Input/Output (I/O) Techniques; Storage Devices

Input/Output (I/O) Techniques

I/O Control Techniques

Programmed I/O (Polling)

# Pseudo-code for polling
while device.status != READY:   # busy waiting!
    pass  # CPU stuck in loop
device.transfer_data()

• CPU wastes cycles checking device repeatedly
• Simple but inefficient
• Used for simple, slow devices

Interrupt-Driven I/O

CPU sends I/O request to device
CPU continues with other tasks
Device finishes I/O → sends INTERRUPT signal
CPU saves current state → handles interrupt (ISR)
CPU resumes previous task

Interrupt types:

• Hardware interrupt — device signaling CPU (keyboard press, NIC packet)
• Software interrupt — program requesting OS service (system call)

Exception — CPU error (divide by zero, page fault, overflow)

DMA (Direct Memory Access)

CPU → "Transfer 1MB from disk to RAM" → DMA Controller
CPU continues other work
DMA handles: disk → DMA buffer → RAM (directly, without CPU)
DMA → Interrupt to CPU: "Transfer complete"

• DMA controller is a special chip that handles I/O independently
• CPU is free to do useful work during data transfer
• Very important for high-speed I/O (disk, network)

Storage Devices

Hard Disk Drive (HDD)

HDD stores data on rotating magnetic platters.

Platter (magnetic disk)
  └── Tracks (concentric circles)
        └── Sectors (pie slices; 512 bytes or 4KB each)
              └── Clusters (groups of sectors)

Components:

• Platter — rotating aluminum disk coated with magnetic material

Read/Write Head — floats nanometers above platter; reads/writes data

• Arm/Actuator — moves heads across platters
• Spindle — motor spinning platters at 5400 or 7200 RPM

HDD Performance:

Solid State Drive (SSD)

SSD stores data in NAND flash memory chips — no moving parts.

Advantages over HDD:

SSD Types: