Bootstrap and Process Management

• The bootstrap process is an initial program a computer uses to start when powered up or rebooted.
• It initializes all aspects of the system, from CPU registers to device controllers and memory contents.
• The bootstrap program locates and loads the OS kernel into memory, which remains there.
• The OS starts executing the first process and waits for an event.

Process Management

• A process, also known as a job, refers to program code loaded into a computer's memory for CPU execution.
• It can be an instance of a running program or an entity assigned to a processor and becomes an active entity when loaded into memory.
• When a compiled program is executed, the OS generates a process.
• Execution starts via GUI mouse clicks or command line entry.
• A program is a passive entity residing in secondary memory as a stored file, and one program can have multiple processes.

Program vs Process Differences

• A program contains instructions for a specific task, while a process is an instance of an executing program.
• A program is passive, residing in secondary memory, whereas a process is active, created during execution and loaded into main memory.
• A program exists permanently until deleted, but a process exists for a limited time until its task is completed.
• A program is a static entity with minimal memory requirements, while a process is dynamic with high resource needs like CPU, memory address, and I/O.
• A process has its own control block, known as the Process Control Block (PCB), while a program does not.

Process States

• New
• Ready
• Running
• Waiting
• Terminated

Process Control Block (PCB)

• The Process Control Block (PCB) is a data structure containing process information, also known as a process descriptor or task control block.
• Essential for process management, the PCB defines the current state of the operating system.
• The PCB stores data like the process state (new, ready, running, blocked/waiting, terminated).
• A process number identifies a particular process.
• The program counter stores the address of the next instruction to be executed.
• Registers used by the process, such as accumulators and index registers.
• List of open files associated with the process.
• CPU scheduling information includes process priority and pointers to scheduling queues.
• Memory management information includes page or segment tables and base/limit registers.
• I/O status information includes lists of I/O devices and files used by the process.
• Accounting information includes time limits, account numbers, and CPU usage.
• The PCB is stored in protected memory, sometimes at the beginning of the kernel stack.

Interrupts

• An interrupt is a signal from hardware or software that requires immediate attention.
• It disrupts the normal program flow, allowing the OS to take immediate action.
• An interrupt handler is a program that determines the action to be taken when an interrupt occurs.
• The interrupt type is described and stored for user notification, and the state of the interrupted process is saved, including the program counter and register contents.
• The interrupt is processed, the error message and state of the interrupted process are sent to the user, and the processor resumes normal operation

Types of Interrupts

• Software interrupts are generated by the system or application programs.
  • Examples include interrupts generated by a running process, a restart interrupt, or an internal interrupt (division by zero).
• Hardware interrupts are generated by physical components, such as external or I/O interrupts from peripheral device controllers.