What is Cache Memory?
Small
section of SRAM added between processor and main memory to speed up execution
process is known as cache memory. Cache memory is a special high-speed
memory. It is buffer between CPU and Main Memory.
It helps
to increase the processing speed of CPU to provide data of program which is
currently in execution mode. A cache memory system includes a small amount of
fast memory (SRAM) and a large amount of slow memory (DRAM).
Direct Mapping Cache Memory
It is the
simplest technique to mapping cache memory to main memory. In this technique,
each block from the main memory has only one possible location in the cache
organization. In this example, the block i of the main memory maps onto block j
(j = i modulo 128) of the cache, as shown on figure. Therefore, whenever one of
memory blocks 0, 128, 256, … is loaded in the cache, it is stored in cache
block 0.
 |
Figure: Direct mapping Cache memory |
Block 1,
129, 257, … are stored in cache block 1 and so on. In general mapping expression is,
j = i modulo m
Where,
i = number of blocks in RAM
j = number of blocks in the cache
m = number of blocks (lines) in the cache
To
implement such cache system, main memory address is divided into three fields, as
shown in diagram. The lower order 4-bits select one of the 16 words in a block.
This field is known as word field. The second field known as block field is
used to distinguish a block from other blocks. Its length is 7-bits since 27
= 128.
When a
new block enters the cache, the 7-bit cache block field determines the cache
position in which this block must be stored. The third field is a tag field. It
is used to store the higher-order 5-bits of memory address of the block. These
are used to identify which of the pages that are mapped into the cache.
The main drawback
of direct mapped cache is that each block of main memory maps to a fixed
location in the cache; therefore, if two different blocks map to the
same location in cache and they are continually referenced, the two blocks will
be continually swapped in and out (known as thrashing in cache memory). So,
direct memory mapping is easy to implement but not flexible.
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