Types of software and interrupts

All computers run on software, which are sets of instructions that tell the hardware what to do. Software is broadly divided into two main types: System Software and Application Software. Think of it like a restaurant: System Software is the kitchen, the chefs, and the waiters – everything needed to make the restaurant run. Application Software is the actual meal you order and eat. You can't get your meal without the kitchen and staff operating correctly.

System Software manages the computer's hardware and provides a platform for other software. Its job is to run and maintain the computer system itself. The most important piece of system software is the Operating System (OS). Other examples include utility programs like anti-virus software and disk defragmenters.

Application Software is designed to perform specific tasks for you, the user. This is the software you interact with directly to get things done. Examples include word processors to write essays, web browsers to surf the internet, or games for entertainment. Application software runs 'on top of' the system software.

The Operating System (OS) is the most important type of system software. It acts as the central coordinator for everything that happens on a computer, managing both hardware and software resources. It's the essential bridge that allows your applications to communicate with the physical hardware. The main functions of an OS are:

• Memory Management: The OS is in charge of the main memory (RAM). It allocates memory to applications that are running and ensures they don't use memory assigned to another program, which would cause crashes. When you close a program, the OS reclaims the memory so it can be used by something else.
• Processor Management (Scheduling): In a modern computer, many processes seem to run at once. The OS manages the CPU's time, switching between processes very quickly to give the illusion of multitasking.
• File Management: The OS is responsible for the file system. It handles the creation, naming, copying, moving, and deletion of files and folders, and keeps track of where they are stored on a hard drive or SSD.
• Input/Output (I/O) Management: The OS manages all communication with peripheral devices, such as the keyboard, mouse, monitor, and printer. It uses programs called 'device drivers' to translate generic commands (like 'print') into specific instructions for a particular piece of hardware.
• Providing a User Interface: The OS gives the user a way to interact with the computer, typically through a Graphical User Interface (GUI) with icons and windows, or a Command Line Interface (CLI) where commands are typed.

An interrupt is a signal sent to the processor that indicates an event needs its immediate attention. This signal causes the processor to pause the task it is currently performing so it can deal with the new, more urgent request. Interrupts are essential for a computer to operate efficiently, as they allow the CPU to perform other tasks instead of waiting for slow events (like a user typing). There are two main categories of interrupts:

• Hardware Interrupts: These are signals generated by a physical hardware device connected to the computer. They are asynchronous, meaning they can happen at any time. Examples include:
• Pressing a key on the keyboard or clicking the mouse.
• A printer signalling it is out of paper or has finished a job.
• A network card indicating a data packet has arrived.
• A hard drive signalling that it has finished reading a requested block of data.

• Software Interrupts: These are generated by a piece of software. They can be caused by an error in a program or when a program needs to request a service from the operating system. Examples include:
• A division by zero error occurring in a calculation.
• An attempt to execute an illegal instruction.
• A program requesting to read data from a file (this is a request to the OS).
• A timer signalling that a certain amount of time has elapsed, used for multitasking.

When the CPU receives an interrupt, it doesn't just stop randomly. It follows a precise sequence of steps called the interrupt handling process to ensure it can deal with the event and then return to its original task without losing its place. Interrupts are checked at the end of each Fetch-Decode-Execute (FDE) cycle.

The process is as follows:

1. Complete Current Cycle: The processor finishes the current FDE cycle it is executing. It will not stop in the middle of an instruction.
2. Check for Interrupts: The processor checks a special pin or internal flag to see if any interrupt requests are pending.
3. Prioritise: If there is an interrupt, the processor checks its priority. Some interrupts (like a power failure warning) are more urgent than others (like a key press). If the interrupt has a high enough priority, the process continues.
4. Suspend Current Process: The CPU suspends the currently running program.
5. Save Context: The current state of the program is saved. This means the contents of all the CPU's registers (like the Program Counter (PC) and Accumulator (ACC)) are copied to a special location in memory called the stack.
6. Load ISR: The processor identifies the cause of the interrupt and loads the starting address of the appropriate Interrupt Service Routine (ISR) into the Program Counter. An ISR is a small, dedicated program written to handle one specific type of interrupt.
7. Execute ISR: The CPU executes the ISR, which deals with the event (e.g., reads the key press from the keyboard buffer, sends an error message to the screen).
8. Restore Context: Once the ISR is complete, the saved state of the original program is retrieved (popped) from the stack and loaded back into the registers.
9. Resume Original Process: The original program continues its execution from the exact point it was interrupted, as if nothing had happened.