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 A+ Essentials (220-901) Cram Notes

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1.3 Compare and contrast RAM types and features  

1. There are basically two important types of RAM (Short for Random Access Memory):

a. SRAM - Static RAM being expensive, primarily used for Cache memory. DRAM, being cheaper, is used for main memory. SRAM is widely used for Level 1, Level 2 or Level 3 cache memory. Level 1 cache is internal to the processor, and level 2 and level 3 caches are external to the processor, it resides on the motherboard.

b. DRAM: Dynamic RAM holds its data if it is continuously accessed by special logic called a refresh circuit. If the memory is not refreshed regularly, then the DRAM will lose its contents. This refreshing action is why the memory is called dynamic. All PCs use DRAM for their main system memory, instead of SRAM, even though DRAMs are slower than SRAMs and require the overhead of the refresh circuitry. The reason that DRAMs are used is that they are much cheaper and take up much less space.

2. RAM is much faster than ROM is, due to the nature of how it stores information. For this reason, RAM is often used to shadow the BIOS ROM to improve performance when executing BIOS code. PROM (Programmable ROM) is also a version of ROM and is slower compared to RAM.

3. The computer main memory usually consists of some type of DRAM. Types of DRAM Packages and DRAM Memory are explained below:

a. 168 pin DIMM (SDRAM): It can run at much higher clock speeds than conventional memory. SDRAM actually synchronizes itself with the CPU's bus and is capable of running at 133 MHz and twice as fast EDO DRAM .

b. 184 pin DIMM (DDR-SDRAM): It supports data transfers on both edges of each clock cycle, effectively doubling the memory chip's data throughput. DDR-SDRAM is also called SDRAM II. DDR stands for Double Data Rate.

c. 240 DIMM (DDR2-SDRAM): It supports higher speeds than it's predecessor DDR SDRAM.DDR2 SDRAM is a double data rate synchronous dynamic random-access memory interface. It supersedes the original DDR SDRAM specification and has itself been superseded by DDR3 SDRAM. DDR2 is neither forward nor backward compatible with either DDR or DDR3.

d. 240 DIMM (DDR3-SDRAM): It supports speeds faster than DDR2 SDRAM. DDR3 SDRAM, an abbreviation for double data rate type three synchronous dynamic random access memory, is a modern kind of dynamic random access memory (DRAM) with a high bandwidth interface, and has been in use since 2007. DDR3 SDRAM is neither forward nor backward compatible with any earlier type of random access memory (RAM) due to different signaling voltages, timings, and other factors

4. PC ratings and corresponding DDR bandwidth are given below:

DDR266 = PC2100

DDR333 = PC2700

DDR400 = PC3200

DDR2-400 = PC2-3200

DDR2-533 = PC2-4200

DDR2-667 = PC2-5300

DDR2-800 = PC2-6400

DDR3-800 = PC3-6400

DDR3-1066 = PC3-8500

DDR3-1333 = PC3-10600

DDR3-1600 = PC3-12800

DR400 memory is also referred to as PC3200 memory. 400MHz DDR2 memory would be listed

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