A1.1.4 Purposes of Different Types of Primary Memory Notes

The Purposes of Different Types of Primary Memory

Primary memory is memory the CPU can access directly and quickly. It holds data and instructions needed right now, not later.

Random-Access Memory (RAM)

Key Characteristics of RAM

1. Volatile Storage: Data is lost when the power is off.
2. High-Speed Access: Faster than secondary storage (e.g., hard drives).
3. Temporary Storage: Holds the operating system, applications, and active data.
4. CPU Access: The CPU can read from and write to RAM freely.

Types of RAM

1. Dynamic RAM (DRAM):
   1. Commonly used in computers.
   2. Requires constant refreshing to maintain data.
2. Static RAM (SRAM):
   1. Faster and more expensive than DRAM.
   2. Used in cache memory.

Read-Only Memory (ROM)

Key Characteristics of ROM

1. Non-Volatile Storage: Retains data without power.
2. Permanent Data: Stores firmware and BIOS.
3. Read-Only: Data is typically written once and not changed.

Uses of ROM

1. Booting the System: Contains the BIOS or firmware that initializes hardware during startup.
2. Embedded Systems: Used in devices like microwaves and printers to store control software.

Cache Memory

Levels of Cache

1. L1 Cache:
   1. Smallest and fastest.
   2. Located inside the CPU core.
   3. Stores critical data and instructions.
2. L2 Cache:
   1. Larger and slightly slower than L1.
   2. Can be shared across cores or dedicated to each core.
3. L3 Cache:
   1. Largest and slowest of the three.
   2. Shared across all CPU cores.

Cache Hits and Misses

1. Cache Hit:
   1. The CPU finds the data it needs in the cache.
   2. Access is fast, improving performance.
2. Cache Miss:
   1. The data is not in the cache, so the CPU must fetch it from RAM or secondary storage.
   2. This increases latency.

Registers

Key Characteristics of Registers

1. Volatile Storage: Data is lost when the power is off.
2. Immediate Access: Directly accessible by the CPU.
3. Limited Capacity: Typically hold 32 or 64 bits of data.

Common Types of Registers

1. Instruction Register (IR): Holds the current instruction being executed.
2. Program Counter (PC): Stores the address of the next instruction.
3. Accumulator (AC): Stores intermediate results of calculations.

The Interaction of the CPU with Different Types of Memory

The CPU interacts with different types of memory in a hierarchical manner to optimize performance.

Memory Hierarchy

1. Registers:
   1. Fastest and closest to the CPU.
   2. Store immediate data and instructions.
2. Cache (L1, L2, L3):
   1. Bridges the gap between registers and RAM.
   2. Stores frequently accessed data.
3. RAM:
   1. Main workspace for the CPU.
   2. Holds the operating system, applications, and active data.
4. ROM:
   1. Stores firmware and boot instructions.
   2. Accessed during startup.

Optimization Techniques

1. Prefetching:
   1. The CPU predicts which data it will need and loads it into the cache before it's requested.
2. Cache Replacement Policies:
   1. Determine which data to evict from the cache when it's full.
   2. Common policies include Least Recently Used (LRU) and First-In-First-Out (FIFO).