Introduction to memory and memory units

Memory is the storage space in a computer where data and instructions are kept. It is divided into small units called cells, and each cell has a unique address.

• Storage element (Cell): Stores 1 bit of data.
• Register: A memory location made of cells.
• Capacity: Total number of bits a memory can store.
• Reading: Retrieving data.
• Writing: Storing data.

Memory is essential because it allows computers to store both temporary data (used while processing) and permanent data (like system programs).

• A word is a group of bits where a memory unit stores binary information. A word with a group of 8 bits is called a byte. 
• A memory unit consists of data lines, address selection lines, and control lines that specify the direction of transfer.

The block diagram of a memory unit is shown below: 

• Data lines provide the information to be stored in memory. The control inputs specify the direct transfer. The k-address lines specify the word chosen. 
• When there are k address lines, 2^k memory words can be accessed. 

Types of Computer Memory

1. Cache Memory: Very fast storage, close to CPU, stores frequently used data.
2. RAM (Random Access Memory): Volatile, read/write, temporary data storage on motherboard.

• DRAM: Uses capacitors, slower, needs refreshing.
• SRAM: Uses flip-flops, faster, retains data as long as power is on.

3. ROM (Read Only Memory): Non-volatile, stores permanent system instructions, read-only.

• MROM: Pre-programmed, low-cost.
• PROM: User programmable once.
• EPROM: Can be erased using UV light and reprogrammed.
• EEPROM: Electrically erasable and reprogrammable, faster, reusable up to ~10,000 times.

4. Virtual Memory: Uses secondary storage as an extension of RAM.
5. Flash Memory: Non-volatile, fast, used in SSDs, USBs, memory cards.
6. Hybrid Memory: Combines RAM and Flash for efficiency and reduced power use.

What is Primary Memory?

It is also referred to as main memory or internal memory. It is a computer system's temporary storage component which is directly accessible by the central processing unit (CPU). It houses data for immediate processing.

Characteristics

• volatile: Data is lost upon power loss.
• High-speed access.
• Limited capacity relative to secondary storage.
• Examples: Random Access Memory(RAM), Read-Only Memory, Cache memory.

Advantages

• High-speed access: Data can be retrieved and stored very quickly.
• Directly accessible by CPU: No intermediate steps are required for data transfer.

Disadvantages

• Volatile: Data is lost when power is turned off.
• Limited storage capacity: compared to secondary storage, primary memory is relatively small.
• Expensive: Cost per unit is higher than secondary storage.

What is Secondary Memory?

Secondary memory or external memory serves as long-term storage for data and programs. Unlike primary memory, it is not directly accessible by the CPU and requires input/output operations.

Characteristics

• Non-volatile: Data persists even when the system is powered off.
• slower access speeds compared to primary memory.
• High storage capacity.
• Examples: Hard Disk Drives(HDD), Solid-State Drives(SSD), Optical drives(CD, DVD, Blu-ray).

Advantages

• Non-volatile: Data persists even when the power is turned off.
• Large storage capacity: can store vast amount of data.
• Relatively Inexpensive: cost-effective for storing large volumes of data.

Disadvantages

• Slower access time: Data retrieval is slower compared to primary memory.
• Requires input/output operations: Data transfer involves additional steps.

Read more about - Difference between Primary and Secondary Memory

Functions of Memory Unit

The memory unit of a computer has several functions:

1. Data Storage: Store temporary data (RAM) and permanent data (ROM).
2. Quick Access: CPU retrieves stored data and instructions for fast processing.
3. Data Transfer: Move data between CPU, RAM, and storage devices.
4. Program Execution: Provide instructions and space for active processes.
5. Reliability: Non-volatile memory (ROM/EEPROM) keeps data safe even without power.

The size of the memory unit affects its speed, power, and capabilities. without a memory unit, the processor would have to wait longer for data retrieval.

Conversions of Units