Exceptions

An application calling read() looks like a normal function call. Illegal instructions, page faults, interrupts, and system calls can also look like “the CPU jumped to another handler.”

But these paths are not normal function calls.

A normal function call stays in the same privilege level and address space. The caller knows the callee address, passes arguments according to the ABI, and returns when the callee finishes. Exceptions and system calls are different: current code cannot freely choose a high-privilege entry. The CPU must switch state according to architectural rules, transfer execution to a controlled entry, and allow the kernel, firmware, or exception handler to decide what happens next.