Memory upgrades are perhaps the simplest to perform, but they can be very confusing without advance planning. Purchasing the right memory for the job is more than half the process of the upgrade. Before installing memory, there are five things to consider:
- Memory chip format
- Memory speed
- EDO RAM
- Cache memory
The best source of information-which should be checked before obtaining memory-is the documentation that comes with the computer's motherboard. This source will generally list the type of memory required, how many SIMMs are required, and their location on the motherboard. If this information is not available, open the case and look. Some documentation provides a chart that includes exactly what memory has been installed and what is needed to upgrade to a given level. The following table gives you an idea of the kind of chart you might come across.
|On-board||Bank 0||Bank 1||Total|
|8 MB||8 MB|
|8 MB||4 MB||4 MB||16 MB|
|8 MB||8 MB||8 MB||24 MB|
|8 MB||16 MB||16 MB||40 MB|
|8 MB||32 MB||32 MB||72 MB|
|Disabled||64 MB||64 MB||128 MB|
You can add memory with SIMMs (single inline memory modules) or DIMMs (dual inline memory modules).
SIMMs are provided in two basic, physical formats: a 30-pin and a 72-pin chip. Format is the first consideration, because the chips must fit into the motherboard. This configuration, along with the size of the processor, determines how many SIMMs are required to fill one bank.
The 30-pin formats contain memory in 8-bit chunks. This means that a 32-bit processor requires four SIMMs to fill one bank. Typical 32-bit processors consist of two banks of SIMMs and therefore eight slots. (See Figure 14.2.)
Figure 14.2 30-pin SIMM
A 72-pin format is larger and supplies memory in 32-bit chunks. Only one SIMM is required for a 32-bit machine. A Pentium processor has a 64-bit data path and requires a 72-pin SIMM. (See Figure 14.3.)
Figure 14.3 72-pin SIMM
Memory is normally sold in multiples of 8 MB. However, some machines will have 8 MB of "on-board" memory (usually soldered in place on the motherboard). When memory is soldered in place, it cannot be changed but this should not be considered a problem; it can be disabled. A computer equipped with this on-board memory can provide 8 MB of memory to the system without having any SIMMs installed in the slots. For such computers, installing 16 MB of RAM too would yield a total of 24 MB of RAM; if 64 MB were to be added, the total RAM would be 72 MB, and so on. In general, the idea of hardwiring memory on a system has died out on desktop PCs.
DIMMs are much easier than SIMMS to install or remove, because they require only one card, which is simply pushed into a module slot. The "key" cut into the edge that goes into the slot prevents the card from being inserted the wrong way. The one problem you face is choosing from the wide variety of memory types available. When ordering a new DIMM, you must know exactly the memory type supported by the system. DIMMs are found in larger memory sizes than SIMMs, ranging to 256MB and beyond for single cards.
Memory speed is the amount of time required to access data and is measured in nanoseconds (ns); each nanosecond equals one billionth of a second. Two important considerations arise when addressing memory speed:
- The lower the number, the faster the chip speed
- All chips in the same computer should run at the same speed.
Typical chip speeds are 50, 60, 70, and 80 nanoseconds. Be sure to check the motherboard documentation or the existing chips to determine the correct speed to use.
The EDO RAM (extended data out read only memory) chip is used extensively with Pentium processors. This chip can improve read times and overall performance by up to 30 percent. This performance gain is possible because the chip continues to output data from one address while setting up a new address.
Parity is used to check the reliability of data. It requires one additional bit (chip). Memory can be purchased with or without parity. With parity, it will cost about 10 percent more. Be sure to check the machine specifications or the existing chips to determine if parity is required. Parity and nonparity chips cannot be mixed; however, some computers allow parity to be turned on or off (BIOS setup).
Cache memory can be found as either L1 or L2. The L1 cache is built into the processor and cannot be changed. The L2 cache, on the other hand, can be either built into the processor, or built onto the motherboard, sometimes both. In most cases, cache memory is fixed, but some machines allow L2 cache to be upgraded or expanded. Cache memory is sometimes found on older motherboard (as DIPPs-dual in-line packages). Check the motherboard documentation to determine what, if any, upgrades can be made to the cache.
You need to take special care when installing DIPP chips. They are sensitive to ESD, can easily be installed backwards (look for pin 1 alignment), and the pins can be broken or bent during insertion.
Installing RAM is a simple process. The only problem is that the slots are not always easily accessible. Sometimes you will need to relocate wires temporarily or even remove expansion cards. This simple procedure usually works:
- Turn off the computer.
- Disconnect all external devices (AC power and monitor power).
- Follow the appropriate ESD safety procedures.
- Remove the cover of the computer.
- Locate the SIMM banks and determine that you have the correct size, speed, and quantity of SIMMs.
- Insert the SIMMs in the slot at a 45 degree angle (backwards) and then snap it into the upright position, as shown in Figure 14.4. Be sure that the "notch" in the SIMM matches the slot. If it doesn't fit easily, it is probably installed incorrectly!
Figure 14.4 Installing a SIMM
- When the SIMM is in an upright position, be sure that the metal retaining clip snaps into position. This clip holds the SIMM in place and must be opened before any SIMM can be removed.
- Replace any temporarily removed or relocated wires or expansion cards. Check others to make sure they have not been loosened or disconnected.
- Replace the cover of the computer.
- Reconnect the power, monitor, and any other needed external devices, and start the computer.
The computer should recognize the new memory and either make the correction or automatically go to the setup program. In many cases, you need only exit setup to save the changes.