Memory Terminology and its General Operations
One of the major advantages that digital systems have over analog systems is the ability to easily store large quantities of digital information and data for short of long periods of time. This memory capability is what makes digital computer so versatile and adaptable to many situations. For example, in a digital computer the internal main memory stores instructions that tell the computer what to do under all the possible circumstances so that the computer will do its job with a minimum amount of human intervention. Magnetic tape and magnetic disk are popular mass storage devices that are much less and expensive in cost per bit that internal memory devices.
Memory Terminology
Memory Cell: This is a device or electrical circuit used to store a single bit (0 or 1)
Memory Word: This is a group of bits (cells) in a memory that represents information or data of some type. Word size in modern computers typically ranges from 4 to 64 bits, depending on the size of the computer.
Byte: This is a special term used for a 8-bit word. In modern digital computers, the word size is usually a multiple of 8-bits such as 2 bytes or 4 bytes.
Memory Capacity: This is a way of specifying how many bits can be stored in a particular memory device or complete memory system. To illustrate, suppose that we have a memory device, which can store 4096 20-bits words. This represents a total capacity of 81,920 bits. We could also express this memory’s capacity as 4096×20. When expressed this way, this first number (4096) is the number of words and the second number (20) is the number of bits per word (word size).
Computer memory is measured in terms of bytes. The larger units are Kilobyte, Megabyte, Gigabyte, Terabyte, Petabyte, Exabyte, Zettabyte and Yottabyte.
1000 bytes = 1 Kilobyte
1000 Kilobyte = 1 Megabyte
1000 Megabyte = 1 Gigabyte
1000 Gigabyte = 1 Terabyte
1000 Terabyte = 1 Petabyte
1000 Petabyte = 1 Exabyte
1000 Exabyte = 1 Yottabyte
Address: This is a number that identifies the location of word in memory. Each word stored in a memory device or system has unique address. Addresses are always specified as a binary number, although octal, hexadecimal and decimal numbers are often used for convenience. Below is the image illustrates a small memory consisting of eight words. Each of these eight words has specific address represented as 3-bit number ranging from 000 to 111. Whenever, we refer to a specific word location in memory, we use its address code to identify it.
Read Operation: This is the operation whereby the binary word stored in a specific memory location (address) is sensed and then transferred to another location. For example, if we want to use ‘word 4′ of the memory above image for some purpose, we have to perform a read operation on address 100. The read operation is often called a fetch operation, since a word is being fetched from memory. We use both terms interchangeable.
Write Operation: This is the operation whereby a new word is placed into a particular memory location. It is also referred to as a store operation. Whenever a new word is written into a memory location, it replaces the word that was previously stored there. The old is lost in the process of writing into this memory location.
Access Time: This is measure of a memory device’s operating speed. It is the amount of time required to perform a read operation. More specifically, it is the time between the memory receiving a read command signal and the data becoming available at the memory output.
Cycle Time: It is another measure of a memory device’s speed. It is the amount of time required for the memory to perform a read or write operation and then return to its original state ready for the next operation. Cycle time is normally longer than access time.
Random Access Memory (RAM): This refers to memories in which the actual physical location of a memory word has no effect on how long it takes to read from or write into that location. In other words, the access time is the same for any address in memory. Most semiconductor memories and magnetic disk memories are random access memories.
Sequential Access Memory It is a type of memory in which the access time is not constant but varies depending on the address location. A particular sorted word is found by sequencing through all address locations until the desired address is reached. This produces access time, which are much longer than those of random access memories. Example of sequential access memory devices includes magnetic tapes.
Read/Write Memory (RWM): It is any memory that can be read from or written into with equal ease.
Read-Only Memory (ROM:) It refers to a broad class of semiconductor memories designed for applications where the ratio of read operation to write operations is very high. Technically, a ROM can be written into (programmed) only once and this operation normally performed at the factory. Thereafter information can only be read from the memory. Other types of ROM are actually read-mostly memories more complicated than the read operation. Therefore the write operation is not performed very often.
Static Memory Devices: These are those semiconductor memory devices in which the stored data will remain permanently stored as long as power is supplied, without the need for periodically rewriting the data into memory.
Dynamic Memory Devices: These are those semiconductor memory devices in which the stored data will not remain permanently stored, even with power applied, unless the data is periodically rewritten into memory. This operation is called a refresh operation.
General Memory Operations
Although each type of memory is different in its internal operation, there are certain basic operating principles that are the same for all memory systems. Every memory system requires several different types of input and output lines to perform the following functions.
- Select the address in memory that is being accessed for a read or write operation.
- Select either a read or write operation to be performed.
- Supply the input data to be stored in memory during a write operation.
- Hold the output data coming from memory during read operation.
- Enable (or disable) the memory so that it will (or will not) respond to the address inputs and read/write commad.