memory
You have many types of Memory. We're only going to look at two types now. The first is ROM (Read Only Memory) and RAM (Random Access Memory).
ROM:
Read only memory is just what it says READ ONLY. This memory is permanent. It is on your mother board as a chip (usually BIOS) and is commonly known as FIRMWARE. This chip is used for the boot process. As the computer goes through its start up, the processor gets its instructions from the firmware and begins its job. Every time you start your computer, ROM is ready to tell the processor how to start up.
RAM:
Random Access Memory is temporary memory and is cleared every time you boot the computer. RAM is the place where the computer holds current programs and data that are in use. If you are working on a document, spreadsheet, database, etc, the information is stored in RAM. Once the computer is rebooted, RAM is cleared out. The amount of RAM you have is an important contributing factor to overall performance. Newer programs require more memory than old ones. These days, 64 megabytes of RAM should be the MINIMUM that you have installed. RAM also comes in many "flavors" so to speak. You have Static RAM (SRAM), Dynamic RAM (DRAM).
Static RAM (SRAM):
Static RAM is a type of RAM that holds its data without external refresh, meaning that as long as power is supplied to the circuit, SRAM will hold all of its data intact unlike dynamic RAM (DRAM), which must be refreshed many times per second in order to hold its data contents. SRAMs are used for specific applications within your computer. SRAM is very fast but its cost is high. SRAMs are used for level 1 cache and level 2 cache memory. Cache memory needs to be very fast, and not very large. Cache (pronounced cash, not cashay) memory will be discussed later.
Dynamic RAM (DRAM):
DRAM is used for system memory. It is a type of RAM that only holds its data if it is continuously accessed by special logic called a refresh circuit. Hundreds of times a second, this circuitry reads the contents of each memory cell, whether the memory cell is being used at that time by the computer or not. The reading action itself refreshes the contents of the memory. If this is not done regularly, then the DRAM will lose its contents, even if it continues to have power supplied to it. This refreshing action is why the memory is called dynamic. All PCs use DRAM for their main system memory, instead of SRAM. DRAMs are used due to simplicity. They are much cheaper to make and take up much less space, typically 1/4 the area of SRAMs or less. DRAMs are smaller and less expensive than SRAMs because SRAMs are made from four to six transistors (or more) per bit, DRAMs use only one, plus a capacitor. The capacitor, when energized, holds an electrical charge if the bit contains a "1" or no charge if it contains a "0". The transistor is used to read the contents of the capacitor. The problem with capacitors is that they only hold a charge for a short period of time, and then it fades away. This is why the refresh circuitry is needed: to read the contents of every cell and refresh them with a fresh "charge" before the contents fade away and are lost. There are many different kinds of DRAM and speeds that they are available in (speed being measured in nano-seconds).
Cache Memory:
Cache Memory is located between the CPU
and the Main Memory. Typically, there is anywhere from 128KB to 512KB of
SRAM cache chips in modern PCs known as Level 1 and Level 2 Cache. Memory
caching is great because programs repeatedly access the same code and data
paths, and much of the processing time is spent in relatively small areas of the
code. By caching this most recently used data in SRAM, the CPU can run much
faster than it could from slower main memory (DRAM).