GATE CS 2001 Q1.12 — System Calls

A software interrupt (often referred to as a system call or trap) is an instruction initiated by a user program to request services from the operating system kernel.

Here is why option C is correct:

• Privileged vs. Non-privileged Mode: User applications run in a restricted environment called user mode (non-privileged mode) to prevent them from accessing hardware directly or executing privileged instructions.
• Switching Modes: When a user application needs to perform an operation requiring privileged access (such as reading from a disk, writing to a file, or allocating memory), it executes a software interrupt instruction.
• Control Transfer: This instruction transitions the processor from user mode to kernel mode (privileged mode) and transfers control to a predefined interrupt handler in the operating system. The OS performs the service safely and then returns control to the user program.

Analysis of other options:

• Option A: While software interrupts can be used for testing, this is not the fundamental architectural reason why a processor needs them.
• Option B: Co-routines are cooperative non-preemptive multitasking blocks that can be implemented entirely in user space using standard jump/call instructions and stack manipulation, without needing OS-level software interrupts.
• Option D: Returning from a standard subroutine is done using a simple return instruction (like RET), which does not require an interrupt.