Write Through And Write Back

Write Through & Write Back

Definitions

• Write Through: Every write operation updates both the cache (if present) and the main memory.

• Write Back: Write operations update only the cache; the main memory is updated later when the block is replaced (if dirty).

• Write Allocate: On a write miss, the block is first loaded into cache, then written into.
• No-write Allocate: On a write miss, data is written directly to memory without loading the block into cache.

1. Write Through (No-write Allocate)

• Read
  • Hit: Take directly from cache.
  • Miss: Read from memory and place the block into cache.
• Write

  • Hit: Write in both cache and main memory (Write Through).

  • Miss: Write directly to main memory without bringing the block into cache (No-write Allocate).

2. Write Back (Write Allocate)

• Read
  • Hit: Take directly from cache.
  • Miss: Read from memory; if the block being replaced is dirty, write it back to memory before replacement.
• Write

  • Hit: Write in cache and set Dirty bit = 1 (Write Back).

  • Miss: Bring block from main memory into cache, then update it in cache (Write Allocate).