How do system calls enable user-space programs to request kernel services, and what is the typical transition mechanism?

• A. User mode handles services directly without transitions
• B. Kernel continuously polls user memory to detect requests
• C. System calls switch to supervisor mode via hardware exception; kernel does nothing
• D. System calls trap into kernel mode via software interrupt or syscall instruction; kernel validates, performs action, then returns to user mode

Answer: (D)

System calls create a protected gateway from user space to the kernel. A user-space program requests a service by invoking a library function that issues a syscall instruction (or an equivalent software interrupt), which traps into kernel mode. This switch changes the processor into kernel (supervisor) mode and switches to the kernel’s address and stack context. In kernel mode, the kernel validates the arguments and permissions, performs the requested action (such as I/O, process management, or memory management), and then returns control to user mode, delivering the result to the program. This mechanism provides isolation and protection, ensuring user programs cannot execute privileged instructions directly.

Directly handling services in user mode isn’t allowed due to protection; polling the kernel for requests would be inefficient; and the kernel does perform work after the transition rather than doing nothing.