Semiconductor Memory - Description

Description

In a semiconductor memory chip, each bit of binary data is stored in a tiny circuit called a memory cell consisting of one to several transistors. The memory cells are laid out in rectangular arrays on the surface of the chip. The 1-bit memory cells are grouped in small units called words which are accessed together as a single memory address. Memory is manufactured in word length that is usually a power of two, typically N=1, 2, 4 or 8 bits.

Data is accessed by means of a binary memory address applied to the memory chip. If the memory address consists of M bits, the number of addresses on the chip is 2M, each containing an N bit word. Consequently, the amount of data stored in each chip is N2M bits. Possible figures are 1, 2, 4, 8, 16, 32, 64, 128, 256 and 512 kilobits, megabits, gigabits and terabits, here defined by binary prefixes. Currently (2012) the largest semiconductor memory chips hold a few gigabits of data, but higher capacity memory is constantly being developed. By combining several integrated circuits, memory can be arranged into a larger word length and/or address space than what is offered by each chip, often but not necessarily a power of two.

The two basic operations performed by a memory chip are "read", in which the data contents of a memory word is read out, and "write" in which data is stored in a memory word, replacing any data that was previously stored there.

In addition to standalone memory chips, blocks of semiconductor memory are integral parts of many computer and data processing integrated circuits. For example the microprocessor chips that run computers contain cache memory to store instructions awaiting execution.