Computer Memory and Types of Memory

Computer memory is any device capable of storing data and information temporarily for processing.

There are two types of Memory in Computer :

  •  Primary memory
  •  Secondary memory
Computer Memory and Types of Memory

A. Primary Memory:

Small amounts of data and information that will be immediately used by the CPU are stored in primary memory or primary storage. It is also known as main memory. It stores data to be processed by the CPU, instructions for the CPU as to how to process data, and operating system programs that manage various aspects of computer?s operation for a short period of time. Primary storage takes place in chips mounted on the motherboard and are located as close as physically possible to the CPU. There are four main types of primary storage: Register, Random Access Memory (RAM), Cache Memory and Read Only Memory (ROM).

1. Register:
• Registers are part of the CPU 
• Have least capacity 
• Stores extremely limited amounts of instructions and data only immediately before and after                  process.

2. RAM 
• stores more information than registers 
• stores less than secondary storage 
• RAM is temporary and volatile memory 
• software program and small amounts of data are stored in RAM when they are brought from secondary storage 
• The two main types of RAM are:
     a. Dynamic RAM (DRAM) and 
     b. Static RAM (SRAM) 
• DRAM offers the greatest capacities and the lowest cost per bit but it is relatively slower.
• SRAM costs more than DRAM but has higher level performance.

3. Cache Memory:
It is also known as CPU memory t is a type of high speed memory that a processor can access more rapidly than RAM. it is faster than RAM because it is located closer to the CPU the basic purpose of cache memory is to store program instructions that are frequently re-referenced by software during operation when a micro-controller processes data, it looks first in cache memory there are mainly 2 types of cache memory : L1 (Level 1) L2 (Level 2) L1 is smaller and faster than L2 and is located in the processor L2 is located on the motherboard but not in the processor nowadays chips are designed with L1 and L2 in the processor and another one called L3 cache on the motherboard.

4. ROM:
• critical instructions are stored in ROM. 
• it is non-volatile.
• instruction in ROM can only be read by the computer and can not be changed by the user. 
• other forms of ROM are Programmable ROM (PROM) and Erasable and Programmable ROM (EPROM).

B. Secondary Memory:

Secondary Memory

• designed to store very large volume of data 
• it is non volatile 
• it is generally electromechanical in nature 
• it is slower than primary storage 
• it is much more cost effective than primary memory 
• it can take place in variety of media. 

1. Magnetic tape: 
• A secondary storage medium on a large open reel or in a smaller cartridge or cassette. 
• it is the cheapest storage medium 
• can store very large amount of data 
• it is a sequential access memory thus data retrieval is very slow 
• Data access in which the compute system must run through data in sequence in order to locate a particular piece is called sequential access Sequential access 
     – Data are stored in binary format 
     – it stores information by giving tiny particles of iron oxide embedded on the tape either a negative or positive polarization.

2. Magnetic disks: 
• A form of secondary storage on a magnetized disk divided into tracks and sectors that provide addresses for various pieces of data; also called hard disks. 
• used for mass storage. 
• data can be accessed in non-sequential manner; uses direct access • in direct access method data can be located using data address. 
• data access is very fast.
• but they are susceptible to mechanical failure.

3. Magnetic Diskette: 
• also known as floppy disks. 
• they are made up of flexible Mylar. 
• slower than hard drives. 
• less storage capacity. 
• highly portable.

4. Optical Storage Device: 

• A form of secondary storage in which a laser reads the surface of a reflective plastic platter. 
• pinpoint laser beam is used to burn tiny holes into the surface of the reflective plastic platter. 
• when information is to be read another laser shines on the surface; and if the light is reflected it is read as 1 and if the light shines on one of the holes resulting in lack of reflection then it is read as 0. 
• Types of optical storage :
    - Compact-disk Read Only Memory (CD-ROM), 
    - Digital Video Disk (DVD), 
    - Fluorescent Multilayer Disk (FMD-ROM) 
• Compact disk, read-only memory (CD-ROM) A form of secondary storage that can be only read and not written on. 
• Digital video disk (DVD): 
      An optical storage device used to store digital video or computer data. 
• Fluorescent multilayer disk (FMD-ROM): 
     An optical storage device with much greater storage capacity than DVDs. It uses multiple layers on an optical disk to store data. All layers of an FMD can be read in parallel, thus it has a high data rate.

5. Memory cards:
Credit-card-size storage devices that can be installed in an adapter or slot in many personal computers.

6. Expandable storage devices:
Removable disk cartridges used as backup storage for internal hard drives of PCs.

7. Enterprise Storage System:
It is an independent, external system with intelligence that includes two or more storage devices. There are three major types of enterprise storage systems: RAID, SAN and NAS.

8. Redundant Array of Independent Disks (RAID):
RAID (redundant array of independent disks; originally redundant array of inexpensive disks) provides a way of storing the same data in different places (thus, redundantly) on multiple hard disks (though not all RAID levels provide redundancy). By placing data on multiple disks, input/output (I/O) operations can overlap in a balanced way, improving performance. Since multiple disks increase the mean time between failures (MTBF), storing data redundantly also increases fault tolerance.

10. Storage Area Network (SAN):
A storage-area network (SAN) is a dedicated high-speed network that interconnects and presents shared pools of storage devices to multiple servers. A SAN moves storage resources off the common user network and reorganizes them into an independent, high-performance network. This allows each server to access shared storage as if it were a drive directly attached to the server. When a host wants to access a storage device on the SAN, it sends out a block-based access request for the storage device.

11. Network Attached Storage (NAS):
Network-attached storage (NAS) is a file-level computer data storage server connected to a computer network providing data access to a heterogeneous group of clients. NAS is specialized for serving files either by its hardware, software, or configuration.

              Input Technologies
               Working of CPU
               Software and Software Crisis

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