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142
I/O Handling
Users of the assembly language system are required to request access from the operating
system. The OS grants access if granting this access does not compromise any previously
granted access.
Devices such as that discussed above which require interrupt service within a specified time
frame are called "synchronous", and need "synchronous" access. Devices with no such time
constraints are called "asynchronous", and need "asynchronous" access.
The regulation of access incorporates the following points -
• When the operating system grants synchronous access to an operation,
it
is guaranteeing
that the requesting process will have its interrupts serviced with maximum priority.
• DMA conflicts with synchronous access since DMA's cycle stealing causes the processor to
run slower and could thus compromise a synchronous process.
• Synchronous access on a low priority select code in conjunction with asynchronous access
on a high priority select code is conflicting since the asynchronous device could interrupt
the synchronous ISR, thus compromising the timing requirements of the synchronous
device.
• Synchronous and asynchronous access on the same priority level is also conflicting.
Remember an interrupt request on the same priority level as a currently executing ISR will
not be processed until the executing ISR completes.
The following table summarizes the granting of access -
Access Already Granted
Abortive
ASYN DMA
SYN
L
H
L
H
H
L
{ Low
"'0
Abortive
Q,l
High
d
d
d
....
(I)
{ Low
Q,l
=
ASYN
a'
High
Q,l
x
x
x
a::
DMA
(I)
x
x
x
(I)
Q,l
{
High
u
SYN
u
ct:
Low
d
x
x
x
x
d
d
x
x
x
x
x
blank
=
Granted
x
=
Not granted
d
=
Dangerous, but granted
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