Memory
When we discuss the memory of a computer it is important to know the following terms.
Capacity – Standard measurement of measuring computer memory capacity is ‘bit’, which is the space required to store one character or one numeric value. Bit, short for binary digit, the smallest unit of information on a machine. The term was first used in 1946 by John Tukey, a leading statistician and adviser to five presidents. A single bit can hold only one of two values: 0 or 1. More meaningful information is obtained by combining consecutive bits into larger units. For example, a byte is composed of 8 consecutive bits.
	8 bit = 1 byte
	1024 byte = 1KB
	1024 KB = 1MB
	1024 MB = 1GB 

Memory address
This represents a memory location where data and instruction can be saved. This factor is very important when retrieving the data from computer memory. 
 There are two types of computer memory. That is,
•	Primary memory
•	Secondary memory
Secondary memory
This memory uses for store the external data. (Examples for external data – user created data, package data, operating system data and so on.) Using these memories you can store data permanently.  
Examples: Hard disk, Floppy Disk, CD/DVD – ROM, Pen drives
Hard disk
Magnetic storage medium for a microcomputer. Hard disks are flat, circular plates made of aluminum or glass and coated with a magnetic material. Hard disks for personal computers can store up to several gigabytes (billions of bytes) of information. Data are stored on their surfaces in concentric tracks. A small electromagnet, called a magnetic head, writes a binary digit (1 or 0) by magnetizing tiny spots on the spinning disk in different directions and reads digits by detecting the magnetization direction of the spots. A computer's hard drive is a device consisting of several hard disks, read/write heads, a drive motor to spin the disks, and a small amount of circuitry, all sealed in a metal case to protect the disks from dust. In addition to referring to the disks themselves, the term hard disk is also used to refer to the whole hard drive.
Floppy disk
A soft magnetic disk. It is called floppy because it flops if you wave it (at least, the 5¼-inch variety does). Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity, but they are much less expensive. And most importantly, they are portable. 
Floppies come in three basic sizes: 
  8-inch: The first floppy disk design, invented by IBM in the late 1960s and used in the early 1970s as first a read-only format and then as a read-write format. The typical desktop/laptop computer does not use the 8-inch floppy disk. 
  5¼-inch: The common size for PCs made before 1987 and the predecessor to the 8-inch floppy disk. This type of floppy is generally capable of storing between 100K and 1.2MB (megabytes) of data. The most common sizes are 360K and 1.2MB. 
  3½-inch: Floppy is something of a misnomer for these disks, as they are encased in a rigid envelope. Despite their small size, microfloppies have a larger storage capacity than their cousins -- from 400K to 1.4MB of data. The most common sizes for PCs are 720K (double-density) and 1.44MB (high-density). Macintoshes support disks of 400K, 800K, and 1.2MB. 
Compact disk (CD)
A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store sound recordings exclusively, but later it also allowed the preservation of other types of data. Audio CDs have been commercially available since October 1982. In 2010, they remain the standard physical storage medium for audio.Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of uncompressed audio (700 MB of data). The Mini CD has various diameters ranging from 60 to 80 mm; they are sometimes used for CD singles or device drivers, storing up to 24 minutes of audio.
Digital Versatile Disk (DVD)
Digital V DVD stands for Digital Versatile/Video Disc, DVDR stands for DVD Recordable and DVDRW for DVD Rewriteable. If you're familiar with regular audio/music CDs or regular DVD-Video discs, then you will know what a recordable DVD looks like. A recordable DVD stores up to 2 hours of very good quality DVD-Video, including several audio tracks in formats like stereo, Dolby Digital or DTS and also advanced menu systems, subtitles and still pictures that can be played by many standalone DVD Players and most computer DVD-ROMs. If you choose to lower the video quality it is possible to store several hours video on a recordable DVD using low bitrates and low resolution with video quality more like VHS, SVHS, SVCD, CVD or VCD. It is also possible to have up to 4.37 GB ordinary data or mix DVD-Video and data on a recordable DVD that can be played by most computer DVD-ROMs. versatile Disk (DVD)
USB pen drive
USB Pen Drive is a small keyring-sized device that can be used to easily transfer files between USB-compatible systems. Available in a range of capacities (and in some cases, with an MP3 player built-in) this handy little gizmo can save all those data-transfer hassles. Plug it into the USB port* of your PC (or Mac!) and watch the system automatically detect the new device. Take at look at your system drives... a new drive has been created! The operating system can now access your USB Pen Drive just like any ordinary Hard Disk Drive.
Copy across all the files you want to the 'new' drive, wait for the Read/Write LED on the USB Pen Drive to stop flashing then disconnect it. That's it. Your files are now safely stored on your USB Pen Drive. If you want to copy those files to another PC/Mac, just plug it in the new machine, wait for it to be detected and copy them off again.  If you don't want to reach round to the back of your PC every time to plug it in, you can use the handy Docking Bay to give you USB Pen Drive access right from your desktop.
Primary Memory
This memory type is used to store data and instructions which relevant to the internal operations of the computer. (Ex. Data and instructions are relevant to the booting process). There are two types of primary memories. Ie,
•	RAM - Random Access Memory
•	RAM- Read Only Memory
RAM
RAM stand for Random Access Memory which is used to store data and instruction temporarily which required to internal operation of the computer. In working situation, the following data available with RAM
•	Document data or User data
•	Package data
•	Operating System data
In the case of power failure, document or user data may be lost and package and operating system data return to proper places. (Current computers package and operating system data is in the hard disk)
RAM contains two sections, named upper memory and conventional memory. In a working situation, document and package data are in the upper memory and operating system data are in the conventional memory. The following diagram explain the working situation of a RAM

Document data or
User data
Package data

Upper memory

Operating system data

Conventional memory

ROM
ROM stand for Read Only Memory.  This memory is used to store data and instructions permanently which relevant to internal operation of the computer. In the ROM its store a special program called BIOS (Basic Input Output System).  BIOS perform the following two functions.
1.	Check all the hardware devices correctly work or not
2.	Load a operating system from secondary device to latter part of RAM(ie, conventional memory) 
Important points
•	The total responsibility of controlling computer is taken by BIOS program before loading the operating system.
•	Content of ROM cannot be deleted by users, only capable of reading it.
•	There is some ROM – BIOS which contents can update, such a BIOS is known as flash BIOS. 
Flash BIOS
Most modern computers are actually supplied with a flash BIOS rather than a ROMBIOS. This chip contains exactly the same type of in-built software, but has the advantage that the software on the chip can be upgraded. This upgrade is achieved by simply running a small program supplied by the computer manufacturer.

Special types of primary memories
o	Cache memory
o	Registers
Cache memory
Cache memory (also called buffer memory) is local memory that reduces waiting times for information stored in the RAM (Random Access Memory). In effect, the computer's main memory is slower than that of the processor. There are, however, types of memory that are much faster, but which have a greatly increased cost. The solution is therefore to include this type of local memory close to the processor and to temporarily store the primary data to be processed in it. Recent model computers have many different levels of cache memory: 
Level one cache memory (called L1 Cache, for Level 1 Cache) is directly integrated into the processor. It is subdivided into two parts: 
the first part is the instruction cache, which contains instructions from the RAM that have been 	decoded as they came across the pipelines. 
the second part is the data cache, which contains data from the RAM and data recently used during processor operations.

Level 1 caches can be accessed very rapidly. Access waiting time approaches that of internal processor registers. 
Level two cache memory (called L2 Cache, for Level 2 Cache) is located in the case along with the processor (in the chip). The level two cache is an intermediary between the processor, with its internal cache, and the RAM. It can be accessed more rapidly than the RAM, but less rapidly than the level one cache. 
Level three cache memory (called L3 Cache, for Level 3 Cache) is located on the motherboard.

All these levels of cache reduce the latency time of various memory types when processing or transferring information. While the processor works, the level one cache controller can interface with the level two controllers to transfer information without impeding the processor. As well, the level two cache interfaces with the RAM (level three cache) to allow transfers without impeding normal processor operation.
Registers
High-speed storage areas used to temporarily hold small units of program instructions and data immediately before, during, and after execution by the CPU
Prepared by
Cpk wijekoon
Lecturer,
Mahaweli National College of Educatuion
Uyanwatta,
Polgolla
Contacts:0718016539