• An interrupt is a request of the processor to suspend its current program and transfer control to a new program called the Interrupt Service Routine (ISR).
• Special hardware mechanisms that are designed for maximum speed force the transfer.
• The ISR determines the cause of the interrupt, takes the appropriate action, and then returns control to the original process that was suspended.
• The structure of the processor of any computer is conceived so it can carry out instructions endlessly. As soon as an instruction has been executed, the next one is loaded and executed.
• Even if it appears the computer is inactive, when it is waiting in the DOS prompt or in Windows for your next action, it does not mean it has stopped working, only to start again when instructed to.
• No, not at all. In fact, many routines are always running in the background independently of your instructions, such as checking the keyboard to determine whether a character has been typed in.
• Thus, a program loop is carried out. To interrupt the processor in its never-ending execution of these instructions, a so-called interrupt is issued. That is why it is possible for you to reactivate the CPU whenever you press a key (fortunately...).
• Another example, this time an internal one, is the timer interrupt, a periodic interrupt, that is used to activate the resident program PRINT regularly for a short time.
• Hardware interrupts, you can distinguish three types of interrupts:
  • Software
  • Hardware
  • Exceptions

1) Software Interrupts

• Software interrupts are initiated with an INT instruction and, as the name implies, are triggered via software. For example, the instruction INT 33h issues the interrupt with the hex number 33h.
• In the real mode address space of the i386, 1024 (1k) bytes are reserved for the interrupt vector table (IVT). This table contains an interrupt vector for each of the 256 possible interrupts.
• Every interrupt vector in real mode consists of four bytes and gives the jump address of the ISR (also known as interrupt handler) for the particular interrupt in segment:offset format.
• When an interrupt is issued, the processor automatically transfers the current flags, the code segment CS and the instruction pointer EIP (or IP in 16-bit mode) onto the stack.
• The interrupt number is internally multiplied by four and then provides the offset in the segment 00h where the interrupt vector for handling the interrupt is located.
• The processor then loads EIP and CS with the values in the table. That way, CS:EIP of the interrupt vector gives the entry point of the interrupt handler. The return to the original program that launched the interrupt occurs with an IRET instruction.
• Software interrupts are always synchronized with program execution; this means that every time the program gets to a point where there is an INT instruction, an interrupt is issued.
• This is very different from hardware interrupts and exceptions as you'll soon find out.

2) Hardware Interrupts

• As the name suggests, these interrupts are set by hardware components (like for instance the timer component) or by peripheral devices such as a hard disk. There are two basic types of hardware interrupts: Non Maskable Interrupts (NMI) and (maskable) Interrupt Requests (IRQ).
• An NMI in the PC is, generally, not good news as it is often the result of a serious hardware problem, such as a memory parity error or a erroneous bus arbitration. An NMI cannot be suppressed (or masked as the name suggests). This is quite easy to understand since it normally indicates a serious failure and a computer with incorrectly functioning hardware must be prevented from destroying data.
• Interrupt requests, on the other hand, can be masked with a CLI instruction that ignores all interrupt requests. The opposite STI instruction reactivates these interrupts. Interrupt requests are generally issued by a peripherical device.
• Hardware interrupts (NMI or IRQ) are, contrary to software interrupts, asynchronous to the program execution. This is understandable because, for example, a parity error does not always occur at the same program execution point. This makes the detection of program errors very difficult if they only occur in connection with hardware interrupts.

3) Exceptions

• This particular type of interrupt originates in the processor itself. The production of an exception corresponds to that of a software interrupt.
• This means that an interrupt whose number is set by the processor itself is issued. When do exceptions occur? Generally, when the processor can't handle alone an internal error caused by system software.

There are three main classes of exceptions which I will discuss briefly.

• Fault: A fault issues an exception prior to completing the instruction. The saved EIP value then points to the same instruction that created the exception. Thus, it is possible to reload the EIP (with IRET for instance) and the processor will be able to re-execute the instruction, hopefully without another exception.

• Trap: A trap issues an exception after completing the instruction execution. The saved EIP points to the instruction immediately following the one that gave rise to the exception. The instruction is therefore not re-executed again. Why would you need this? Traps are useful when, despite the fact the instruction was processed without errors, program execution should be stopped as with the case of debugger breakpoints.

• Abort: This is not a good omen. Aborts usually translate very serious failures, such as hardware failures or invalid system tables. Because of this, it may happen that the address of the error cannot be found. Therefore, recovering program execution after an abort is not always possible.