Random Access Memory - Cache

• Cache RAM is high speed RAM working at a speed to match the processor; not usually user-changeable, or controlled.
• It holds data recently accessed from external storage ready for use

For Example: Caching a Hard disk To enable a high speed connection between a fast CPU and (relatively) slow Hard Disk, some Cache RAM working at the CPU speed may be placed between. This can increase your drives effective speed, since disk write instructions may be transferred to the Cache at CPU speed, then transferred to the Hard Disk at its speed, while the CPU continues with other functions.

• Two things determine the effectiveness of the RAM cache
  • The speed of the chips - the faster the better
  • The algorithm that guesses at what the next data to fetch will be.

If the code or data is not in level 1 cache, the motherboard chipset requests data from level 2 cache. If the code/data is in level 2 cache it is sent to the CPU in a rapid burst of four or more chunks. (Often called burst fill), this is when the level 2 architecture makes a difference.

• Synchronous level 2 cache
  • Superfast and expensive RAM
  • Can always give the CPU the code/data as fast as the required.
• Pipelined burst level 2 cache
  • Takes longer than synchronous to get the first chunk of data/code, but prepares the next whilst sending the previous, so after the first chunk no time is lost for subsequent chunks.
• Asynchronous Level 2 cache
  • May make the CPU wait one or more extra cycles for each chunk.

External cache may account for 10% of a systems overall performance, but raw power and internal cache size are more important.

(Glass, 1996)

[Rev: 26/7/98] Jun-97 © 1997-98 V/2-Com (Verhaart), P O Box 8415, Havelock North, New Zealand.