Embedded System

Memories..

Memories are most important part of any computer system. There are various kinds of memory available in market which can be categorized differently based on certain aspects like speed, cost and efficiency. The names as we know in computer languages are given below in their access order…

 

7-3

Now the important issue is what makes L1, L2 or L3 caches much faster than the main memory as both are random access memory(means they loose their values after power off). The answer is their basic construction, Cache is SRAM and Main Memory is DRAM mostly. Here is the difference between the two:

There are two types of Random Access Memory or RAM, each has its own advantages and disadvantages compared to the other. SRAM (Static RAM) and DRAM (Dynamic RAM) holds data but in a different ways. DRAM requires the data to be refreshed periodically in order to retain the data. SRAM does not need to be refreshed as the transistors inside would continue to hold the data as long as the power supply is not cut off. This behavior leads to a few advantages, not the least of which is the much faster speed that data can be written and read.

The additional circuitry and timing needed to introduce the refresh creates some complications that makes DRAM memory slower and less desirable than SRAM. One complication is the much higher power used by DRAM memory, this difference is very significant in battery powered devices. SRAM modules are also much simpler compared to DRAM, which makes it easier for most people to create an interface to access the memory. This makes it easier to work with for hobbyists and even for prototyping.

Structurally, SRAM needs a lot more transistors in order to store a certain amount of memory. A DRAM module only needs a transistor and a capacitor for every bit of data where SRAM needs 6 transistors. Because the number of transistors in a memory module determine its capacity, a DRAM module can have almost 6 times more capacity with a similar transistor count to an SRAM module. This ultimately boils down to price, which is what most buyers are really concerned with.

Because of its lower price, DRAM has become the mainstream in computer main memory despite being slower and more power hungry compared to SRAM. SRAM memory is still used in a lot of devices where speed is more crucial than capacity. The most prominent use of SRAM is in the cache memory of processors where speed is very essential, and the low power consumption translates to less heat that needs to be dissipated. Even hard drives, optical drives, and other devices that needs cache memory or buffers use SRAM modules.

Summary:

1. SRAM is static while DRAM is dynamic
2. SRAM is faster compared to DRAM
3. SRAM consumes less power than DRAM
4. SRAM uses more transistors per bit of memory compared to DRAM
5. SRAM is more expensive than DRAM
6. Cheaper DRAM is used in main memory while SRAM is commonly used in cache memory

 

Below are some very good resources to read on memory:

Chapter7.1-Memory1-old

cpumemory

Chapter7.2-Memory2-old

Chapter7.1-Memory1

10-Memory

Malloc_tutorial

The Memory hierechy

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