Because the price for RAM has dropped so drastically this year, more and more companies are upgrading the memory in their existing systems. It makes sense when you compare the savings afforded by giving a system a performance upgrade with more memory to purchasing a new computer. However, there are many types of RAM out there, so before you run off and buy it, you need to be sure you get the correct type. Let’s review the different types of RAM available and some tips to help you correctly identify whether or not a system needs more memory and, if so, what type to install.

Basic RAM facts
The computer industry is chock-full of acronyms. Let’s review some of the acronyms related to computer memory that you might come across. Terms such as single inline memory module (SIMM) and dual inline memory module (DIMM) may be familiar to you, but there are others that are often misunderstood.

When manufacturers first invented memory chips, they were called dynamic random access memory, or DRAM. Similar DRAM packages were used on the memory modules in the personal computer until just a few years ago.

The printed circuit boards that are used to hold the DRAM chips are called sticks. Although sticks of RAM are commonly referred to as DIMMs or SIMMs, the actual reference is being made to the form factor, which defines the size, pin configuration, and other specifications of the module. Thus, when ordering memory to upgrade a computer, you would ask for a 256-MB stick of RAM, rather than a DIMM or SIMM.

Determining capacity
When manufacturers calculate the capacity of a RAM stick, they first determine the capacity of the DRAM chip in megabits. This can be confusing since a stick of RAM is normally measured in megabytes. However, some simple math will help eliminate confusion.

Think of DRAM chips as very large spreadsheets, each one made up of millions of cells that hold one bit of data. Determining the organization of the cells will allow you to calculate the capacity of the chip. For example, a chip described as 8 Mb x 8 is telling you the chip is eight million rows deep and eight columns wide. If you multiply the depth by the width, you come up with the density, or capacity, of the chip. In this example, the capacity of the chip is 64 Mb.

Once you know the capacity of one chip, you can calculate the capacity of the entire stick. If your stick of RAM has eight 64-Mb chips, the total capacity is 512 Mb. To convert this into megabytes, simply divide the total chip capacity by eight, which is the total number of bits that are in a byte. Thus, 512 Mb divided by eight bits equals 64 MB.

Types of RAM
Early PCs used DRAM memory. These chips were manufactured individually, and each one had to be installed on the motherboard by hand. In those days, the packaging used for DRAM was called the dual inline pin package, or DIPP. Each chip had two rows of pins that could be easily bent when inserted into the socket on the motherboard.

In an effort to make memory installation easier, computer manufacturers began to package DRAM chips on printed circuit boards, similar to what we are used to seeing today. The first generation of this type of DRAM packaging was called the single inline pin package, or SIPP. These 30-pin modules were a step in the right direction, but they still contained the same protruding pins as the DIPP.

The trend to simplify the upgrade process continued with the next generation of DRAM packaging called the single inline memory module, or SIMM. SIMMs were designed to use a special 30-pin edge connector socket, called a SIMM socket, on the motherboard. These modules were much easier to install and looked similar to the RAM installed on today’s computer systems.

When the Pentium processor was released, the motherboards were changed to include a 72-pin SIMM socket on the motherboard. In addition to having more pins or edge connectors, the new RAM sticks were also 32 bytes wide, which increased the RAM’s performance.

A later generation of 72-pin RAM was developed called extended data out, or EDO RAM, in an attempt to improve the speed at which memory operates. This type of memory helped reduce latency by allowing the DRAM chips to be refreshed less often. Although this did improve the performance of the system, it also caused some problems for those of us who work on computer hardware. For example, EDO and non-EDO RAM may look the same, but they cannot be mixed in the same computer system without causing memory address errors. If you have a computer that uses EDO RAM or if you have EDO SIMMs, be sure to label them to avoid any confusion.

The next and current generation of memory began using the much faster synchronous DRAM chips, or SDRAM. This type of memory is so named because it was tied, or synchronized, to the system clock. For example, if the CPU wants a piece of data that’s stored in memory, it sends a request to the SDRAM, waits for a certain number of clock ticks, and then retrieves the data when it has been located. Because SDRAM is tied to the system clock, the CPU will not waste time trying to access the data before it has been found. Also, to combat the bandwidth and latency issues inherent in memory access, SDRAM uses the “divide and conquer” theory by breaking up the RAM into two banks of memory. While one bank is servicing one request, the other bank will be receiving another request. This process is also synchronized to the system clock, with each request and reply occurring on the tick of the system clock.

SDRAM chips are packaged in dual inline memory modules, or DIMMs. This package uses a 168-pin socket and is 64 bytes wide. Because SDRAM is tied to the system clock, its speed is measured in MHz, just like the processor. You can find SDRAM sticks that work with 66-MHz, 100-MHz, 133-MHz, 166-MHz, 200-MHz, and 266-MHz bus speeds. Also, some of the higher speed SDRAM is backward-compatible with slower bus speeds.

The latest version of SDRAM to hit the market is double data rate SDRAM, or DDR SDRAM. It is somewhat similar to SDRAM in that both break the RAM into smaller chunks for simultaneous and synchronized request-and-reply access. However, DDR SDRAM performs the request and reply on both the rise and fall of the clock cycle, effectively doubling the bandwidth and increasing the speed at which the system can access data in memory.

DDR SDRAM modules use a 184-pin interface. Thus, to use DDR SDRAM, you must have a motherboard with a DDR SDRAM DIMM interface. DDR SDRAM is offered in two speeds. PC1600 is used with a 100-MHz memory bus, and PC2100 will work with a 133-MHz memory bus. Because DDR SDRAM effectively doubles the speed of memory access, the stated memory bus speeds will be doubled, making the 100-MHz bus seem like a 200-MHz bus and the 133-MHz bus seem like a 266-MHz bus.

Another type of memory that has not yet gained widespread use is called Direct Rambus DRAM, or DRDRAM. This new design is completely different from traditional SDRAM because it uses a high-speed, 16-bit bus that runs at a clock rate of 400 MHz and uses the Rambus inline memory module (RIMM) socket. Because memory access is performed on the rise and fall of the clock cycle, DRDRAM can theoretically run at speeds of over 1 GHz. Although the bus is only 16 bits wide, as opposed to the 64-bit bus used with SDRAM, the narrow channel allows the data to flow at a higher speed than if the bus were wider.

Determining whether or not a computer needs more RAM
The amount of RAM that is in a computer system contributes greatly to the overall performance of the machine. Insufficient memory can cause the system to run as much as 50 percent below its capability.

One of the telltale signs that a computer needs more memory is when the hard drive is accessed more often. When this occurs, the computer is reading and writing information to the hard disk because the RAM is full. Doing this is much slower than accessing the information in RAM and will definitely degrade the system’s performance.

More obvious signs are virtual memory errors. If a user begins to receive these types of errors, you should check the virtual memory configuration on the system. To do this in Windows 2000, go to the Advanced tab of the System Properties dialog box and click the Performance Options button, as shown in Figure A.

Figure A
When you open the Performance Options dialog box, you can view the current virtual memory settings and change them.

To make changes, click Change, as shown in Figure B.

Figure B

Figure C shows the Virtual Memory dialog box, which allows you to set the paging file size. These settings should be near the total amount of RAM currently in the computer.

Figure C
In the Paging File Size For Selected Drive section, enter a value and click Set to adjust the file size.

Things to think about before upgrading
If you decide that a computer needs a memory upgrade, the first thing you should reach for is the motherboard’s system book. If you don’t have the book for the system, you can probably find the information on the manufacturer’s Web site. The two things you are looking for are the type and total amount of memory the system can use.

After determining this information, take a moment to think about the future use of the PC. If the system can use PC 100 RAM even though the bus speed is 66 MHz, you might want to get the faster sticks; you will be able to use the same memory in another computer with a faster bus, which would allow you to reuse the RAM.

You should also consider the number of sockets on the motherboard. If the system has only a couple of RAM slots, you will be better off buying a larger stick of RAM, such as a 256-MB stick. Although adding a 64-MB or 128-MB stick of RAM may meet the user’s current needs, when you want to upgrade to, say 512-MB or more, you will be forced to buy two larger sticks. Also, you may be unable to use the smaller stick of RAM on that system unless it has an extra socket on the motherboard. Besides, with the current prices of memory, even a 256-MB stick of RAM should be able to fit into most IT budgets.

Upgrading PC memory isn’t too difficult. If you purchase the correct type of RAM for the system and consider the future use of the computer, a memory upgrade will improve the system performance and might take only a few minutes of your valuable work time.