Memory (RAM)
Memory Overview
Computer memory is electronic circuitry that holds binary data. It does this by setting the positions of microscopic electronic switches to on or off. The on position represents the binary 1, and the off position represents the binary 0. Each 0 and 1 is called a bit. The switches are divided into groups of eight bits, called bytes. A byte is an 8-bit number that can equal any decimal number between 0 (all 0s) and 255 (all 1s), and is the basis of all computer data. For more information on binary numbers, go to http://computer.howstuffworks.com/bytes.htm.
Main memory in computers comes in modules, often called sticks, which can contain billions of switches. Memory sticks are measured in megabytes (MB), and soon, gigabytes (GB). One MB equals, oddly enough, 1,048,576 bytes. One GB equals 1,1,073,824 bytes.
The acronym RAM stands for random access memory. This means that any part of memory can be accessed directly, as opposed to data stored on tape, which requires fast winding to the appropriate segment.
When selecting memory for a computer, there are several items to consider:
Type: Motherboards can accept certain types of memory, such as SDRAM, DDR SDRAM, Rambus, EDO RAM, and burst-EDO RAM. Check the motherboard documentation or the motherboard manufacturer's Web site. We discuss these types later in this chapter.
Speed: Motherboards can accept memory modules in certain ranges of speed; for example, 100 or 133 MHz. The faster the memory, the faster the performance of the computer. As in memory type, all memory installed in a single computer should be the same speed. If you install two different speeds of memory on the same motherboard, all chips will run at the slower speed.
Quantity (as measured in megabytes): Unlike medication, with memory, more is better, although you can reach a point of diminishing return. The motherboard documentation will specify the maximum amount of memory it can accept. We discuss how to determine the optimum amount of memory later in this chapter.
Quality: Memory rarely fails unless it is exposed to static electricity. The most common problem with memory is when brand new modules are bad—so a good warranty is essential. Kingston, Crucial, and PNY are all good brands.
Error detection: Memory comes in ECC or parity, or non-ECC or non-parity. ECC and parity are systems for detecting and correcting memory errors. Parity memory can compensate for single-bit errors. This parameter is specified by the motherboard manufacturer, but is changeable in some BIOSs. If the BIOS is set for ECC/parity memory, only ECC/parity memory will work. Again, even if the motherboard will accept either, it likely will accept only one of these at a time. If you have a DIMM (see the next item in this list) and you want to determine if it is ECC/parity or non-ECC/parity, simply count the number of black chips soldered to the module. If the number of chips is evenly divisible by three or five, then the module contains ECC or parity memory. If the number of chips is not evenly divisible by three or five, you have non-ECC/parity memory.
Physical module size and pin layout: Almost all currently used memory comes in Dual Inline Memory Module (DIMM) form (see Figure 5.1). However, size and number of pins vary. This parameter must match that of the motherboard. Rambus memory comes in Rambus Inline Memory Modules (RIMMS), which are Rambus' version of DIMMs. Prior to DIMMs, Single Inline Memory Modules (SIMMs) were prevalent. Notesite computers take very small DIMMs, called SODIMMs.
Figure 5.1: A 184-pin DIMM.
CAS Latency: Measured in numbers such as CL2 and CL3; make sure these match the requirements of the motherboard.
Serial Presence Detect (SPD): This is memory with an additional chip that contains information used by some motherboards to set certain memory parameters. This can be used on any motherboard, but if the motherboard requires this type of memory and non-SPD memory is used, the computer will display an error message when attempting to boot. We discuss the error message later in this chapter.
Single- or double-sided module: Some motherboards take either kind, but with a restriction. The Intel D815EEA motherboard, for example, has four memory slots. You can use up to four single-sided modules, but the motherboard will recognize only two double-sided modules. If you install two double-sided modules on slots 0 and 1, any modules in slots 2 or 3 will be ignored. Interestingly, you might not be able to determine this parameter by looking at the module. For more information, go to kingston.com and use the Memory Search function to search for memory for the Intel D815EEA motherboard.
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