I/O Systems

Wikipedia · Input/output · CC BY-SA 4.0

Polling vs interrupts

Polling: the CPU repeatedly checks a status register. Simple, but burns CPU cycles. Good only for fast devices where data arrives immediately. Interrupts: the device sends a signal when ready. The CPU does other work until then. Better for slow devices, but interrupt handling has overhead.

Direct Memory Access (DMA)

DMA lets a device transfer data to/from memory without CPU involvement. The CPU sets up the transfer (source, destination, count), then the DMA controller handles it. When done, the controller interrupts the CPU. Bulk transfers (disk, network) would be impractical without DMA.

Device drivers

A device driver is the OS module that knows the specifics of a particular hardware device. The OS provides a uniform interface (open, read, write, close). The driver translates these into the actual hardware commands (port I/O, register writes, command queues).

Disk scheduling

When multiple I/O requests target different locations on a spinning disk, the order of service matters. The disk arm must physically move to each track. Minimizing total arm movement reduces latency.

FCFS: serve in arrival order. Simple, but the arm may zigzag. SCAN (elevator): the arm sweeps one direction, servicing requests, then reverses. C-SCAN: like SCAN, but only services requests in one direction, then jumps back to the start.

; SCAN disk scheduling simulation
; Head starts at position, moves in one direction, then reverses

(define (scan-schedule requests start max-track)
  (let* ((sorted (sort requests <))
         (right (filter (lambda (r) (>= r start)) sorted))
         (left (filter (lambda (r) (< r start)) sorted))
         (order (append right (list max-track) (reverse left))))
    (display "Service order: ") (display order) (newline)
    ; Calculate total head movement
    (define (total-movement seq prev total)
      (if (null? seq) total
          (total-movement (cdr seq) (car seq)
                          (+ total (abs (- (car seq) prev))))))
    (let ((movement (total-movement order start 0)))
      (display "Total head movement: ") (display movement))))

(scan-schedule '(20 40 80 110 160) 50 199)

# SCAN (elevator) disk scheduling
def scan(requests, start, max_track=199):
    right = sorted(r for r in requests if r >= start)
    left = sorted((r for r in requests if r < start), reverse=True)
    order = right + [max_track] + left

    movement = sum(abs(order[i] - (order[i-1] if i > 0 else start))
                   for i in range(len(order)))

    print(f"Service order: {order}")
    print(f"Total head movement: {movement}")

scan([20, 40, 80, 110, 160], start=50)