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A+ Essentials  Technologies

1.Memory

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

      1. SRAM - Static RAM, and
      2. DRAM - Dynamic RAM, 

SRAM, 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. 

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. 

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.

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

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 . 

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.

240 DIMM (DDR2-SDRAM): It supports higher speeds than it's predecessor DDR SDRAM

240 DIMM (DDR3-SDRAM): It supports speeds faster than DDR2 SDRAM.

PC Memory cards:

SIMMs (Single Inline Memory Modules) have 72 pins on each side of the stick. SIMMs are 32 bits wide, and you need two 72-pin SIMM sticks (Minimum) on a Pentium class computer. This is because the bus width is 64 bits in a Pentium class computer. Note that each side of each pin on a SIMM stick is same; where as each side of each pin on a DIMM (Dual Inline Memory Module) has separate signal flowing.

A DIMM (Dual-Inline Memory Module) has two rows of connecting fingers; one row on each side, and the total number of contacts is 168 for SDRAM, 184 pins for DDR, 240 pins for DDR2 and DDR3 memories.

144-pin small outline DIMM (soDIMM) is commonly used in notebook computers. 144-pin micro-DIMM is still smaller than the so-DIMM and used in sub-notebook computers. 72-pin SODIMM was used in older laptops.

A Secure Digital (SD) card is a small memory card used to make storage portable among various devices, such as cellular phones, PDAs, digital cameras, music players, and personal computers. It uses flash memory to provide nonvolatile storage, which means that a power source is not required to retain stored data.

PCMCIA Cards:

    1. Type I: 3.3 mm thick. Used for memory upgrade cards.
    2. Type II: 5 mm thick. Used for modem and network cards. Some are combination         Modem/NIC cards.
    3. Type III: 10.5 mm thick. Used in PC card hard drives.

The two most important features of PCMCIA are its Plug and Play and Hot Swapping capabilities

The following are usually hot pluggable devices:

    a.eSATA (even SATA is hot pluggable under XP and Vista)
    b.USB
    c.Expresscard/54

But you need to follow proper procedures if you want to remove a USB or eSATA device while the computer is on. The Personal Computer Memory Card International Association (PCMCIA) developed the both the ExpressCard standard and the PC card standards. The host device supports both PCI Express and USB 2.0 connectivity through the ExpressCard slot; cards can be designed to use either mode. The cards are hot-pluggable.

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