PC Hardware in a Nutshell, Second Edition

Memory Access Methods

PC memory may use the following access methods:

Asynchronous DRAM

Asynchronous DRAM , which was used in all PCs until the late 1990s, uses a window of fixed minimum duration to determine when operations may occur. If the CPU has transferred data while a window is open, and if a subsequent clock cycle occurs while that window remains open, the CPU cannot transfer additional data until the next window opens, thereby wasting that clock cycle. Asynchronous operation forces the CPU to conform to a fixed schedule for transferring data, rather than doing so whenever it wishes.

All forms of asynchronous DRAM are now obsolete. Although asynchronous DRAM is still available, it costs so much per megabyte that it seldom makes sense to buy it. For example, in late 2001, SDRAM DIMMs sold for about $0.15 per megabyte while asynchronous DRAM SIMMs sold for between $1.50 and $12.00 per megabyte, depending on capacity and type. Asynchronous DRAM is available in the following types:

Fast Page Mode (FPM) DRAM

FPM was commonly used on 486 and earlier systems, and may be installed in early Pentium systems. FPM is not supported by recent chipsets. Although you can migrate FPM DRAM from an old Socket 5 or Socket 7 system to a newer Socket 7 system, it is good for little else. You may be able to install surplus FPM DRAM in your laser printer.

Extended Data Out (EDO) DRAM

EDO, also sometimes called Hyper Page Mode DRAM, is marginally faster than FPM, is still available in all common ...