The initial part was , a later A suffix version was upward compatible and usable with the or processor. The combines multiple interrupt input sources into a single interrupt output to the host microprocessor, extending the interrupt levels available in a system beyond the one or two levels found on the processor chip. The was introduced as part of Intel's MCS 85 family in However, while not anymore a separate chip, the A interface is still provided by the Platform Controller Hub or Southbridge chipset on modern x86 motherboards. Up to eight slave s may be cascaded to a master to provide up to 64 IRQs.

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Without it, the x86 architecture would not be an interrupt driven architecture. The function of the A is to manage hardware interrupts and send them to the appropriate system interrupt. This allows the system to respond to devices needs without loss of time from polling the device, for instance. The PIC controls the CPU's interrupt mechanism, by accepting several interrupt requests and feeding them to the processor in order. For instance, when a keyboard registers a keyhit, it sends a pulse along its interrupt line IRQ 1 to the PIC chip, which then translates the IRQ into a system interrupt, and sends a message to interrupt the CPU from whatever it is doing.

Part of the kernel's job is to either handle these IRQs and perform the necessary procedures poll the keyboard for the scancode or alert a userspace program to the interrupt send a message to the keyboard driver.

Without a PIC, you would have to poll all the devices in the system to see if they want to do anything signal an event , but with a PIC, your system can run along nicely until such time that a device wants to signal an event, which means you don't waste time going to the devices, you let the devices come to you when they are ready. It is unlikely that any of these single-PIC machines will be encountered these days.

This was possible due to the A's ability to cascade interrupts, that is, have them flow through one chip and into another.

This gives a total of 15 interrupts. Why 15 and not 16? That's because when you cascade chips, the PIC needs to use one of the interrupt lines to signal the other chip. This two-chip architecture is still used and available in modern systems, and hasn't changed except for the advent of the above-mentioned APIC architecture.

Each of the two PICs in modern systems have 8 inputs. When any of the inputs is raised, the PIC sets a bit internally telling one of the inputs needs servicing.

It then checks whether that channel is masked or not, and whether there's an interrupt already pending. If the channel is unmasked and there's no interrupt pending, the PIC will raise the interrupt line.

On the slave, this feeds IRQ 2 to the master, and the master is connected to the processor interrupt line. When the processor accepts the interrupt, the master checks which of the two PICs is responsible for answering, then either supplies the interrupt number to the processor, or asks the slave to do so. The PIC that answers looks up the "vector offset" variable stored internally and adds the input line to form the requested interrupt number.

After that the processor will look up the interrupt address and act accordingly see Interrupts for more details. Each chip master and slave has a command port and a data port given in the table below. When no command is issued, the data port allows us to access the interrupt mask of the PIC. These default BIOS values suit real mode programming quite well; they do not conflict with any CPU exceptions like they do in protected mode.

It was an IBM design mistake. Consequently it is difficult to tell the difference between an IRQ or an software error. A common choice is to move them to the beginning of the available range IRQs For that, we need to set the master PIC's offset to 0x20 and the slave's to 0x For code examples, see below.

This is just a set of definitions common to the rest of this section. When you enter protected mode or even before hand, if you're not using GRUB the first command you will need to give the two PICs is the initialise command code 0x This command makes the PIC wait for 3 extra "initialisation words" on the data port.

These bytes give the PIC:. This is done via:. It is 8 bits wide. This register is a bitmap of the request lines going into the PIC. When a bit is set, the PIC ignores the request and continues normal operation. Note that setting the mask on a higher request line will not affect a lower line. The IRR tells us which interrupts have been raised. This is a command sent to one of the command ports 0x20 or 0xa0 with bit 3 set.

To read the ISR or IRR, write the appropriate command to the command port, and then read the command port not the data port. To read the IRR, write 0x0a. To read the ISR, write 0x0b. Note that these functions will show bit 2 0x as on whenever any of the PIC2 bits are set, due to the chained nature of the PICs.

Also note that it is not necessary to reset the OCW3 command every time you want to read. The chip remembers what OCW3 setting you used. Disclaimer: I have not tested this last part, but that's what the spec says. This is a spurious IRQ. There are several reasons for the interrupt to disappear. In my experience the most common reason is software sending an EOI at the wrong time.

Also note that some operating systems e. Linux keep track of the number of spurious IRQs that have occurred e. This can be useful for detecting problems in software e. Jump to: navigation , search. Category : Interrupts. Personal tools Log in. Namespaces Page Discussion. Views Read View source View history. About This site Joining Editing help Recent changes. In other languages Deutsch. This page was last modified on 3 May , at This page has been accessed , times.

Contents 1 What does the PIC do?


8259A Interrupt Controller



Intel 8259



Intel 8259A Programmable Interrupt Controller


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