Skip to content

I/O Interface

Short Notes

How the CPU communicates with external peripherals.

Data Transfer Modes

  • Programmed I/O: CPU polls the device in a loop. CPU is 100% busy.
  • Interrupt-driven I/O: Device signals CPU via interrupt. CPU only interacts when needed.
  • DMA (Direct Memory Access): Device transfers data directly to/from memory without continuous CPU involvement.

Key Theories & Formulas

1. Cycle Stealing in DMA

DMA "steals" a bus cycle from the CPU during the transfer.

  • DMA Time per character: \(T_{dma} = \text{Preparation Time} + \text{Transfer Time}\).
  • CPU stall time: percentage of cycles stolen.

2. Interrupt latency

The time from the occurrence of an interrupt to the start of the execution of the Interrupt Service Routine (ISR).

Example Problems

Problem: A disk transfers data at 1 Mbps. CPU clock is 1 GHz. DMA transfer is in 1-byte bursts. How many clock cycles are stolen per byte?

  1. Time for 1 byte: \(8 \text{ bits} / 10^6 \text{ bps} = 8 \mu\text{s}\).
  2. CPU clock period: \(1 / 10^9 = 1 \text{ ns}\).
  3. If 1 DMA burst takes 1 memory cycle (10 ns typical):
  4. Every \(8 \mu\text{s}\), the CPU is stalled for \(10 \text{ ns}\).
  5. Stolen cycles = \(10 \text{ cycles}\) per Byte.

Hardest GATE Questions

Topic: DMA Overhead and CPU Idle time Tricky Question (GATE 2013/2015): A DMA controller transfers 64-character blocks from a device to memory. The device rate is 200 KBps. The CPU runs at 1 MHz. Each DMA transfer (of one character) steals one bus cycle. What is the percentage of time the CPU is busy?

  • Analysis:
  • Device makes a request every \(1/200K = 5 \mu\text{s}\).
  • CPU cycle time = \(1 / 1M = 1 \mu\text{s}\).
  • Every \(5 \mu\text{s}\), one cycle (\(1 \mu\text{s}\)) is stolen.
  • Stolen fraction = \(1/5 = 0.20\) or \(20\%\).
  • CPU Busy time = \(100\% - 20\% = 80\%\).
  • The "Trap": Sometimes the question mentions "Burst Mode" vs "Cycle Stealing Mode". In Burst Mode, the CPU is stalled for the duration of the entire block transfer.
  • Hard Aspect: When multiple devices are connected, and you must calculate the aggregate stolen cycles or determine if the bus saturates

References