Lecture 3: I/O Structure, Memory, and Caching/DMA

1. I/O Structure Comparison

Feature	Synchronous I/O (Blocking)	Asynchronous I/O (Non-blocking)
Control Return	After I/O completes (CPU waits).	Immediately after I/O starts (CPU continues).
CPU Behavior	CPU waits or loops, wasting cycles (No I/O overlap).	CPU executes other tasks while I/O runs (I/O overlap occurs).
Result	Slow performance, poor multitasking.	Better system performance and multitasking.

• Device-status Table: Maintained by the OS; stores information (type, address, state) about each I/O device to help management.

2. Storage Definitions

• Bit: The basic unit of computer storage (0 or 1).
• Byte: 8 bits; the smallest convenient chunk of storage.
• Word: The computer architecture's native unit of data (e.g., 64-bit word = 8 bytes).

3. Storage Hierarchy

Storage systems are organized based on Speed, Cost, and Volatility.

• Hierarchy (Fastest to Slowest): Registers → Cache → Main Memory (RAM) → Solid-State Disk (SSD) → Hard Disk → Optical Disk → Magnetic Tapes.
• Main Memory (RAM): Volatile; temporary storage for active programs and data.
• ROM (Read-Only Memory): Non-volatile; permanent storage for firmware.

4. Caching and DMA

• Caching: Temporarily storing frequently used data in faster storage (cache) to reduce access time.
  • Example: Main memory acts as a cache for secondary storage.
• Direct Memory Access (DMA):
  • Function: Allows high-speed I/O devices (like disks) to transfer blocks of data directly to/from main memory without constant CPU intervention.
  • Benefit: Reduces CPU workload by generating only one interrupt per block transferred, instead of one interrupt per byte.

Audio Summary