PC memory may use the following access methods:
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. Asynchronous DRAM is available in the following types:
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.
EDO, also sometimes called Hyper Page Mode DRAM, is marginally faster than FPM, is still available in all common package types, and was commonly installed on new systems until late 1998. EDO DRAM now costs so much that it is often less expensive to replace the existing motherboard, processor, and memory with current products than to buy EDO memory. That said, you may be able to upgrade the memory in an EDO-based system economically. Many EDO-based systems ...