Memory Terminologies

What is memory cell?

This is a device or electrical circuit used to store a single bit (0 or 1)

What is 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.

What is 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.

What is the memory capacity or unit of memory?

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 this 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 x 20. When expressed this way the 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 and terabyte. The relationship between these is shown below:

1 Byte             = 8 Bit

1 Kilobyte   = 1204 Bytes

1 Megabyte   = 1024 kilobytes (1048576 bytes)

1 Gigabyte   =  1024 megabyte (1048576 kilobytes)

1 Terabyte   = 1024 Gigabyte (1048576 megabyte)

What is memory address?

Memory address is a number that is assigned to each byte in a computer’s memory that the CPU uses to track where data and instructions are stored in RAM. Each byte is assigned a memory address whether or not it is being used to store data. The computer’s CPU uses the address bus to communicate which memory address it wants to access and the memory controller reads the address and then puts the data stored in that memory address back onto the address bus for the CPU to use.

What is read operation?

When reading from memory the CPU specifies an address in memory plus the number of consecutive bytes it wants usually 1, 2, 4, 8 bytes are the only possibilities starting at the address specified plus the fact that it wants to read from memory. The address is sent to memory via the address bus. The number of bytes can be sent in a signal through the control bus. Also the fact that the computer wants to do a read operation can be sent through the control bus. Once memory has received the signals it retrieves the data from memory and outputs the data to the data bus and alerts the CPU that the quantity is now available at which point the CPU will then read the data off the data bus and store the information internally in a register. 

What is write operation?

For a write operation the CPU specifies an address in memory plus the number of consecutive bytes it wants to store usually 1, 2, 4, 8 bytes are the only possibilities starting at the address specified plus the fact that it wants to write from memory plus the data it wants to write to memory. The address is sent to memory via the address bus. The number of bytes can be sent in a signal through the control bus. The CPU also informs memory it wants to do a write operation through the control bus. Finally it places the data from the CPU onto the data bus usually from a register. Once memory has received the signals it reads the data off the data bus and then stores the data at the address specified.

What is memory access time?

The average period of time in nanoseconds, it takes for RAM to complete one access and begin another. Access time is composed of latency the time it takes to initiate a request for data and prepare to access it and transfer times. DRAM chips for personal computers have accessing times of 50 to 150 nanoseconds (billionths of a second). Static RAM (SRAM) has access times as low as 10 nanoseconds. Ideally the accessing times of memory should be fast enough to keep up with the CPU. If not the CPU will waste a certain number of clock cycles which makes it slower. A nanosecond (ns or nsec) is = 10-9 one billionth of a second.

What is memory cycle time?

Memory cycle time is a measurement of how quickly two back-to-back accesses of a memory chip can be made. A DRAM chip's cycle time is usually much longer than its access time which measures only a single access. This is because there is latency between successive memory accesses. Cycle time is the time usually measured in nanosecond s between the start of one random access memory (RAM) access to the time when the next access can be started. Cycle time consists of latency the overhead of finding the right place for the memory access and preparing to access it and transfer time. Cycle time should not be confused with processor clock cycles or clock speeds which have to do with the number of cycles per second in megahertz or MHz to which a processor is paced.

What is the difference between random access memory and sequential access memory?

Random Access Memory

This memory is used for temporary storage. This is volatile memory all the data vanishes from the RAM as the computer off. Random access means that because each memory location has an individual the computer can go directly to the instruction and data it wants using that address rather than searching each individual location one after another. RAM memory is both readable and writeable meaning that the contents of any memory location can be changed and read at any time. RAM is volatile meaning that is a computer loses power the contents of its memory are lost. Therefore it is important to frequently save any valuable work to secondary disk storage. RAM is farther divided into: SRAM Static Random Access Memory, DRAM Dynamic Random Access Memory. Random access memory (RAM) is what is most often referred to when PC memory is discussed. RAM is the form of volatile memory used to hold temporary instructions and data for manipulation while the system is running. The term random is applied because the CPU can access or place data to and from any addressable RAM on the system. If power to the system is lost, all RAM is lost as well. Usually, when referring to RAM, we are speaking of some variation of DRAM (Dynamic RAM) or the newer SDRAM (Synchronous DRAM). These are the most common forms of RAM used in the modern PC.

Sequential Access Memory

Sequential Access Memory (SAM) is a class of data storage devices that read their data in sequence. This is in contrast to Random Access Memory (RAM) where data can be accessed in any order. Sequential access devices are usually a form of magnetic memory. While sequential access memory is read in sequence accesses can still be made to arbitrary locations by seeking to the requested location. This operation however is often relatively inefficient. Magnetic sequential access memory is typically used for secondary storage in general-purpose computers due to their higher density at lower cost compared to RAM as well as resistance to wear and non-volatility. Examples of SAM devices still in use include hard disks, CD-ROMs and magnetic tapes.

What are difference between Static memory devices and dynamic memory devices?

Static Memory Device

These are those semiconductor memory devices in which the stored data will remain permanently stored as long power is supplied without the need for periodically rewriting the data into memory.

Dynamic Memory Device

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.

What is the memory test?

A computer memory test evaluates your computer’s memory to see if it is working correctly. The computer memory is the part of the computer in which files and programs are stored and retrieved for the user’s use. Therefore, it is important to be sure that the computer's memory is working correctly. A computer memory test should be done if a user suspects that there is a problem with their computer’s memory. It should also be run when a user purchases new memory.