupt request. The Return from Interrupt Instruction
CRT!) is used to reverse the action of the interrupt sequence. The top two words
on the stack are popped into the PC and PS, returning control to the interrupted
sequence.
1.2.3 Programming Example
A paper tape reader interrupt service could appear as follows:
First the user must initialize the service routine by specifying an address pointer
and a word count
INIT: MOV # BUFADR,POINTER ;set address pointer
MOV #COUNT,COUNTR
MOV # 101,PRS
;set counter
;enable reader program continues until interrupt
When the interrupt occurs and is acknowledged, the processor stores the current
PC and PS on the stack. Next it goes to the interrupt vector and picks up the new
PC and PS beginning at location 70. When the program was loaded the address of
PRSER would be put in location 70 and 200& in 72 (to set priority at 4). The next
instruction executed is the first instruction of the device service routine at PRSER.
PRSER: TST PRS
DONE: RT[
BMIERROR
MOVB PRB,@POINTR
INC POINTR
DEC COUNTR
BEQ DONE
INC PRS
;test for error
;branen if bit 15 set
;move character to buffer
;increment pointer
;decrement character count
;branch when input dOne
;start reader for next character
;retum to interrupt program
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