What does 4x32 (etc.) mean?

Common 30Pin SIMMs are 1MB - 1x8 (non-parity), 4MB - 4x8 (non-parity) and 1MB 10 (parity), 1MB -1 x9 (parity), 4MB - 4x3 (parity), 4MB - 4x9 (parity). Common types of 72Pin SIMMs are 4MB - 1x32, 8MB - 2x32 (non-parity), 16MB - 4x32 (non-parity), and 4MB - 1x36 (parity), 8MB - 2x36 (parity), 16MB - 4x36 (parity). The "1x32" part is the SIMM specification, also referred to as 'architecture'. It indicates the SIMM design in terms of number of chips on the SIMM, and density of the chips. If you multiply the 2 numbers out, you get the total number of megabits and then divide by 8 (for non-parity SIMMs) or 9 (for parity SIMMs) to get the size in megabytes. They will also differ visually in having a notch in the middle of the connector edge.

Do I need Parity or Non-Parity?

The motherboard manual will usually specify the type of memory your system needs. The manual may refer to parity as 9bit (for systems that use 30Pin memory) or 36bit (for systems that use 72Pin memory). Non-parity modules are usually referred to as 8bit and 32bit respectively. If your manual is still unclear on this, you can count the number of chips on the modules that you presently have in your system. If the modules in your system have 2, 4, 8, or 16 chips, the modules are non-parity. If the modules have 3, 9, 12, or 24 chips, the module is parity.

What is a "parity" SIMM?

Parity vs. Non-parity: Most Macintoshes use non-parity SIMMs (x8, x32), and most PCs use parity SIMMs (x9, x36). However, the recent trend is toward using non-parity SIMMs on many Pentium boards (such as the Intel) because the chances of a memory error are so small, it might be years before the first error occurs. If there is an error with a non-parity SIMM, you won't know about it unless you spot it in your computer files, or output data, etc. When an error is detected in a parity SIMM, the computer grinds to a halt, losing whatever was on the screen at that time. Another type is ECC (error checking and correcting) used on high-end servers, and these will self-check and fix any errors.

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What is a 'logic parity' or 'logic generator' SIMM?

Important.- There is another type of parity SIMM known as 'logic parity', and these modules are often advertised and sold as "parity", but they are not "true parity". The parity generation tricks the system board into thinking they are parity SIMMs, without doing any error checking. They're inferior and incompatible with many system boards (especially name-brand units), and are sold as cheaper parity solutions. Other inexpensive SIMMs (off-brand makes) are turning up in the market, and many of these are not built to the industry standard spec. They may work okay on many common boards, but they have bee known to fail to work on the higher-quality," higherperformance boards, such as Compaq, HP, IBM, Gateway, etc.

How many modules do I need in order to upgrade my system's memory?

Memory is placed in computers in what is called memory banks. 486 CPU's address their memory in 32bit blocks. You need 32bits per memory bank. This usually translates into four 30Pin SIMMs or one 72Pin SIMM per bank. Pentium class systems address their memory in 64bit blocks. You need 64bits per bank. This means that 72Pin SIMMs must be installed in pairs (two 32bit 72Pin SIMMs). Parity Data is grouped together in 8bit chunks called bytes. Error detection methods add additional data bits in order to verify if the data byte has been transmitted correctly. With the error detection called parity, an additional data bit, called a parity bit, is generated and added onto each data byte. This parity bit may be set to a 1 or a 0 depending upon the number of 1 data bits found in the byte. If Even Parity is selected, the parity bit that is added will make the total number of 1bits in the byte equal an even number. If Odd Parity is selected, the parity bit that is added will make the total number of 1 bits in the byte equal an odd number.

Parity error detection can only detect single-bit errors that may occur during the transmission of data. If more than one bit has been altered, the parity check may not detect a problem. An example of this is shown with a data byte of 1001 0111, a parity bit of 1, using even parity. As the data is transmitted, it is altered so that the information is received as 0 110 0 111, with a parity bit of 1. In this example, even parity was used, and the parity bit of the resulting byte is still 1, not indicating any change in data, even though there was. It is apparent from this example, that this parity method does not work if two bits are in error, because two errors do not change the number of 1's in the data byte. However, the likelihood of a double error is so small that the parity bit method of error detection is considered adequate in today's computer systems.

The following table gives a "rule of thumb" for the minimum number of SIMMs required for different CPUs:

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What is a bank of memory?

Computers will usually take memory in "banks." Each bank carries the minimum number of slots to be filled required by the computer.

• A bank in a 486 machine would consist of either four 30Pin SIMMs, or one 72Pin SIMM.
• A bank in a Pentium TM or PowerPC system is two 72Pin SIMMs
• If a memory upgrade is performed, SIMMs must be added to fill a bank. Banks are normally labeled 0, 1, 2 etc.

Note: All SIMMs in each bank must be of the same capacity.

What do the beeps mean when my system boots, but nothing appears on the screen?

Errors can occur during the POST (Power On Self Test), which is performed every time the system is powered on. Fatal errors, which prevent the system from continuing the boot process, are communicated through a series of audible beeps. The following list shows the beep codes and corresponding error messages for Intel-manufactured motherboards that use the AMI BIOS:

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