Miscellaneous Additional Factors

Particularly with older memory types, there are additional factors that can influence the reliability of specific memory modules in specific motherboards. These factors include

• Jumper settings Early 80486 and earlier computers often used jumpers on the motherboard to identify the amount of RAM installed. Later 80486 and later computers gener- E ally auto-detect the amount and, where applicable, type of memory. If you have an older O motherboard, you might need to adjust jumper settings when you add or remove RAM. Y

• Number of chips In the mid-1990s, there was a great deal of discussion about 2- or 3-chip 30-pin SIMMs as opposed to 8- or 9-chip 30-pin SIMMs. The 2- or 3-chip SIMMs used higher-capacity chips than did their 8- or 9-chip counterparts of the same capacity, and this difference resulted in some subtle electronic differences. The 2- and 8-chip SIMMs lacked parity, whereas the 3- and 9-chip variants were parity SIMMs. In general, it's best to keep the number of chips on each SIMM the same, particularly within a single bank, on motherboards of this era. More recent motherboards that use 72-pin SIMMs and 168-pin DIMMs are less troubled by such effects.

• Sidedness Memory modules can have chips mounted on one or both sides. This characteristic is correlated with electronic differences, and some motherboards have specific requirements for single- or double-sided SIMMs or DIMMs. These requirements are usually documented in the motherboard manual, so you should check there for further information.

• Parity If you plan to expand your memory, try to use parity memory if your system currently uses it, or non-parity, if not. Most computers report whether the memory is parity or non-parity in a brief display during bootup.

You can see the BIOS's hardware information display, including a summary of the memory type, by placing a blank formatted but not bootable floppy disk in the floppy disk drive and rebooting the computer. Be sure the BIOS is set to boot from the floppy disk before doing this, or the computer might try the hard disk first and boot Linux. Normally, when the BIOS encounters an unbootable floppy, it prints a short message on the screen and then stops, leaving the hardware information on display for you to read at your leisure. (The unbootable floppy message is actually contained on the floppy disk itself, and so varies depending upon how you formatted the floppy disk.)

• Memory holes Some motherboard BIOSes can be set to create or not create memory holes at particular locations, such as 16MB or 64MB. These memory holes are discontinuities in the memory map—small areas that aren't usable as RAM. Linux generally works with or without the memory holes, but if you experience problems in which Linux doesn't recognize all your RAM, you might want to check and change such settings in your BIOS.

• Kernel parameters Linux kernels prior to the 2.2.x series couldn't automatically detect the amount of memory past 64MB, so if you have an older kernel and a lot of memory, you might need to tell Linux how much memory you've got. You do this by adding a line like the following to your /etc/lilo.conf file and then rerunning lilo to reinstall the Linux boot loader:

append="mem=nnnM"

The nnn value is the number of megabytes, as in 128 for 128MB of RAM. In some cases, it's necessary to use this command even with 2.2.x kernels, although these kernels can usually detect the memory correctly.

As a general rule, you should read your motherboard's manual to learn about any further peculiarities it might have with respect to memory. A few boards require peculiar RAM variants, and you should be careful to track down appropriate RAM for such computers.

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