Each word of a memory has a numerical address, starting at 0. The term random access refers to the fact that any word can be retrieved or changed in the same amount of time, regardless of its address. This is different from tape drives where the first word could be accessed immediately if the tape were rewound, but the last word would take a long time. Both RAM and ROM memories share this random access property, so their names are sort of misleading. We will refer to the central RAM of a computer as its main memory.

Memory words are accessed by specifying the address and an operation. The address is stored in a special register called the MAR, or Memory Address Register. Another register, the MBR, or Memory Buffer Register, is used to hold the contents of the memory cell. The operation is either read or write, and another control wire called memory active is used to tell the memory if it is to read or write.