Double-sided RAM is a type of random-access memory which has its chips divided into two sides (called "ranks"), 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 rank, and can no longer read or write to the first half until it switches back again. Single-sided RAM refers to a RAM expansion with a single "rank" of chips, which the computer can access all at once. The physical expansion card may have chips on both of its sides, or only on one side, but is considered to be single sided because the computer can "see" all of its memory at once.