PC Users Bible

Increasing the amount of memory in your computer is almost always worth the time and expense involved. A computer with enough memory to handle everything you want it to do can open and save files, refresh on-screen images, and respond to your commands more quickly than the same computer with a faster processor but not enough memory. Whether you do it yourself or pay a service shop to do it for you, you should start by learning how much memory is already installed in the computer, and what kind of memory modules your system uses.

How much RAM do you have?

To learn how much memory your computer has, follow these steps:

1. From the Windows desktop, right-click the My Computer icon.

2. Choose the Properties command from the pop-up menu. This command is usually at or near the bottom of the menu. The System Properties window appears.

3. If it's not already visible, click the General tab to open the display shown in Figure 7.1. The amount of memory installed in your computer appears near the bottom of the text on the right side of the window.

   Figure 7.1: The arrow points to the line indicating that this computer has 512MB of RAM.

What kind of memory?

As computer processors have become faster and more complex, so has RAM. Your computer's chipset is designed to work with one or more specific memory designs:

• FPM (Fast Page Mode): FPM DRAM was used with the earliest Intel x86 processors. It's often identified just as DRAM. FPM memory locates and reads each bit before it starts to look for the next one. FPM memory is not used in computers that can run Windows XP or other recent operating systems.

• EDO (Extended Data Out): EDO memory is faster than FPM because it starts looking for each bit as soon as it has found the previous one, without waiting for the memory controller to read the first bit.

• SDRAM (Synchronous DRAM): The speed of an SDRAM module should match the clock speed of the CPU and performs one operation per clock cycle.

• DDR SDRAM (Double Data Rate Synchronous DRAM): DDR SDRAM performs operations on the rising and falling of the clock cycle or clock edges. It doubles the data transfer rate between RAM and the CPU.

• DDR2 SDRAM (Double Data Rate 2 Synchronous DRAM): DDR2 is an improved version of DDR technology that operates at higher speed while consuming less power and producing less heat.

• RDRAM (Rambus DRAM): Rambus modules, sometimes called RIMMs, transfer data 16 bits at a time through a high-speed Direct Rambus Channel. In some systems, Rambus memory is significantly faster than SDRAM. Because this approach produces a great deal of heat, RIMMs usually have aluminum heat spreaders covering the printed circuit board.

Before you buy new memory for your computer, it's essential to identify the type you need. Table 7.2 lists the most commonly used packages for RAM modules.

Module types

Memory modules come in a confusing number of different physical sizes, speeds, and other characteristics. In order to make it impossible to install incompatible memory modules on a motherboard, the industry has established several different physical standards for modules and sockets. Each type has a specific number of electrical contacts on the socket and the module, a different size, and positioning slots in specific locations on the connectors. Figure 7.2 shows the relative sizes and shapes of several module types.

Figure 7.2: From top to bottom: 72-pin SIMM, 168-pin DIMM, 184-pin DIMM, 144-pin SODIMM, 200-pin SODIMM

Memory speed

The speed of a memory chip (or a module that contains several chips) is the minimum amount of time necessary for the memory to find and read each bit. The speed of older memory modules was rated in nanoseconds (ns), so a smaller number meant a faster memory. However, the speed of SDRAM modules is often listed in MHz (frequency, or millions of cycles per second), so faster SDRAM has a higher number.

SDRAM operates at the same speed as the CPU if it can, so it's important to use modules that are rated at the same or greater speed as your computer's clock. If you use memory that is rated for a faster speed than your computer can use, the actual access time is the system's internal clock speed. On the other hand, if you use memory that can't run as fast as the CPU, it can't synchronize itself with the processor, and the computer's overall performance suffers.

Unfortunately, each type of memory uses a different system to specify memory speed, so you must know which type of memory your computer uses before you choose the right speed for your system. Table 7.3 shows the speeds available for each type of memory module. In this table, access speed is the amount of time that it takes for a bit to move between RAM and the CPU, expressed in nanoseconds (ns); system bus speed is the maximum data transfer speed between the CPU and the L2 cache, expressed in millions of cycles per second, or megahertz (MHz); and peak bandwidth is the maximum amount of data that the CPU and RAM can exchange in one second, expressed in megabits per second (MB).

Latency and other complications

Along with the type of memory and the speed, you may often see some additional numbers included in the description of a memory module. For example, one module's specification looks like this:

DDR PC2700 CL=2.5 Unbuffered Non-ECC

A similar module from another company has these specifications:

PC2700 Latency: 2.5-3-3-7 Parity Unbuffered

Latency is the amount of time it takes for a memory module to respond to a command, measured in clock cycles. Some manufacturers show this value as CAS (Column Access Strobe) Latency, or CL.

Some memory specifications show latency as a series of three, four, or five numbers separated by dashes, like this: A-B-C, A-B-C-D or A-B-C-D-E. The B, C, and D values are based on the Row Access Strobe time, which is another element in the process of finding and reading a specific address in response to an instruction from the memory controller. In order, these values are:

• CAS Latency: The amount of time it takes for a memory module to respond to a command, measured in clock cycles.

• tRCD (RAS-to-CAS delay): The minimum number of clock cycles between the active command and the read/write command.

• tRP (RAS precharge time): The minimum number of clock cycles between the precharge command and the active command.

• tRAS (Row Active Time): The minimum number of clock cycles between activating a row and deactivating it.

• CMD rate: The minimum number of cycles after chip selection before a command can accept a command.

Normally, the BIOS sets these and other DRAM settings automatically during startup, but it's also possible to use the BIOS Setup Utility to change them.

Each of these values reflects part of a memory module's response time. To make sense of them, it's important to understand that each bit on a memory module has a physical location with a specific address, arranged in columns and rows. The memory controller uses a Column Access Strobe (CAS) and a Row Access Strobe (RAS) to find each bit. For most users, the important thing to know is that smaller latency values indicate faster memory performance.

Parity is an error-checking method that adds an extra bit to every 8-bit byte of data. In some systems, the parity bit is set to 1 when the number of binary 1s in the byte are even, and 0 when the number of 1s is odd. Other systems use the opposite values (the parity bit is 0 when the number of 1s is odd, and 0 when the number of 1s is even). When a memory module uses parity, the CPU counts the number of 1s in each byte and confirms that the parity bit is correct; if the parity bit is wrong, the CPU rejects the byte and instructs the memory module to resend it.

ECC (Error Correction Code) not only finds memory errors but fixes them as well. An ECC circuit attaches a series of ECC bits to the data bits and transmits them to the CPU along with the data. If one ECC bit is corrupted, the CPU corrects the error; if two or more bits are corrupted, it rejects the data.

Most desktop and laptop computers don't need memory with parity or ECC error checking, but they can improve performance in network servers and other high-volume systems. When you add memory to a computer, the new modules should have the same error checking and ECC specifications as the modules already in place.

Registered and buffered memory uses two or three register chips on each module to intercept and hold address and control signals. Registered memory is used in motherboards that have a relatively large number of slots for memory modules, such as the ones used for servers. Buffered memory works the same way, but it was used with older EDO and Fast Page Mode memory. On an unbuffered module, the signals go directly to the RAM chips. If the chipset on your computer's motherboard requires registered memory, it won't work with unbuffered modules; if it doesn't need them, the motherboard won't work with buffered modules. If you're not sure which type to use, consult the computer or motherboard manual, or the manufacturer's Web site.

Identifying your memory type

The Memtest86 program (you can download it from http://www.memtest86.com) is primarily a memory test, but it also displays the CPU type and clock speed, the sizes of the L1 and L2 caches, the amount of RAM installed in the computer, the chipset's make and model, RAM speed and type, and the memory's CAS values.

To run Memtest86, follow these steps:

1. Place a formatted diskette in your floppy disk drive. Run the install.bat program from the memtest86 folder. The program creates a boot floppy with the Memtest86 program on it.

2. Leave the floppy disk in the drive and restart your computer. The computer automatically loads Memtest86.

If your computer doesn't have a floppy disk drive, download the ISO image file from http://www.memtest86.com and follow the instructions supplied with the file to create a bootable Memtest86 CD.

Installing memory modules

Before you open up your computer, look in the computer or motherboard manual, or use one of the selector tools described earlier in this section to identify the type of memory modules that your computer uses. Either order one or more new modules directly from an online supplier, or buy them from a local retailer.

When you're ready to add more memory to a desktop computer, follow these steps:

1. If you have a manual for your computer's motherboard, find it and open to the diagram that shows the locations of components. If you don't have a manual, go to the manufacturer's Web site and download a copy. Print the page with the layout diagram.

2. Turn off your computer. Unplug the power cable.

3. Remove the cover and put it aside. If you have an anti-static grounding strap, put it on now and ground yourself to a metal part of the computer's case.

4. Examine the motherboard to locate the memory module sockets. At least one socket has a module mounted in it, as shown in Figure 7.3. If you can't find the memory sockets, refer to the diagram in the manual.

   Figure 7.3: The memory modules mount in sockets near the CPU. This motherboard has two modules in place.

5. To remove a module from its socket, move the plastic latch at each end away from the module itself. Figure 7.4 demonstrates.

   Figure 7.4: The plastic latches at each end of the memory sockets hold the modules firmly in place. Pull them away from the module to release a module from its socket.

6. If there are no empty sockets, remove all of the memory modules from their sockets. Handle the modules carefully, holding them by their edges. Read the labels to identify the module with the smallest amount of RAM. That's the one you replace. If the computer or motherboard manual instructs you to replace modules in pairs, find the pair of modules with the smallest amount of RAM on each one.

7. Line up the notches at the bottom of the first module and insert it into a socket. Push each end of the module toward the motherboard until the latch on that end closes. Repeat for each additional module.

8. Turn on the computer. If the memory modules are properly seated in their sockets, the computer starts up normally. If the computer beeps as soon as you turn it on, the BIOS is producing an error code, either because the modules are not in the sockets correctly, or you're using one or more incompatible memory modules.

9. When Windows has started, right-click the My Computer icon in the desktop. Choose Properties from the pop-up menu. The System Properties window should show the new amount of RAM (go back to Figure 7.1).

10. When you are convinced that the computer has recognized the new memory and is working properly, replace the cover.

It's somewhat more difficult to explain how to add memory to a laptop computer because each make and model has a different design. Some provide access from the bottom of the case, and others place the RAM modules underneath the keyboard. Fortunately, most laptop makers offer their own detailed instructions in the manuals supplied with their products and on their Web sites. Don't try to open up a laptop case without instructions.