Structure of Magnetic Disk | Structure of Hard Disk

 

Magnetic Disk

A magnetic disk is a storage device that uses a magnetization process to read, write, rewrite and access data. Hard disk, Floppy disk and Zip disk are types of magnetic disk.

 

Figure: Cross Section of Hard Disk

A magnetic disk is a thin, circular metal plate. It is coated with a thin magnetic film, usually on both sides. Digital information is stored on the magnetic disk by magnetizing process.

 

Figure: Mechanical Structure of Magnetic Disk

The disks are mounted on a rotary drive so that the magnetized surface moves in close proximity to magnetizing coil or head as shown in diagram.

 

Figure: Magnetic Disk Read/Write Head

The head consists of a magnetic yoke and the magnetic coil. Digital information can be stored on the magnetic film by applying current to the magnetizing coil. Head is used for read and write data on magnetic disk.

 

Figure: Magnetic Disk Layout

The data on the disk is organized in a concentric set of rings as shown in diagram. This concentric set of rings are called as track. The tracks are further divided in to sector. Each sector stores a block of data which can be transferred to or from the disk. Data bits are stored serially on each track. The same number of bits are stored on each sector in a single track. To avoid magnetic interference between two adjacent sectors the gap is introduced between two adjacent sectors. Storage capacity of the track is increases as we move from the inner most track to the outer most track.

Figure: Working of Hard Disk 

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Classification of Auxiliary Memory | Types of Secondary Memory | Types of Permanent Memory

 

What is memory?

A memory unit is the collection of storage units or devices together. The memory unit stores the binary information in the form of bits.

Types of Memory

 

Figure : Conventional types of memory

Volatile Memory: It loses its data, when power is switched off.

Non-Volatile: This is a permanent storage and does not lose any data when power is switched off.

 

Figure : Classification of Memory

Primary Memory: It is also called Temporary or Main memory.

Secondary Memory: It is also called Permanent or Auxiliary memory.

Classification of Auxiliary Memory

 

Figure : Classification of Auxiliary Memory

Characteristics of auxiliary memory

Non-volatile memory − Data is not lost when power is cut off.

Reusable − The data stays in the secondary storage on permanent basis until it is not overwritten or deleted by the user.

Reliable − Data in secondary storage is safe because of high physical stability of secondary storage device.

Convenience − With the help of a computer software, authorised people can locate and access the data quickly.

Capacity − Secondary storage can store large volumes of data in sets of multiple disks.

Cost − It is much lesser expensive to store data on a tape or disk than primary memory.

Memory Hierarchy in Computer

Figure : Memory Hierarchy in Computer

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Tri State Buffer | Three State Buffer

Tri State Buffer

A common bus system can be implemented using tri-state or three-state gates instead of multiplexers. A tri-state gate is a digital circuit that can have three output states High, Low and High Impedance. The high impedance state behaves like an open circuit, which mean that the output is disconnected.

We know that bus is common connection between number of registers and other units. The data transfer through tri-state gates may require larger sinking and sourcing of current. The tri-state gate which provides more sinking and sourcing capacities is called tri-state buffer.

 

Figure : Tri State Buffer

Connection of Tri State Buffer with Register

How the data transfer takes place between register and common bus. Here, 4-bit register is shown with tri-state buffer at input and output sides. Each data line requires two tri-state buffers one at the input side and one at the output side. In diagram, all tri-state buffers at the input side are control by a common control signal R_in, and output side are control by a common control signal R_out.

 

Figure : Connection of Tri state buffer with register

Common Bus Structure (Using Tri State buffer)

Figure shows the bus structure for the data transfer between various register and the common bus. As shown in figure each register has input and output tri state buffers and these tri state buffers are controlled by corresponding control signals. Control signals R_i in is set to 1, the data available on the common data bus is loaded into register R_i. Similarly, when Ri out is set to 1, the contents of register R_i are placed on the common data bus. The signals R_i in and R_i out are commonly known as input enable and output enable signals of registers, respectively.

Figure : Common bus structure using tri state buffer

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Time and Space Diagram

 

Time and Space Diagram

Pipelines are nothing more than assembly lines in computing that can be used either for instruction processing or, in a more general sense, for performing any complex operation. To achieving pipelining, one must subdivide the input task (process) into sequence of subtasks. In advance computers, pipelines are applied for instruction execution, arithmetic computation, and memory accessing operations. A processor supporting such a hardware architecture is known as pipeline processor. The concurrent execution of subtask increasing throughput and improve system performance.

 

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Application of Logical Micro-operation

Logical Micro Operation

Micro-operation is an elementary operation performed with the data stored in registers.

Logic micro-operation perform logic operations such as AND, OR, complement and XOR on the strings of bits stored in registers. Special symbols are used for the logic micro-operations AND, OR and complements to different them from the corresponding symbol used to express Boolean functions.

Figure : Hardware Implementation of Logic Circuit

Application of Logical Micro Operation

There are four applications of logical micro-operation: Selective Set Operation, Selective Complement Operation, Selective Clear Operation, Mask Operation

Selective Set Operation:

In this operation, the bits in register A will change where corresponding register B bit is 1. The resultant bit is set to 1 in register A. If register B bit has 0, it will not affect register A bit.

The OR microoperation can be used to selectively set bits of a register.

Selective Complement Operation

In this operation, the bits in register A will complement, where corresponding register B bit is 1. If register B bit has 0, it will not affect register A bit.

The ex-OR microoperation can be used to selective complement bits of A register.

Selective Clear Operation

In this operation, the bits in register A will clear (set 0), where corresponding register B bit is 1. If register B bit has 0, it will not affect register A bit.

The corresponding microoperation is A ß A ∧ B’.

Mask Operation

The mask operation is similar to the selective clear operation. But in this operation, the bits in register A will clear (set 0), where corresponding register B bit is 0. If register B bit has 1, it will not affect register A bit.

The mask operation is an AND microoperation.

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Flynn's Taxonomy | Flynn's Classification

 

Flynn’s Taxonomy

This taxonomy proposed by Michael J. Flynn in 1966. Flynn's taxonomy is a “specific classification of parallel computer architectures.” Classifications are based on the number of concurrent instruction (single or multiple) and data streams (single or multiple) available in the architecture. The sequence of instructions read from memory is called an instruction stream. The operations performed on the data in the processor is called a data stream.

 

SISD (Single Instruction Single Data Stream)

Most conventional machines with one CPU containing a single arithmetic – logic unit. SISD computers and sequential computers are same. SISD represents the organization of a single computer containing a control unit, a processor unit, and a memory unit. Instructions are executed sequentially and the system may or may not have internal parallel processing capabilities. In SISD computer instructions are executed sequentially but may overlap in their execution stages.

Figure : Single Insturction Single Data (SISD)

SIMD (Single Instruction Multiple Data Stream)

This category corresponds to array processors. They have multiple processing/execution units and one control unit. SIMD represents an organization that includes many processing units under the supervision of a common control unit. All processors receive the same instruction from the control unit but operate on different items of data.

Figure : Single Insturction Multiple Data (SIMD)

MISD (Multiple Instruction Single Data Stream)

There is no computer at present that can be classified as MISD. Mostly used in pipelined computers. Each receiving distinct instructions operating over the same data stream and its derivatives. The result of one processor become the input of the next processor in the micro-pipe. MISD structure is only of theoretical interest since no practical system has been constructed using this organization.

Figure : Multiple Insturction Single Data (MISD)

MIMD (Multiple Instruction Multiple Data Stream)

Most multiprocessor system and multiple computer system can be classified in this category. In MIMD, there are more than one processor unit having the ability to execute several programs simultaneously. MIMD organization refers to a computer system capable of processing several programs at the same time. Contains multiple processing units. Execution of multiple instructions on multiple data.

Figure : Multiple Insturction Multiple Data (MIMD)

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How to find primitive roots of prime number | Application of Primitive roots in cryptography

In this post I have explain two methods: 1. Primitive roots for short prime number 2. Primitive roots for large number.

How to find primitive roots for short prime number?

Step – 1: If ‘a’ is a primitive root of ‘q’. Where, q is prime number. Where a=1 to q-1.

Step – 2: aⁿ mod q. Where, n = 1 to q-1. It produces each integer from 1 to q-1 exactly once.

 

How to find primitive roots for large prime number?

Step – 1: Express Ø(q) = as possible powers of prime number (say a^x, b^y, etc.…)

Step – 2: Find A = Ø(q)/a, B = Ø(q)/b, … Let it be A, B, C, ….

Step – 3: Choose any number X. If n1 = X^A mod q, n2 = X^B mod q, ...      Where, n1 & n2 … ≠ 1, then X is a primitive root of q.

 

Application of Primitive roots in cryptography

Primitive root concepts are used in Diffie-Hellman key exchange algorithm.

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Instruction Format (With reference to address)

 

Instruction Format (With reference to address)

Instruction contains some specific fields: opcode, source/destination operand, source operand address, destination operand address, next instruction address. 

Computers may have instructions of several different length containing varying number of addresses.  According to address reference, there are mainly four types of instruction:

1. Three address instruction

2. Two address instruction

3. One address instruction

4. Zero address instruction

Three Address Instruction

Syntax of three address instruction is,  

Operation Destination, Source 1, Source 2

For Example:  ADD A, B, C

Where A, B, C are the operands. These variable names are assigned to distinct locations in the memory. In above example, operands B and C are called source operands and operand A is called destination operand and ADD is the operation to be performed on the operands. Bits required the three memory addresses of the three operands. If n-bit are required to specify one memory address, 3n bits are required to specify three memory addresses.

Two Address Instruction

Syntax of two address instruction is,

Operation   Destination, Source

For Example: ADD A, B

Where A, B are the operands. These variable names are assigned to distinct locations in the memory. In above example, operands B are called source operands and operand A is serves as both source & destination operand.  ADD is the operation to be performed on the operands. Bits required the two memory addresses of the two operands. If n-bit are required to specify one memory address, 2n bits are required to specify two memory addresses.

One Address Instruction

Syntax of one address instruction is,

Operation Source

For Example: ADD A

This instruction adds the contents of variable A into the processor register called accumulator and stores the sum back into the accumulator destroying the previous content of the accumulator. In this instruction the second operand is assumed implicitly in a unique location accumulator.

Example of one address instruction:

LOAD A (Copy content of memory location A into accumulator)

STORE B (Copies the contents of accumulator into memory location B)

Zero Address Instruction

In this instruction, the locations of all operands are defined implicitly. Such instructions are found in machines that stores operands in a structure called a pushdown stack. Though all the functionalities (like push, pop, TOS...) of stack is included in the instruction execution.

The instruction with only one address will require a smaller number of bits to represent it, and instruction with three addresses will require a greater number of bits to represent it. Three addresses instruction requires more memory accesses (more time required to execute) and one address requires less memory accesses (less time required to execute).

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