LOGO Chapter 1 Interrupt handling

hardware interrupt Under x86, hardware interrupts are called IRQ's. When the CPU receives an interrupt, it stops whatever it's doing (unless it's processing a more important interrupt, in which case it will deal with this one only when the more important one is done), saves certain parameters on the stack and calls the interrupt handler.

Hardware interrupts architecture in x86 IRQ Numbers rior to plug-and-play devices, users had to set IRQ values of devices manually when adding the device, such as a modem or printer, to a system. The following list of IRQ numbers specifies what each of the 16 IRQ lines are used for.plug-and-playIRQ

Example IRQ signal sources IRQ Number Typical UseDescription IRQ 0System timer This interrupt is reserved for the internal system timer. It is never available to peripherals or other devices. IRQ 1Keyboard This interrupt is reserved for the keyboard controller. Even on devices without a keyboard, this interrupt is exclusively for keyboard input. IRQ 2 Cascade interrupt for IRQs This interrupt cascades the second interrupt controller to the first. IRQ 3Second serial port (COM2)serial portCOM2 The interrupt for the second serial port and often the default interrupt for the fourth serial port (COM4). IRQ 4First serial port (COM1) This interrupt is normally used for the first serial port. On devices that do not use a PS/2 mouse, this interrupt is almost always used by the serial mouse. This is also the default interrupt for the third serial port (COM3).PS/2 mouse

IRQ 5Sound card This interrupt is the first choice that most sound cards make when looking for an IRQ setting. IRQ 6Floppy disk controller This interrupt is reserved for the floppy disk controller. IRQ 7First parallel portparallel port This interrupt is normally reserved for the use of the printer. If a printer is not being used, this interrupt can be used for other devices that use parallel ports. IRQ 8Real-time clock This interrupt is reserved for the system's real-time clock timer and can not be used for any other purpose. IRQ 9Open interrupt This interrupt is typically left open on devices for the use of peripherals. IRQ 10Open interrupt This interrupt is typically left open on devices for the use of peripherals. IRQ 11Open interrupt This interrupt is typically left open on devices for the use of peripherals. IRQ 12PS/2 mouse This interrupt is reserved for the PS/2 mouse on machines that use one. If a PS/2 mouse is not used, the interrupt can be used for other peripherals, such as network card.

How hardware interrupt works in PC? A table of interrupt vectors (pointers to routines that handle interrupts).pointersroutines interrupts Use of the interrupt vector table can be triggered by three types of events: hardware interrupts, software interrupts and exceptions, which together are often simply referred to as "interrupts". In total up to 256 of these can be defined, each with their own unique identifier which is called a vector. On PCs, the interrupt vector table consists of byte pointers, and resides in the first 1 K of addressable memory. (after 80386, the interrupt table can be anywhere in the memory, specified by a register called IDTR) Each interrupt number is reserved for a specific purpose. For example, 16 of the vectors are reserved for the 16 IRQ lines.PCs pointersKIRQ

The mapping between IRQs and Interrupt Vector Table This is a list showing what the different IRQ lines in the system is going to be used for. Its also listed what the different IRQ are normally used for, and what Interrupt they are going to be routed to. Here is the list: IRQ Usage IRQIDTNormaly Used ForMost Likely To Be Used For NMI 1 02hMemory parity control 050hSystemtiming 151hKeyboard 252hThe PIC-2 is cascaded to this IRQThe PIC-2 cascade IRQ 8-15 is connected to PIC-2. When PIC-2 receives an interrupt request it uses its own IRQ line to signal PIC-1. When the lowest IRQ has the highest priority, any interrupt request (8-15) coming through the PIC-2 cascade must have a higher priority than interrupt request from 3 to 7. This table has the highest IRQ priority on the top. 853hRTCAvailable for PnP Configuration 954hNetworkAvailable for PnP Configuration 1055hQuaternary IDEAvailable for PnP Configuration 1156hQuaternary IDE / Tertiary IDEAvailable for PnP Configuration 1257hTertiary IDEAvailable for PnP Configuration 1358hExceptions in FPU calculationsAvailable for PnP Configuration 1459hPrimary IDEAvailable for PnP Configuration 155AhSecondary IDEAvailable for PnP Configuration 35BhCOM 2Available for PnP Configuration 45ChCOM 1Available for PnP Configuration 55DhLPTAvailable for PnP Configuration 65EhFloppyAvailable for PnP Configuration 75FhLPTAvailable for PnP Configuration

When an hardware interrupt occurs 1.The CPU stops what it is doing 2.Store its return address (like a call) and status (flags, etc) 3.Retrieve the pointers in the IVT (Interrupt vector table) 4.Jump to the address of the pointer 5.Executing code in the handler 6.The handler usually return by IRET 7.IRET returns the saved address and status (flags) 8.CPU continues its execution from where it is stopped.

Software Interrupt The address in a IVT can be also called by programs, using INT n, where n is the interrupt number In a protected O.S. software interrupt instructions are used as a trap instruction. O.S. uses it to open a gate for user programs to communicate so that protection mode changed can be invoked.

The difference between CALL and INT In x86 assembly language, there are the call (call a subroutine) and ret (return from subroutine) instructions. Before transferring control to the subroutine, call pushes the segment offset address of the instruction following the call onto the stack; ret pops this value off the stack, and jumps to it, effectively returning the flow of control to that part of the program.. There are also two similar instructions, int (interrupt), which saves the current register values on the stack, then performs a far call, except that instead of an address, it uses an interrupt vector, an index into a table of interrupt handler addresses. The matching return from interrupt instruction is iret, which restores the register values after returning. Soft Interrupts of the type described above are used by some operating systems for system calls, and can also be used in debugging hard interrupt handlers. Hard interrupts are triggered by external hardware events.