Loops and Conditionals

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Source https://www.cs.cmu.edu/~15110-n15/lectures/unit03-Algorithm-1.pdf

In this section we will see how to multiply two matrices. The matrix multiplication can only be performed, if it satisfies this condition. Suppose two matrices are A and B, and their dimensions are A (m x n) and B (p x q) the resultant matrix can be found if and only if n = p. Then the order of the resultant matrix C will be (m x q).

Algorithm

matrixMultiply(A, B):
Assume dimension of A is (m x n), dimension of B is (p x q)
Begin
   if n is not same as p, then exit
   otherwise define C matrix as (m x q)
   for i in range 0 to m - 1, do
      for j in range 0 to q – 1, do
         for k in range 0 to p, do
            C[i, j] = C[i, j] + (A[i, k] * A[k, j])
         done
      done
   done
End

Example

 Live Demo

#include<iostream>
using namespace std;
int main() {
   int product[10][10], r1=3, c1=3, r2=3, c2=3, i, j, k;
   int a[3][3] = {
      {2, 4, 1},
      {2, 3, 9},
      {3, 1, 8}
   };
   int b[3][3] = {
      {1, 2, 3},
      {3, 6, 1},
      {2, 4, 7}
   };
   if (c1 != r2) {
      cout<<"Column of first matrix should be equal to row of second matrix";
   } else {
      cout<<"The first matrix is:"<<endl;
      for(i=0; i<r1; ++i) {
         for(j=0; j<c1; ++j)
            cout<<a[i][j]<<" ";
         cout<<endl;
      }
      cout<<endl;
      cout<<"The second matrix is:"<<endl;
      for(i=0; i<r2; ++i) {
         for(j=0; j<c2; ++j)
            cout<<b[i][j]<<" ";
         cout<<endl;
      }
      cout<<endl;
      for(i=0; i<r1; ++i)
         for(j=0; j<c2; ++j) {
            product[i][j] = 0;
         }
      for(i=0; i<r1; ++i)
         for(j=0; j<c2; ++j)
            for(k=0; k<c1; ++k) {
               product[i][j]+=a[i][k]*b[k][j];
            }
      cout<<"Product of the two matrices is:"<<endl;
      for(i=0; i<r1; ++i) {
         for(j=0; j<c2; ++j)
            cout<<product[i][j]<<" ";
         cout<<endl;
      }
   }
   return 0;
}

Output

The first matrix is:
2 4 1
2 3 9
3 1 8
The second matrix is:
1 2 3
3 6 1
2 4 7
Product of the two matrices is:
16 32 17
29 58 72
22 44 66

Source : https://www.tutorialspoint.com/matrix-multiplication-algorithm

An interrupt is a signal emitted by hardware or software when a process or an event needs immediate attention. It alerts the processor to a high-priority process requiring interruption of the current working process. In I/O devices, one of the bus control lines is dedicated for this purpose and is called the Interrupt Service Routine (ISR).

When a device raises an interrupt at the process, the processor first completes the execution of an instruction. Then it loads the Program Counter (PC) with the address of the first instruction of the ISR. Before loading the program counter with the address, the address of the interrupted instruction is moved to a temporary location. Therefore, after handling the interrupt, the processor can continue with the process.

While the processor is handling the interrupts, it must inform the device that its request has been recognized to stop sending the interrupt request signal. Also, saving the registers so that the interrupted process can be restored in the future increases the delay between the time an interrupt is received and the start of the execution of the ISR. This is called Interrupt Latency.

A single computer can perform only one computer instruction at a time. But, because it can be interrupted, it can manage how programs or sets of instructions will be performed. This is known as multitasking. It allows the user to do many different things simultaneously, and the computer turns to manage the programs that the user starts. Of course, the computer operates at speeds that make it seem like all user tasks are being performed simultaneously.

An operating system usually has some code that is called an interrupt handler. The interrupt handler prioritizes the interrupts and saves them in a queue if more than one is waiting to be handled. The operating system has another little program called a scheduler that figures out which program to control next.

Types of Interrupt

Interrupt signals may be issued in response to hardware or software events. These are classified as hardware interrupts or software interrupts, respectively.

1. Hardware Interrupts

A hardware interrupt is a condition related to the state of the hardware that may be signaled by an external hardware device, e.g., an interrupt request (IRQ) line on a PC, or detected by devices embedded in processor logic to communicate that the device needs attention from the operating system. For example, pressing a keyboard key or moving a mouse triggers hardware interrupts that cause the processor to read the keystroke or mouse position.

Hardware interrupts can arrive asynchronously for the processor clock and at any time during instruction execution. Consequently, all hardware interrupt signals are conditioned by synchronizing them to the processor clock and act only at instruction execution boundaries.

In many systems, each device is associated with a particular IRQ signal. This makes it possible to quickly determine which hardware device is requesting service and expedite servicing of that device.

On some older systems, all interrupts went to the same location, and the OS used specialized instruction to determine the highest priority unmasked interrupt outstanding. On contemporary systems, there is generally a distinct interrupt routine for each type of interrupt or each interrupts source, often implemented as one or more interrupt vector tables. Hardware interrupts are further classified into two types, such as:

• Maskable Interrupts:Processors typically have an internal interrupt mask register which allows selective enabling and disabling of hardware interrupts. Each interrupt signal is associated with a bit in the mask register; on some systems, the interrupt is enabled when the bit is set and disabled when the bit is clear, while on others, a set bit disables the interrupt. When the interrupt is disabled, the associated interrupt signal will be ignored by the processor. Signals which are affected by the mask are called maskable interrupts.
  The interrupt mask does not affect some interrupt signals and therefore cannot be disabled; these are called non-maskable interrupts (NMI). NMIs indicate high priority events that need to be processed immediately and which cannot be ignored under any circumstances, such as the timeout signal from a watchdog timer.
  To mask an interrupt is to disable it, while to unmask an interrupt is to enable it.
• Spurious Interrupts:
  A spurious interrupt is a hardware interrupt for which no source can be found. The term phantom interrupt or ghost interrupt may also use to describe this phenomenon. Spurious interrupts tend to be a problem with a wired-OR interrupt circuit attached to a level-sensitive processor input. Such interrupts may be difficult to identify when a system misbehaves.
  In a wired-OR circuit, parasitic capacitance charging/discharging through the interrupt line’s bias resistor will cause a small delay before the processor recognizes that the interrupt source has been cleared. If the interrupting device is cleared too late in the interrupt service routine (ISR), there won’t be enough time for the interrupt circuit to return to the quiescent state before the current instance of the ISR terminates. The result is the processor will think another interrupt is pending since the voltage at its interrupt request input will be not high or low enough to establish an unambiguous internal logic 1 or logic 0. The apparent interrupt will have no identifiable source, and hence this is called spurious.
  A spurious interrupt may result in system deadlock or other undefined operation if the ISR doesn’t account for the possibility of such an interrupt occurring. As spurious interrupts are mostly a problem with wired-OR interrupt circuits, good programming practice in such systems is for the ISR to check all interrupt sources for activity and take no action if none of the sources is interrupting.

2. Software Interrupts

The processor requests a software interrupt upon executing particular instructions or when certain conditions are met. Every software interrupt signal is associated with a particular interrupt handler.

A software interrupt may be intentionally caused by executing a special instruction that invokes an interrupt when executed by design. Such instructions function similarly to subroutine calls and are used for various purposes, such as requesting operating system services and interacting with device drivers.

Software interrupts may also be unexpectedly triggered by program execution errors. These interrupts are typically called traps or exceptions.

Handling Multiple Devices

When more than one device raises an interrupt request signal, additional information is needed to decide which device to consider first. The following methods are used to decide which device to select first,

1. Polling
   In polling, the first device encountered with the IRQ bit set is to be serviced first, and appropriate ISR is called to service the same. It is easy to implement, but a lot of time is wasted by interrogating the IRQ bit of all devices.
2. Vectored Interrupts
   In vectored interrupts, a device requesting an interrupt identifies itself directly by sending a special code to the processor over the bus. This enables the processor to identify the device that generated the interrupt. The special code can be the starting address of the ISR or where the ISR is located in memory and is called the interrupt vector.
3. Interrupt Nesting
   In this method, the I/O device is organized in a priority structure. Therefore, an interrupt request from a higher priority device is recognized, whereas a lower priority device is not. The processor accepts interrupts only from devices/processes having priority more than it.
   Processors priority is encoded in a few bits of PS (Process Status register), and it can be changed by program instructions that write into the PS. The processor is in supervised mode only while executing OS routines, and it switches to user mode before executing application programs.

Interrupt Handling

We know that the instruction cycle consists of fetch, decode, execute and read/write functions. After every instruction cycle, the processor will check for interrupts to be processed. If there is no interrupt in the system, it will go for the next instruction cycle, given by the instruction register. If there is an interrupt present, then it will trigger the interrupt handler. The handler will stop the present instruction that is processing and save its configuration in a register and load the program counter of the interrupt from a location given by the interrupt vector table.

After processing the interrupt by the processor, the interrupt handler will load the instruction and its configuration from the saved register. The process will start its processing where it’s left. This saves the old instruction processing configuration, and loading the new interrupt configuration is also called context switching. There are different types of interrupt handlers.

1. First Level Interrupt Handler (FLIH) is a hard interrupt handler or fast interrupt handler. These interrupt handlers have more jitter while process execution, and they are mainly maskable interrupts.
2. Second Level Interrupt Handler (SLIH) is a soft interrupt handler and slow interrupt handler. These interrupt handlers have less jitter.

Source : https://www.javatpoint.com/what-is-interrupt-in-os

There can be a huge amount of files in our system. The operating system is responsible for managing the files. The operating system is used to manage computer files. The File Management in the operating system manages all files with various extensions.

Before we dive into file management in the operating system, let us first recap what a file is.

A file is a collection of specific information stored in a computer system’s memory. There are various types of files in our computer-like text files, images, audio/video files, etc. All these files have different extensions. The file management system is responsible for the efficient storage, retrieval, and manipulation of files.

File management is one of the fundamental and crucial components of an operating system. The operating system manages computer system files. Operating systems control all files with various extensions.

File management is formally defined as manipulating files in a computer system, which includes creating, modifying, and deleting files. Therefore, one of the simple but crucial features offered by the operating system is file management. The operating system’s file management function entails software that handles or maintains files (binary, text, PDF, docs, audio, video, etc.) included in computer software.

The operating system’s file system has the ability to manage both single files and groups of files that are present in a computer system. All of the files on the computer system with different extensions(such as .exe, .pdf, .txt, .docx, etc.) are managed by the operating system’s file management.

The file management system is also known as the file system. It is responsible for file management in any system. The various functions involved in file management are as follows:

• It is responsible for creating new files in the computer system and placing them in specific locations.
• It is responsible for locating the existing files in the computer system.
• It facilitates keeping the files in separate folders known as directories. These directories allow users to quickly search for files or organize files based on their types of uses.
• It enables users to change the data of files or the name of files in directories.

The components of the file management system in the operating system are as follows:

• File Attribute
• File Operations
• File Access Permissions
• File Systems

File Attribute

It specifies file characteristics such as type, date of last modification, size, location on disk, and so on. File attributes assist the user in comprehending the value and location of files. It is used to describe all of the information about a specific file.

File Operations

It defines the actions that can be taken on a file, such as opening and closing it.

File Access Permissions

It specifies a file’s access permissions, such as read and writes. Without permission, a file cannot be accessed.

File Systems

It defines the logical method of storing files in a computer system. FAT and NTFS are two of the most commonly used file systems.

The various operation on files that can be performed by a user and is facilitated by the file management in the operating system are:

• Creating a File
• Reading a File
• Writing a File
• Deleting a File
• Truncating a File

Creating a File

In order to create a new file, there should be ample space in the file system. The new file’s directory entry must then be created. This entry should include information about the file, such as its name, location, etc.

Reading a File

The system call must specify the file’s name and location to read from a file. A read pointer should be present at the location where the read should occur. The read pointer should be updated once the read process is completed.

Writing a File

To write to a file, the system call should specify the file’s name as well as the contents to be written. A write pointer should be present at the location where the write should occur. The write pointer should be updated after the write process is completed.

Deleting a File

In order to delete a file, it must be present in the directory. When the file is deleted, all the file space is deleted so it can be reused by other files.

Truncating a File

This removes the data from the file without erasing all of its attributes. Only the file length is reset to zero, and the contents of the file are erased. The remaining characteristics remain unchanged.