What is a memory unit?

A collection of connected storage devices form the memory unit. It is a space to store data and instructions required for processing. Information is stored in a memory unit in the form of bits. There are two types of memory namely:

• Memory that is volatile: The data is erased when the system is turned off.
• Memory that is non-volatile: It is permanent storage where the data remains even after the system is turned off.

Memory hierarchy

• Register memory
• Cache memory
• Main memory
• Magnetic disk
• Magnetic tapes

The memory hierarchy system is from slow auxiliary memory to fast main memory and also has a smaller cache memory. The hierarchy has auxiliary memory at the bottom. The access time of the auxiliary memory is 1000 times the main memory that is why it is at the bottom of the hierarchy system. When the central processing unit (CPU) requires a program that resides in the main memory, the program is brought to the main memory from auxiliary memory. The currently running programs are stored in the main memory. The programs which are not currently running are sent to auxiliary memory to free the space in the main memory. The data and instructions associated with the currently executing program are stored in cache memory. The ratio of the access time of cache memory and the main memory is 1 to 7~10.

Register memory

Register memory is the smallest and fastest memory located within the CPU of the computer. It is the memory associated with the registers which are directly accessed by the CPU. Since it is not a part of the main memory, it is the fastest accessible memory. It is the smallest memory that holds the data and address associated with the instruction currently executing by the CPU.

Cache memory

The data which is frequently required by the CPU from the main memory is stored in cache memory. The data is stored in cache memory for easier access of data in a shorter time. While executing a process, the CPU first checks the cache memory for the required data block. If the data block is not present in the cache memory, the block is transferred to the cache memory from the main memory and then read by the CPU. The search and fetch time of data in the cache memory is less compared to other memories. In cache memory, the old data are deleted simultaneously as the new data keeps entering the cache memory.

Main memory

Computers have a CPU for processing data and auxiliary and cache memory for storing data. These components communicate through the main memory. Main memory is also known as the central storage unit of the system. Main memory is a quite large and fast memory that stores the data and instructions corresponding to the currently executing programs. Main memory consists of RAM (random access memory) and ROM (read-only memory).

RAM

A form of computer memory that is used to store the instructions and data that can be accessed in any sequence. It is a volatile memory that stores the data temporarily. The types of RAM are listed below;

DRAM: It is a dynamic RAM. It is made up of transistors and capacitors. It refreshes every 10 ~100ms. Dynamic RAM is so cheap and slow compared to SRAM.

SRAM: It is called static RAM. It retains data in its six transistor circuits in each cell. Data gets vanished if the power cuts off.

NVRAM: It is called a non-volatile RAM. The data does not vanish even though the power is turned off. For example, flash memory.

ROM

ROM means Read-Only Memory. ROM is a non-volatile memory. It stores the data permanently. The bootstrap loader program is loaded in from the ROM. Whenever the computer system turns on or resets, ROM starts the operating system. The three common types of ROM are listed below,

PROM-Programmable ROM

EPROM-Erasable programmable ROM

EEPROM-Electrically erasable programmable ROM

Auxiliary memory

Auxillary memory provides backup storage. Devices used for auxiliary memory are tapes and magnetic disks. A few more examples of auxiliary memory are optical disks, magnetic bubble memory, and magnetic drums. The CPU cannot access auxiliary memory directly therefore it is accessed through input or output channels.

Hit Ratio

The cache memory’s performance is considered in terms of a quantity called hit ratio. While accessing data from the cache memory, if the data is found then it is called a hit. If the data is not found in the cache memory then it is called a miss. Hit ratio is calculated using the formula below,

$\mathrm{Hit} \mathrm{ratio}=\frac{\mathrm{Hit}}{\left(\mathrm{Hit}+\mathrm{miss}\right)}$

Associative memory

Associative memory is called content addressable memory (CAM). Associative memory is a memory chip in which each bit position can be compared. The comparison of each bit position allows a fast glance through the table(data stored in table format). The data in the associative memory is stored without considering the addressing scheme as each position might be compared. It has a lower storage capacity compared to other memory chips.

Memory access methods

Memory must be accessed to find and locate a specific block of data. There are various methods to access the memory which are listed below,

Random Access: In this method, the data is accessed from the memory in random order which means that memory can be accessed in any order. Access to a memory location is independent of its address in the memory or any other factor.

Sequential Access: In this method, the memory is accessed in a linear order starting from the first memory location to the last memory location. To access a particular memory location, all the memory locations before the desired location need to be accessed.

Direct Access: In this method, the data is accessed using both random and sequential access. It is mainly used with hard disks where the memory is organized into tracks and sectors. Each track has a separate read and write head that can be accessed directly, and the data within the tracks are accessed sequentially.

Context and Applications

This topic is important for postgraduate and undergraduate courses, particularly for,

• Bachelors in computer science engineering.
• Associate of science in computer science.

Practice Problems

Question 1: _____ is an electronic storage space from which the data can be accessed and stored when it is required.

1. Disk
2. Memory
3. Drive
4. None of these

Answer: Option B is correct.

Explanation: Memory is an electronic storage space for storing and accessing data. Disk and drive are the storage devices. 

Question 2: Which of the following is the fastest accessible memory?

1. Virtual memory
2. Registers
3. Cache
4. None of these

Answer: Option B is correct.

Explanation: Registers are the fastest accessible memory. Registers are located within the processer and hold the data that CPU is processing.

Question 3: ______ memory is implemented with semiconductor chips.

1. Main
2. Registers
3. Cache
4. None of these

Answer: Option A is correct.

Explanation: The main memory is implemented with semiconductor chips because the main memory is present on the motherboard where the ram, ROM, and other circuits are built.

Question 4: Expand EPROM.

1. Erasable Programmable Read Only Memory
2. End Programmable Read Only Memory
3. Escape Programmable Read Only Memory
4. None of these

Answer: Option A is correct.

Explanation: Erasable programmable read-only memory is a type of ROM. It is a non-volatile memory.

Question 5: Whose memory size depends upon the size of the address bus?

1. Cache
2. Main
3. Secondary
4. None of these

Answer: Option B is correct.

Explanation: The main memory’s size depends upon the address bus size of the CPU. The main memory includes RAM and ROM.