To be useful, memory cells must be readable and writable. Within the RAM device, multiplexing and demultiplexing circuitry are used to select memory cells. Typically, a RAM device has a set of address lines <math>A_0, A_1,...A_n</math>, and for each combination of bits that may be applied to these lines, a set of memory cells are activated. Due to this addressing, RAM devices virtuallyalmost always have a memory capacity that is a power of two.
Usually several memory cells share the same address. For example, a 4 bit "wide" RAM chip has four memory cells for each address. Often, the width of the memory and that of the microprocessor are different,; for a 32-bit microprocessor, eight 4-bit RAM chips would be needed.
OftenOccasionally, more addresses are needed than can be provided by a device. In that case, external multiplexorsmultiplexers to the device are used to activate the correct device that is being accessed. RAM is often byte-addressable, although it is also possible to make RAM that is word-addressable.<ref>{{cite book | url=https://books.google.com/books?id=QGPHAl9GE-IC&dq=size+of+a+memory+address&pg=PA321 | isbn=978-0-7637-3769-6 | title=The Essentials of Computer Organization and Architecture | date=2006 | publisher=Jones & Bartlett Learning }}</ref><ref>{{cite book | url=https://books.google.com/books?id=-vQCEAAAQBAJ | title=Foundations of Computer Technology | isbn=978-1-000-15371-2 | last1=Anderson | first1=Alexander John | date=25 October 2020 | publisher=CRC Press }}</ref>
