Memory Basics

Memory is the workspace for the computer's processor. It is a temporary storage area where the programs and data being operated on by the processor must reside. Memory storage is considered temporary because the data and programs remain there only as long as the computer has electrical power or is not reset.

Before being shut down or reset, any data that has been changed should be saved to a more permanent storage device (usually a hard disk) so it can be reloaded into memory in the future.

Memory often is called RAM, for random access memory. Main memory is called RAM because you can randomly (as opposed to sequentially) access any location in memory. This designation is somewhat misleading and often misinterpreted.

Read-only memory (ROM), for example, is also randomly accessible, yet is usually differentiated from the system RAM because it maintains data without power and can't normally be written to. Disk memory is also randomly accessible, but we don't consider that RAM either.

Over the years, the definition of RAM has changed from a simple acronym to become something that means the primary memory workspace the processor uses to run programs, which usually is constructed of a type of chip called dynamic RAM (DRAM).

One of the characteristics of DRAM chips (and therefore RAM in general) is that they store data dynamically, which really has two meanings. One meaning is that the information can be written to RAM repeatedly at any time.

The other has to do with the fact that DRAM requires the data to be refreshed (essentially rewritten) every 15ms (milliseconds) or so. A type of RAM called static RAM (SRAM) does not require the periodic refreshing. An important characteristic of RAM in general is that data is stored only as long as the memory has electrical power.

When we talk about a computer's memory, we usually mean the RAM or physical memory in the system, which is mainly the memory chips or modules the processor uses to store primary active programs and data.

This often is confused with the term storage, which should be used when referring to things such as disk and tape drives (although they can be used as a form of RAM called virtual memory).

RAM can refer to both the physical chips that make up the memory in the system and the logical mapping and layout of that memory. Logical mapping and layout refer to how the memory addresses are mapped to actual chips and what address locations contain which types of system information.

People new to computers often confuse main memory (RAM) with disk storage because both have capacities that are expressed in similar megabyte or gigabyte terms. The best analogy to explain the relationship between memory and disk storage I've found is to think of an office with a desk and a file cabinet.

In this popular analogy, the file cabinet represents the system's hard disk, where both programs and data are stored for long-term safekeeping. The desk represents the system's main memory, which allows the person working at the desk (acting as the processor) direct access to any files placed on it.

Files represent the programs and documents you can "load" into the memory. To work on a particular file, it must first be retrieved from the cabinet and placed on the desk. If the desk is large enough, you might be able to have several files open on it at one time; likewise, if your system has more memory, you can run more or larger programs and work on more or larger documents.

Adding hard disk space to a system is similar to putting a bigger file cabinet in the office—more files can be permanently stored. And adding more memory to a system is like getting a bigger desk—you can work on more programs and data at the same time.

One difference between this analogy and the way things really work in a computer is that when a file is loaded into memory, it is a copy of the file that is actually loaded; the original still resides on the hard disk.

Because of the temporary nature of memory, any files that have been changed after being loaded into memory must then be saved back to the hard disk before the system is powered off (which erases the memory). If the changed file in memory is not saved, the original copy of the file on the hard disk remains unaltered.

This is like saying that any changes made to files left on the desktop are discarded when the office is closed, although the original files are still preserved in the cabinet. Memory temporarily stores programs when they are running, along with the data being used by those programs.

RAM chips are sometimes termed volatile storage because when you turn off your computer or an electrical outage occurs, whatever is stored in RAM is lost unless you saved it to your hard drive. Because of the volatile nature of RAM, many computer users make it a habit to save their work frequently. (Some software applications can do timed backups automatically.)

Launching a computer program brings files into RAM, and as long as they are running, computer programs reside in RAM. The CPU executes programmed instructions in RAM and also stores results in RAM. RAM stores your keystrokes when you use a word processor and also stores numbers used in calculations. Telling a program to save your data instructs the program to store RAM contents on your hard drive as a file.

Physically, the main memory in a system is a collection of chips or modules containing chips that are usually plugged into the motherboard. These chips or modules vary in their electrical and physical designs and must be compatible with the system into which they are being installed to function properly.

Next to the processor and motherboard, memory can be one of the more expensive components in a modern PC, although the total amount spent on memory for a typical system has declined over the past few years.

Before the big memory price crash in mid-1996, memory had maintained a fairly consistent price for many years of about $40 per megabyte. A typical configuration back then of 16MB cost more than $600. In fact, memory was so expensive at that time that it was worth more than its weight in gold.

These high prices caught the attention of criminals and memory module manufacturers were robbed at gunpoint in several large heists. These robberies were partially induced by the fact that memory was so valuable, the demand was high, and stolen chips or modules were virtually impossible to trace.

After the rash of armed robberies and other thefts, memory module manufacturers began posting armed guards and implementing beefed-up security procedures. By the end of 1996, memory prices had cooled considerably to about $4 per megabyte—a tenfold price drop in less than a year.

Prices continued to fall after the major crash until they were at or below 50 cents per megabyte in 1997. All seemed well, until events in 1998 conspired to create a spike in memory prices, increasing them by four times their previous levels.

The main culprit was Intel, who had driven the industry to support a then-new type of memory called Rambus DRAM (RDRAM) and then failed to deliver the supporting chipsets on time.

The industry was caught in a bind by shifting production to a type of memory for which there were no chipsets or motherboards to plug into, which then created a shortage of the existing (and popular) SDRAM memory. An earthquake in Taiwan during that year served as the icing on the cake, disrupting production and furthering the spike in prices.

Since then, things have cooled considerably, and memory prices have dropped to all-time lows, with actual prices of under 20 cents per megabyte.

In particular, 2001 was a disastrous year in the semiconductor industry, prompted by the dot-com crash as well as worldwide events, and sales dropped well below that of previous years. This conspired to bring memory prices down further than they had ever been and even forced some companies to merge or go out of business.

Memory is less expensive now than ever, but its useful life has become much shorter. New types of memory are being adopted more quickly than before, and any new systems you purchase now most likely will not accept the same memory as your existing ones.

In an upgrade or a repair situation, that means you often have to change the memory if you change the motherboard. The chance that you can reuse the memory in an existing motherboard when upgrading to a new one is slim.

Because of this, you should understand all the various types of memory on the market today, so you can best determine which types are required by which systems, and thus more easily plan for future upgrades and repairs.

To better understand physical memory in a system, you should see where and how it fits into the system. Three main types of physical memory are used in modern PCs:

  • ROM. Read-only memory

  • DRAM. Dynamic random access memory

  • SRAM. Static RAM


Read-only memory, or ROM, is a type of memory that can permanently or semipermanently store data. It is called read-only because it is either impossible or difficult to write to. ROM also is often referred to as nonvolatile memory because any data stored in ROM remains there, even if the power is turned off.

As such, ROM is an ideal place to put the PC's startup instructions—that is, the software that boots the system. Note that ROM and RAM are not opposites, as some people seem to believe. Both are simply types of memory. In fact, ROM could be classified as technically a subset of the system's RAM.

In other words, a portion of the system's random access memory address space is mapped into one or more ROM chips. This is necessary to contain the software that enables the PC to boot up; otherwise, the processor would have no program in memory to execute when it was powered on.