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I/O Handling
141
If the linked select code is low-priority, the statement is -
The service routine is exited with a RET 1 instruction.
Here is an example of a short interrupt service routine which simply reads a word of data from
the interface -
Access
':::;TH
?CT
:
.C
IS
,~
a line feed?
NOTE
Utilities cannot be called from an interrupt service routine.
Attempts to do so may lock up the machine.
The operating system (OS) contains a mechanism to regulate requests for hardware capabilities
in order to eliminate conflicting uses of these capabilities. For instance, since there is only one
DMA channel, it is necessary that there be a mechanism to prohibit two simultaneous DMA
transfers.
The OS mechanism which regulates the use of DMA (and also interrupt) transfers either grants
or does not grant what is called "access". Before starting either an interrupt or DMA operation,
access should be requested from the operating system.
Another example - suppose a device operating on a high priority select code has a relatively
slow data rate. This is an ideal situation in which to use interrupt driven I/O. Suppose further
that the device operates in such a fashion that the data must be transferred within a fixed time
period following its issuance of an interrupt or the data is lost (the internal tape drive is such a
device.) If there are other interrupt type transfers operating concurrently on other high priority
I
select codes, it may not be possible to service our slow device within the necessary time frame.
When the operating system grants access, this type of conflict is impossible.
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