Computer Buses

Definition: Electronic pathways for high-speed communication between the CPU, memory, and other devices.
Function: Facilitate data and instruction transfer throughout the computer system.

Types of Buses

System Bus (Internal Bus):
- Connects the CPU to main memory (RAM).
- Initially a single bus (Von Neumann architecture) led to bottlenecks.
- Modern systems use three separate buses:
  - Data Bus: Moves data between the CPU, main memory, and registers. It is bidirectional.
  - Address Bus: Carries memory addresses of data locations. It is unidirectional (CPU to RAM).
  - Control Bus: Transmits instructions (e.g., read, write) between components. It is unidirectional (CPU to RAM).
Expansion Bus (External Bus):
- Connects the CPU to input/output (I/O) devices (via ports).
- Managed by a bridge that connects it to the system bus.
- Communication is generally slower than the system bus.

Bus Width (Word Size):
- Refers to the number of wires in a bus; each wire transmits one bit of information at a time.
- A larger bus width allows more data to be transmitted simultaneously, increasing system speed.
- A system's "x-bit" designation (e.g., 32-bit system) usually indicates its word size, meaning the register size, data bus size, and address bus size are all x bits.
Impact of Word Size on RAM: The word size, specifically the address bus width, determines the maximum amount of RAM a system can directly address.
- Maximum addressable locations = 2^{ ext{number of bits in address bus}}.
- Examples (assuming 1 byte per addressable location):
  - 8-bit address bus: 2^8 = 256 bytes maximum RAM.
  - 16-bit address bus: 2^{16} = 65,536 bytes (64 KB) maximum RAM.
  - 32-bit address bus: 2^{32} \approx 4.3 billion addresses (4 GB) maximum RAM.
  - 64-bit address bus: 2^{64} addresses maximum RAM.

Unidirectional: Data moves in one direction only (e.g., Address Bus, Control Bus).
Bidirectional: Data can move in both directions (e.g., Data Bus, allowing data to be written to or read from memory).