Process Creation | System Calls | Mode & Context Switching | Overhead | Easy Learning IT Classroom
Objectives;
• Process creation + System Calls
• Dual Mode Execution (Mode Switching)
• Processing a System Call
• Context Switching
• Overhead
System Calls:
• System calls allow processes running at the user mode to access kernel functions that run under the kernel mode
• Prevent processes from doing bad things, such as
• Halting the entire operating system
• Modifying the MBR
System Calls Contd.:
• Serve as an entry point to OS code
• Allows users to request OS services
• API’s/library functions usually provide an interface to system calls
• e.g, language-level I/O functions map user parameters into system-call format
• Thus, the run-time support system of a prog. language acts as an interface between programmer and OS interface
Process Creation:
• In general-purpose systems, some way is needed to create processes as needed during operation. There are four principal events led to processes creation.
• System initialization.
• Execution of a process Creation System calls by a running process.
• A user request to create a new process.
• Initialization of a batch job.
Process Creation: Following are some reasons for creation of a process
• User logs on.
• User starts a program.
• Operating systems creates process to provide service, e.g., to manage printer.
• Some program starts another process, e.g., Netscape calls xv to display a picture.
Mode Switching
• System calls allow boundary to be crossed
• System call initiates mode switch from user to kernel mode
• User Mode Kernel Mode
• Special instruction – “software interrupt” – calls the kernel function
• transfers control to a location in the interrupt vector table
• OS executes kernel code, mode switch occurs again when control returns to user process
Processing a System Call:
• Before we discuss processing of system call, one should know what is Context Switch and an Overhead
• Context switching is an Overhead.
• Overhead, time that doesn’t involve any user process execution, but it is needed to accomplish various important tasks.
Context Switching: Switching the CPU to another process requires saving the state of the old process and loading the saved state for the new process
Processing a System Call contd.:
• Switching between kernel and user mode is time consuming
• Kernel must
• Save registers so the executing process can resume execution
• Other overhead is involved; e.g. cache misses, & prefetch
• Verify system call name and parameters
• Call the kernel function to perform the service
• On completion, restore registers and return to caller
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Lecture 1 - Introduction to C++ (HINDI/URDU) Part 1 | Easy Learning IT Classroom–
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