What does a trap instruction do in an operating system?

Question

While studying operating systems concepts, I saw the term trap instruction. "A TRAP instruction is performed to switch from user mode to kernel mode." I failed to understand what a trap instruction does.

Answer 1

There are two ways to enter kernel mode:

1. Interrupt
2. Exception

When either occurs, the processor dispatches to the appropriate handler in its interrupt dispatch table (or similar mechanism). This table is defined by the operating system.

A trap is an exception where the instruction cannot be restarted. In contrast, a fault is an exception where the instruction can be restarted.

Every processor I am aware of has some instruction that explicitly causes an exception in order to get into kernel mode. Such instructions are used to implement operating system services.