Cache Memory Concept:

Cache memory is a high-speed memory buffer that temporarily stores data the

processors needs, allowing the processor to retrieve that data faster than from

main memory.

Additional feature of a cache over simple buffer, and that is intelligence. A

cache is a buffer with a brain.

Cache holds the data the processor is most likely to need in advance of it

actually being needed.

Cache memory is made up of static RAM (SRAM) memory.

Two levels of cache are used in PC.

1. Level 1 (L1) (Internal cache)

2. Level 2 (L2) (External cache)

3. L3 cache

(1) Level 1 (L1) (Internal cache) :

L1 or primary cache, is a small, high speed cache incorporated right onto the processor’s chip. The L1 cache typically ranges from 8 KB to 64 KB and uses the high-speed SRAM (static RAM) instead of the slower and cheaper DRAM (dynamic RAM) used for main memory. Using memory cache to hold memory values, or the most recently used data and instructions means the processor can retrieve the data from the cache instead of the system's main memory, which is much slower than the cache memory.(2) Level 2 (L2) (External cache) :

L2 or secondary cache, is memory between the RAM and the CPU (but not on the CPU chip itself) and is bigger than the primary cache (typically to 64 KB to 2 MB). L2 ATC (Advanced Transfer Cache) uses micro-architectural improvements, which provide a higher data bandwidth interface between the L2 cache and the processor core, and is completely scaleable with the processor core frequency. The L2 cache is also a unified, non-blocking cache, which improves performance over cache-on-motherboard solutions through a dedicated 64-bit cache

Memory hierarchy(3) L3 CACHE :

Since more manufacturers are beginning to include L2 cache into their architectures, L3 cache is slowly replacing the L2 cache function – the extra cache built into the motherboards between the CPU and the main memory (old L2 cache definition) is now being called the L3 cache. Some manufacturers have proprietary L3 cache designs already, but most desktop and notebook computers do not offer this feature yet. Micron has developed a chip set with 8MB of

on-chip DRAM in the north bridge chip that acts as an L3 cache, but offering an L3 cache as standard equipment is still a future prospect

Advantages of cache : 1) The cache memory enhances the speed of system or improving performance. 2) Cache memory reduces a traditional system bottleneck. 3) As the cache memory lies on the same chip (For L1 cache) the access time is

very small. 4) The same block of data which are stored on the main memory resides on the

cache. Thus the instructions take less time to execute. 5) The CPU and the cache are connected with a local bus which is of high capacity

and speed due to which the data transfer is quick.6) Cache memory is intelligent memory. 7) It holds current working set of code and data. 8) It reduces wait state or no wait states (L1 cache) in system.

Disadvantages: 1) Size is very small. 2) Cost is very high.