Device driver; FCFS, SSF, SCAN numerical

Working mechanism of a device driver

A device driver is the device-specific software (usually in the kernel) that knows how to talk to one type of device controller. It accepts abstract requests from the device-independent software above it ("read block n") and issues the commands to the controller registers.

 User process
     | read()
 Device-independent I/O software
     | read block n
 Device driver  <---- interrupt handler
     | commands (registers)
 Device controller -- Device

Working:

1. The driver checks the request (valid parameters) and translates the abstract block number into device terms (cylinder, head, sector).
2. If the device is busy, the request is queued (for a disk, a scheduling algorithm orders the queue).
3. It writes the commands in the controller registers (seek, read, address, count) and blocks the calling process while the hardware works.
4. When the operation is finished, the controller raises an interrupt; the interrupt handler wakes the driver.
5. The driver checks for errors, passes data/status back and starts the next queued request. It also handles power management, initialisation and re-entrancy; drivers are loaded as modules.

Disk scheduling numerical

200 cylinders (0 to 199), head at 53, queue: 98, 183, 37, 122, 14, 124, 65, 67. The direction is not stated, so the head is taken as moving towards higher cylinder numbers for SCAN.

FCFS: service order (head at 53): 98, 183, 37, 122, 14, 124, 65, 67

   53 -> 98    = 45
   98 -> 183   = 85
  183 -> 37    = 146
   37 -> 122   = 85
  122 -> 14    = 108
   14 -> 124   = 110
  124 -> 65    = 59
   65 -> 67    = 2
Total = 45 + 85 + 146 + 85 + 108 + 110 + 59 + 2 = 640

SSTF: service order (head at 53): 65, 67, 37, 14, 98, 122, 124, 183

   53 -> 65    = 12
   65 -> 67    = 2
   67 -> 37    = 30
   37 -> 14    = 23
   14 -> 98    = 84
   98 -> 122   = 24
  122 -> 124   = 2
  124 -> 183   = 59
Total = 12 + 2 + 30 + 23 + 84 + 24 + 2 + 59 = 236

SCAN: service order (head at 53): 65, 67, 98, 122, 124, 183, 37, 14

   53 -> 65    = 12
   65 -> 67    = 2
   67 -> 98    = 31
   98 -> 122   = 24
  122 -> 124   = 2
  124 -> 183   = 59
  183 -> 199   = 16
  199 -> 37    = 162
   37 -> 14    = 23
Total = 12 + 2 + 31 + 24 + 2 + 59 + 16 + 162 + 23 = 331

If SCAN moves towards 0 first: 53 -> 37 -> 14 -> 0 (53 cylinders) then 0 -> 183 (183 cylinders) = 236 cylinders.

Answer: FCFS = 640, SSF = 236, SCAN = 331 cylinders (236 if the arm initially moves toward cylinder 0).