I/O Organization & DMA

Input/Output Organization

The computer system's I/O architecture is its interface to the outside world. I/O devices (like keyboards, monitors, and hard drives) operate at much slower speeds than the CPU and Memory. Therefore, we need specific techniques to handle data transfer between the fast CPU and the slow I/O devices without wasting too much CPU time.

1. I/O Interfaces

An I/O Interface (or I/O Controller) is a piece of hardware that sits between the system bus (CPU/Memory) and the peripheral device. It handles the difference in data transfer speeds, data formats, and operating modes.

2. Modes of Data Transfer

2.1 Programmed I/O

• How it works: The CPU initiates the I/O operation and then constantly checks (polls) the status of the I/O device to see if it is ready for the next byte of data.
• Drawback: Polling wastes a huge amount of CPU time. The CPU gets trapped in a continuous loop waiting for the slow device to get ready, meaning it cannot do any other useful work during this time.

2.2 Interrupt-Initiated I/O

• How it works: To solve the polling problem, the CPU issues an I/O command and then goes off to do other useful work. When the slow I/O device is finally ready to send or receive data, it sends a hardware signal called an Interrupt to the CPU.
• Response: The CPU temporarily stops what it was doing, handles the data transfer, and then resumes its previous task.
• Advantage: No CPU time is wasted on polling.

2.3 Direct Memory Access (DMA)

• How it works: DMA bypasses the CPU entirely for the actual data transfer.
• A special chip called the DMA Controller takes control of the system buses.
• The CPU simply tells the DMA Controller: "Transfer 1GB of data from the Hard Drive to RAM starting at address X, and let me know when you are done."
• The DMA Controller manages the entire transfer directly between the I/O device and Memory. The CPU is free to do heavy processing during this time.
• Advantage: Extremely fast for large blocks of data.

3. Bus Arbitration and Cycle Stealing

• Cycle Stealing: The DMA controller steals a single memory cycle from the CPU to transfer a word of data, and then gives control back to the CPU. It slows down the CPU slightly, but keeps both working simultaneously.
• Burst Mode: The DMA controller takes over the bus and transfers the entire block of data all at once before giving control back to the CPU.