INTERRUPT JOURNEY FROM HARDWARE TO SOFTWARE - 1

Interrupt is a very vast topic. This post is about How Interrupt is generated from Peripheral and routed to the processor. Exactly till we get an IRQ number.

INTERRUPT: 

    An event external to the currently executing process that causes a change in the normal flow of instruction execution, usually generated by hardware devices external to the CPU sometimes by software.

We will deal software interrupts and exceptions in a separate post.

Hardware interrupts are asynchronous in nature.

How it is asynchronous ? 

Interrupt will not wait for nay other CPU routine to complete. It can be raised even in the middle of CPU execution.

Interrupt Vs Polling

Polling is a technique used by CPU to check for events in the peripheral device in a periodic manner.

Pros:

Efficient if interrupts arrive frequently.

Cons:

Takes precious CPU time even when there is no request from external device.

Each hardware interrupt has an interrupt level, trigger, and interrupt priority.

The following sections describe various interrupt components:

Interrupt Level

The interrupt level defines the source of the interrupt and is often referred to as the interrupt source. There are basically two types of interrupt levels: system and bus. The bus interrupts are generated by the devices on the buses (such as PCI, ISA, VDEVICE, and PCI-E). Examples of system interrupts are the timer and Environmental and Power Off Warning (EPOW).

Interrupt Trigger

There are two types of trigger mechanisms, level-triggered interrupts and edge-triggered interrupts.

Level-Triggered: A level-triggered interrupt module always generates an interrupt whenever the level of the interrupt source is asserted.

A device sends a signal (Setting this line to active), and keeps it at that state until the interrupt is serviced.
Typically, the processor samples the interrupt input at predefined times during each bus cycle
Pro:Level-triggered interrupt is favored by some because it is easy to share the interrupt Request line without losing the interrupts, when multiple shared devices interrupt at the same time. Upon detecting assertion of the interrupt line, the CPU must search through the devices sharing the interrupt Request line until one who triggered the interrupt is detected.
Con: As long as any device on the line has an outstanding request for service the line remains asserted, so it is not possible to detect a change in the status of any other device.

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An edge-triggered interrupt is an interrupt signalled by a level transition on the interrupt line, either a falling edge (high to low) or a rising edge (low to high). A device, wishing to signal an interrupt, drives a pulse onto the line and then releases the line to its inactive state.
Hybrid.

Some systems use a hybrid of level-triggered and edge-triggered signalling. The hardware not only looks for an edge, but it also verifies that the interrupt signal stays active for a certain period of time.

A common use of a hybrid interrupt is for the NMI (non-maskable interrupt) input.

A message-signaled interrupt does not use a physical interrupt line. Instead, a device signals its request for service by sending a short message over some communications medium, typically a computer bus. The message might be of a type reserved for interrupts, or it might be of some pre-existing type such as a memory write. This is been used by PCI Express devices.

To generate an interrupt request PCI Express device initiates a memory write transaction targeting north bridge (APIC Interrupt controller). The data written is a unique interrupt vector associated with device generating the interrupt.

Interrupt controller interrupts the CPU through North bridge and the vector is delivered to the CPU.

We will have a separate post on MSI.

doorbell interrupt is often used to describe a mechanism whereby a software system can signal or notify a computer hardware device that there is some work to be done. Typically, the software system will place data in some well known and mutually agreed upon memory location(s), and "ring the doorbell" by writing to a different memory location.

Interrupt Priorities

The interrupt priority defines which of a set of pending interrupts is serviced first.

INTERRUPT STATES

Inactive An interrupt that is not active or pending.
Pending An interrupt from a source to the PIC that is recognized as asserted in hardware or generated by software and is waiting to be serviced by a target processor.
Active An interrupt from a source to the PIC that has been acknowledged by a processor, and is being serviced but has not completed.
Active and pending A processor is servicing the interrupt and the PIC has a pending interrupt from the 
same source.

 

 programmable interrupt controller (PIC) is a device that is used to combine several sources of interrupt onto one or more CPU lines, while allowing priority levels to be assigned to its interrupt outputs. 

INTERRUPT CONTROLLER INTERNALS

Delivering the interrupt from Interrupt controller to processor completes with Interrupt acknowledgement sequence. As a result Processor will be getting the IRQ number.

1. Eight interrupt lines IR0-IR7 are connected to the interrupt request register (IRR). The IRR is eight bits wide, where every bit corresponds to one of the lines IR0-IR7. The device requiring service signals the PIC via one of the eight pins IR0-IR7 setting it to a high level. The 8259A then sets the corresponding bit in the IRR.
2. At the same time, the PIC activates its output INT line to inform the processor about the interrupt request. The INT line is directly connected to the INTR input of the processor. This starts an interrupt acknowledge sequence.
3. The CPU receives the INT signal, finishes the currently executing instruction and outputs a first interrupt acknowledge (INTA) pulse if the IE flag is set (that is, if the interrupts are not masked at the CPU).
4. Upon receival of the first INTA pulse from the CPU the highest priority in the IRR register is cleared and the corresponding bit in the in-service register (ISR) register is set.There is no PIC activity on the data bus in this cycle.
5. The processor initiates a second INTA pulse and thus causes the 8259A to put an 8-bit number onto the data bus. This 8 bit number is the IRQ number. The CPU reads this number as the number of the interrupt handler to call, through IDT-Interrupt Descriptor Table, which is then fetched and executed.
6. In the Automatic End Of Interrupt (AEOI) Mode the ISR bit is reset at the end of the second INTA pulse. Otherwise, the CPU must issue an End of Interrupt (EOI) to the 8259A PIC when executing the interrupt handler to clear the ISR bit manually.

The above procedure will give you a clear picture on how Interrupt is routed from Peripheral device to the Processor.

***More to be posted on Interrupt descriptor Table and how OS handles it.