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COMPUTER | Memory(Primary & Secondary)

Primary Memory Explained

There are two types of physical memory: primary and secondary memory. Primary memory is volatile memory. Secondary memory (secondary storage) is non-volatile.
Types of memory that retain their contents when power is turned off. ROM is nonvolatile, whereas RAM is volatile. This term often refers to the CMOS memory in PCs that holds the BIOS.
The main functions of primary memory  (also called main memory or primary storage) is to execute program code and store temporary data. Primary memory is the memory that the processor accesses first. The memory is on chips located on the motherboard. The primary memory  stores applications to run the operating system (OS), the user interface and installed software utilities.
The computer can manipulate only the data that is in main memory. Every program executed and every file accessed must be copied from a storage device into main memory. The amount of main memory in a computer system determines how many programs can be executed at one time and how much data can be readily available to a program.
Did You Know...?  A small program (called a boot loader or bootstrap loader) is stored in primary memory. It loads the operating system into the computer's memory when the system is booted and the program also starts the operating system. This process is known as "booting up" the computer.

Primary Memory: RAM and ROM

Examples of primary memory include RAM and ROM.

What is RAM?

Random access memory (RAM) is a type of volatile memory and is the most common type of memory found in computers and other devices, such as printers. RAM requires a flow of electricity to retain data (e.g., the computer is powered on).

What is ROM?

Read-only memory (ROM) is a type of non-volatile memory and is computer memory on which data has been prerecorded. ROM will retain data without the flow of electricity (e.g., when the computer is powered off).

Difference Between RAM and ROM

RAM is Random Access Memory.  
ROM is Read Only Memory.
RAM is the memory available for the operating system, programs and processes to use when the computer is running.
ROM is the memory that comes with your computer that is pre-written to hold the instructions for booting-up the computer.
RAM requires a flow of electricity to retain data (e.g. the computer powered on).
ROM will retain data without the flow of electricity (e.g. when computer is powered off).
RAM is a type of volatile memory. Data in RAM is not permanently written. When you power off your computer the data stored in RAM is deleted. 
ROM is a type of non- volatile memory. Data in ROM is permanently written and is not erased when you power off your computer.
There are different types of RAM, including DRAM (Dynamic Random Access Memory) and SRAM (Static Random Access Memory).
There are different types of ROM, including PROM (programmable read-only memory) that is manufactured as blank memory (e.g. a CD-ROM) and EPROM (erasable programmable read-only memory).
There are many differences between RAM and ROM memory but there are also a couple similarities (and these are very easy to remember).  Both types of memory used by a computer, and they are both required for your computer to operate properly and efficiently.

Secondary Memory

Secondary memory (also called auxiliary memory) is storage devices: hard drives, solid state drives, removable storage media -- including flash drives and DVDs.
Secondary memory is not accessed directly by the CPU as it is with primary memory. Instead, data from secondary memory is loaded into RAM then sent to the processor. It transfers the requested data to an intermediate area in primary storage. While secondary memory is much slower than primary memory, it offers greater storage capacity.

Persistent Storage

Persistent storage (storage that doesn't lose its data after it loses its power supply) is extremely important in a computer system. It is needed to store data in a non-volatile device during and after the running of a program to keep files and data for later use. The hard disk drive is a common example of persistent storage.

The Purpose of Cache Memory

Cache memory (also called CPU cache) is a high-speed storage mechanism that is a reserved section of main memory or an independent high-speed storage device. The CPU cache reduces the time required to access data from the main memory. Modern CPUs have different independent caches, such as instruction and data caches.
There are two types of caching commonly used in computer systems: memory caching and disk caching.

What is Memory Cache?

Memory cache, sometimes called RAM cache, is a portion of memory made of high-speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is effective because most programs access the same data or instructions over and over.

What is Disk Caching?

Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.

The Machine Instruction Cycle

The instruction cycle refers to the time period during which one instruction is fetched from memory and executed when a computer is given an instruction in machine language. The CPU carries out the following four stages of an instruction cycle:
1. Fetch the instruction from memory. This step brings the instruction into the instruction register, a circuit that holds the instruction so that it can be decoded and executed.
2. Decode the instruction. Mathematical and logical operations used in reference to data.
3. Read the effective address from memory if the instruction has an indirect address.
4. Execute the instruction. This combines all steps.
Note: Steps 1 and 2 are called the fetch cycle and are the same for each instruction. Steps 3 and 4 are called the execute cycle and will change with each instruction.


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