Read And Write Memory (RAM)

A memory unit is a collection of storage cells together with associated circuits needed to transfer information in and out of the device. Memory cells can be accessed for information transfer to or from any desired random location and hence the name random access memory, abbreviated RAM.

A memory unit stores binary information in groups of bits called words. A word in memory is an entity of bits that move in and out of storage as a unit. A memory word is a group of 1's and 0's and may represent a number, an instruction, one or more alphanumeric characters, or any other binary-coded information. A group of eight bits is called a byte. Most computer memories use words that are multiples of 8 bits in length. Thus, a 16-bit word contains two bytes, and a 32-bit word is made up of four bytes. The capacity of a memory unit is usually stated as the total number of bytes that it can store.

The communication between a memory and its environment is achieved through data input and output lines, address selection lines, and control lines that specify the direction of transfer .The n data input lines provide the information to be stored in memory and the n data output lines supply the information coming out of memory. The k address lines specify the particular word chosen among the many available. The two control inputs specify the direction of transfer desired: The write input causes binary data to be transferred into the memory, and the read input causes binary data to be transferred out of memory.

The memory unit is specified by the number of words it contains and the number of bits in each word. The address lines select one particular word. Each word in memory is assigned an identification number, called an address, starting from 0 and continuing with 1, 2, 3,upto 2k - 1, where k is the number of address lines. The selection of a specific word inside the memory is done by applying the k-bit binary address to the address lines.

Write and Read Operations

The two operations that a random-access memory can perform are the write and read operations. The write signal specifies a transfer-in operation and the read signal specifies a transfer-out operation. On accepting one of these control signals, the internal circuits inside the memory provide the desired function. The steps that must be taken for the purpose of transferring a new word to be stored into memory are as follows:

1. Transfer the binary address of the desired word to the address lines.

2. Transfer the data bits that must be stored in memory to the data input lines.

3. Activate the write input.

The memory unit will then take the bits from the input data lines and store them in the word specified by the address lines. The steps that must be taken for the purpose of transferring a stored word out of memory are as follows:

1. Transfer the binary address of the desired word to the address lines.

2. Activate the read input.

The memory unit will then take the bits from the word that has been selected by the address and apply them to the output data lines. The content of the selected word does not change after reading.

Types of Memories

Integrated-circuit RAM units are available in two possible operating modes, static and dynamic. The static RAM consists essentially of internal flip-flops that store the binary information. The stored information remains valid as long as power is applied to the unit. The dynamic RAM stores the binary information in the form of electric charges that are applied to capacitors. The capacitors are provided inside the chip by MOS transistors. The stored charge on the capacitors tends to discharge with time and the capacitors must be periodically recharged by refreshing the dynamic memory. Refreshing is done by cycling through the words every few milliseconds to restore the decaying charge. Dynamic RAM offers reduced power consumption and larger storage capacity in a single memory chip, but static RAM is easier to use and has shorter read and write cycles