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The process looks something like this -
"original"
routine
c:
.Q
"B-----,
Q)
x
Q)
'0
~
:;::::
interrupt
--O---~
detected
The sequence of events in interrupt I/O can be detailed as follows -
I/O Handling
139
1. The interface sends a request for service to the backplane which passes it along to the
processsor.
2. The processor alters the flow of execution so that the routine associated with that inter-
rupting source can be executed. The processor saves its place
~n
the interrupted routine
so that it can later return to it.
3. The interrupt service routine is executed, performing whatever functions are desired.
Frequently these functions involve some form of programmed I/O or direct memory
access. The service routine may signal an end-of-line BASIC branch, indicating to
BASIC that some condition occurred (discussed below).
4. The service routine returns the processor to the interrupted routine so that the "original"
process can resume.
The uses for interrupt I/O are so diverse that it is difficult to generalize about them. However,
one particular use is fairly well-defined and of general applicability - data transfers.
Interrupt I/O is normally used in data transfers whenever a particular data device has a transfer
rate which is significantly slower than that of the computer. Since the 9835A/ B has a transfer
rate of around 10 000 characters per second, peripheral devices with transfer rates slower than
this number are candidates for interrupt I/O.
The usual approach is to transfer a word to or from the peripheral device, then go away to do
some other processing whil-'e waiting for the device to interrupt by becoming "ready" for
another transfer.
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