Page 177 - DCAP403_Operating System

Operating System

Notes Device Reservation

Coordination to provide exclusive access to a device
1. System calls for allocation and deallocation

2. Watch out for deadlock

Error Handling

1. OS can recover from disk read, device unavailable, transient write failures
2. Most OS’s return an error number or code when I/O request fails
3. System error logs hold problem reports

I/O Protection

User process may accidentally or purposefully attempt to disrupt normal operation via illegal
I/O instructions

1. All I/O instructions defined to be privileged

2. I/O must be performed via system calls
3. Memory-mapped and I/O port memory locations must be protected too

Task For complete the I/O operation you need two I/O operation and these operations
are.

9.6 Disk Scheduling

In order to satisfy an I/O request the disk controller must first move the head to the correct

track and sector. Moving the head between cylinders takes a relatively long time so in order to
maximise the number of I/O requests which can be satisfied the scheduling policy should try to

minimise the movement of the head. On the other hand, minimising head movement by always
satisfying the request of the closest location may mean that some requests have to wait a long
time. Thus, there is a trade-off between throughput (the average number of requests satisfi ed in
unit time) and response time (the average time between a request arriving and it being satisfi ed).
Various different disk scheduling policies are used:

First Come First Served (FCFS)

The disk controller processes the I/O requests in the order in which they arrive, thus moving
backwards and forwards across the surface of the disk to get to the next requested location
each time. Since no reordering of request takes place the head may move almost randomly
across the surface of the disk. This policy aims to minimise response time with little regard for
throughput.
Each time an I/O request has been completed the disk controller selects the waiting request
whose sector location is closest to the current position of the head. The movement across the
surface of the disk is still apparently random but the time spent in movement is minimised. This
policy will have better throughput than FCFS but a request may be delayed for a long period if
many closely located requests arrive just after it.

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