Difference Between Interrupt and Trap

Interrupt vs Trap

In any computer, during its normal execution of a program, there could be events that can cause the CPU to temporarily halt. Events like these are called interrupts. Interrupts can be caused by either software or hardware faults. Hardware interrupts are called (simply) Interrupts, while software interrupts are called Exceptions or Traps. An Exception is an automatically generated software interrupt, while a Trap is a software-invoked interrupt initiated by the programmer. Once an interrupt (software or hardware) is raised, the control is transferred to a special subroutine called ISR (Interrupt Service Routine) that can handle the conditions that are raised by the interrupt.

What is an Interrupt?

The term Interrupt is usually reserved for hardware interrupts. They are program control interruptions caused by external hardware events. Here, external means external to the CPU. Hardware interrupts usually come from many different sources such as timer chip, peripheral devices (keyboards, mouse, etc.), I/O ports (serial, parallel, etc.), disk drives, CMOS clock, expansion cards (sound card, video card, etc). That means hardware interrupts almost never occur due to some event related to the executing program. For example, an event like a key press on the keyboard by the user, or an internal hardware timer timing out can raise this kind of interrupt and can inform the CPU that a certain device needs some attention. In a situation like that the CPU will stop whatever it was doing (i.e. pauses the current program), provides the service required by the device and will get back to the normal program. When hardware interrupts occur and the CPU starts the ISR, other hardware interrupts are disabled (e.g. in 80×86 machines). If you need other hardware interrupts to occur while the ISR is running, you need to do that explicitly by clearing the interrupt flag (with sti instruction). In 80×86 machines, clearing the interrupt flag will only affect hardware interrupts.

What is a Trap?

A Trap can be identified as a transfer of control, which is initiated by the programmer. The term Trap is used interchangeably with the term Exception (which is an automatically occurring software interrupt). But some may argue that a trap is simply a special subroutine call. So they fall in to the category of software-invoked interrupts. For example, in 80×86 machines, a programmer can use the int instruction to initiate a trap. Because a trap is always unconditional the control will always be transferred to the subroutine associated with the trap. The exact instruction, which invokes the routine for handling the trap is easily identified because an explicit instruction is used to specify a trap.

What is the difference between Interrupt and Trap?

Interrupts are hardware interrupts, while traps are software-invoked interrupts. Occurrences of hardware interrupts usually disable other hardware interrupts, but this is not true for traps. If you need to disallow hardware interrupts until a trap is served, you need to explicitly clear the interrupt flag. And usually the interrupt flag on the computer affects (hardware) interrupts as opposed to traps. This means that clearing this flag will not prevent traps. Unlike traps, interrupts should preserve the previous state of the CPU.