"Double sided" RAM has its chips divided into two sides (called "banks"), only one of which can be seen at a time by the computer. Initially, these were created by essentially attaching two single-sided SIMM cards to the same PCB, but more modern chips use different wiring. Pins 33 and 45 on the board are used by double-sided memory, and can sometimes be an indicator as to whether a given module is single or double-sided. To use the second half of the storage available, the computer must switch to the second bank, and can no longer read or write to the first half until it switches back again.
Higher-end computer systems may wish to avoid using double-sided memory because of the reduced performance that results from the time needed to switch banks, however it is not a noticeable decrease for the majority of personal uses.
lol ctrl-c + ctrl-v off wikipedia....
