MEMORY TYPES AND CHARACTERISTICS

Many forms of memory have been available since the first computer was introduced. Table 17.2 provides a quick reference to the conceptual aspects of memory that you should be aware of before taking the A+ core test.

Table 17.2: Memory Concepts

Memory Type	Packaging	Volatile	Nonvolatile
Main DRAM	DIP	X
Main SDRAM	DIMM	X
Main DDRAM	DIMM	X
Main RDRAM	RIMM	X
Main VRAM	Adapter	X
Cache Level 1	IC Card	X
Cache Level 2	IC Card	X
Cache Level 3	IC Card	X
ROM BIOS	Chip		X
Virtual memory	Swap file on hard drive		X

Read-Only Memory

You should recall from Chapter 15 that ROM contains small programs installed at the factory. ROM is installed on the motherboard and on some types of expansion boards. ROM chips contain the system BIOS, whose main function is to carry out boot operations by communicating with I/O devices and programs.

Dynamic Random Access Memory (DRAM)

DRAM is the most common type of memory in use today. It is considered affordable, and it is the memory most often used by modern CPUs. DRAM is volatile memory that will lose all its stored information if it is disconnected from the power source. A DRAM chip is made up of little storage units called cells. Each cell contains a capacitor. A capacitor is an electronic device that can hold an electrical charge that can be positive or negative. If the charge is positive, the capacitor registers a binary digit value of 1. If the charge is negative, the capacitor registers a binary digit value of 0. The capacitors in DRAM must be electronically refreshed continuously in order to hold their information. DRAM is considered a very slow type of memory, with speeds of approximately 50ns. Originally, DRAM chips were mounted on the motherboard using Dual Inline Packages (DIPs). DIPs are long chips with flimsy pins that are very difficult to install on the motherboard. DIPs tended to heat up quickly due to thermal cycling and often caused the DRAM chips to creep out of their sockets. This creeping effect is known as chip creep. Today, DRAM chips are soldered onto integrated circuit boards that are inserted into the motherboard more securely.

Static Random Access Memory

SRAM or static RAM is memory that also holds data as long as there is power available to the chip. Power is provided to the SRAM chip by the system battery. SRAM does not require the use of capacitors and does not need to be constantly refreshed, as does DRAM. Instead, SRAM uses a flip-flop method of regenerating its contents by means of transistors.

Through the use of its own internal clock, SRAM is synchronized with the motherboard’s bus speed, thereby helping SRAM achieve higher speeds than DRAM.

Level 2 memory cache (fast memory frequently accessed by the processor) is stored on SRAM chips. SRAM typically comes in sizes of 128MB to 4MB and is more expensive than DRAM.