Computer with Single Processor and Cache

Cache

• CPUs became much faster at processing instructions
• However, RAM access speed did not improve very much
• The CPU could not process instructions during data transfers
  • The CPU spent a lot of time waiting for RAM to respond
  • Memory latency became a problem
• Computers started using small amounts of "cache" memory

Cache Memory

• Cache Memory

  • Physically close to the CPU
  • Stores copies of current and recently used data
  • Reduces the time spent communicating with main memory

• Uses "static" RAM

  • This is faster than the "dynamic" RAM used in main memory
  • More expensive
  • Uses more power
  • Usually much smaller than main memory

Cache Fetch

• The CPU wants to fetch data
• Does the cache have a copy of the data?
• Yes
  • There is a "cache hit"
  • The CPU uses that copy
• No
  • There is a "cache miss"
  • The data is fetched from main memory

Cache Write

• The CPU wants to store data
• It writes the data to the cache
• The data in the cache is written to main memory
  • The CPU can continue working while this happens

Cache Controller

• The process is managed by a "cache controller"
• The data is transferred in fixed-size blocks
  • Known as "cache lines"
  • Typically 64 bytes on modern hardware (8 words)
• Each cache line relates to an address in main memory