Priority Interrupt Controller Background A priority interrupt controller PIC is used to place interrupt requests into a hierarchy: If an interrupt request at a certain level in the hierarchy is being serviced, then that servicing cannot be interrupted by requests at the same level or lower. Requests for interrupts that are at higher levels in the hierarchy will suspend the servicing of interrupts at lower levels. These port addresses depend on the particular PIC being accessed. Our machines have only one PIC, whose first port address is 20H.

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Among all these, only INTR is a non-vectored type of interrupt, rest are vectored interrupts. We know vectored interrupts are those interrupts whose ISR address is known to the processor. Or we can say in case of vectored interrupts, the processor holds the address of the memory location where ISR is stored. So, in this case, the interrupt generating device provides the ISR address to the microprocessor.

An has 5 major interrupts for which a fixed number of lines are present in the chip. But there are many devices connected to a processor. So, for such a case the processor must have more number of lines to handle several interrupts. But it is not practically possible to increase the number of lines each time with the increase in the number of interrupts.

So, to overcome this problem PIC chip is used. As we have already discussed that the processor holds the address of ISR in case of vectored interrupts. So, it is not possible to combine a non-vectored interrupt with a vectored one. Therefore, is used to combine various interrupts which are non-vectored in nature. Also, suppose in some way or the other, two devices generate interrupt simultaneously through a common line without the involvement of This shows the necessity of The programmable interrupt controller tells the microprocessor about the interrupt.

Basically the external devices initially interrupt the and further the interrupts the microprocessor. Features of The programmable interrupt controller has 8 interrupt pins thus can handle 8 interrupt inputs. The priority of interrupts in can be programmed. The priority of interrupts is decided by the different operating modes. We know that a single can handle 8 interrupt inputs but by cascading multiple , it can handle maximal 64 interrupt inputs. If multiple interrupts are generated, then holds the status of interrupts that are masked, in-service and pending.

It reduces the software and real-time overhead generated due to handling multilevel priority interrupts. Architecture of The figure below shows the architectural representation of programmable interrupt controller: It has an 8-bit of data bus. As we have already discussed that never services the interrupt, it simply forwards the interrupts to the microprocessor. Thus, the above architecture has different units that combinely functions to increase the interrupts handled by the processor.

Let us understand the operation performed by each unit in detail: 1. Data Bus Buffer: has tri-stated bidirectional 8-bit data bus buffer i. It holds initialization command word register and operation command word register inside which various control formats exist that are needed for the device operation. Basically, these pins are used by the processor for read and write operations. A low signal at CS i. Control Logic: This unit is the heart of the architecture of It controls the overall operation of the system by sending the INTR signal to the processor whenever an interrupt request is generated.

Also, it receives INTA signal by the processor when microprocessor demands for the address of the interrupt service routine. The control logic is responsible for sending the address of the desired interrupt service routine through the data bus. Interrupt request register IRR : This unit stores the interrupt requests generated by the peripheral devices. We know that has 8 interrupt request pins i. So, the unit can store 8 interrupt requests that are requesting the service from the processor.

Priority Resolver: This logic unit decides that among all the interrupt request present in the IRR which holds the highest priority and needs to be executed first. Suppose at the time of servicing an interrupt, another incoming interrupt request gets generated then that request will be ignored as the one in-service is holding the highest priority. But in case the incoming request has greater priority than the one which is being in current execution then that respective bit will be set in ISR and INTR signal is sent to the microprocessor.

This simply means that only the interrupt holding the highest priority will be forwarded by the to the processor. In-service register: Here the name of the unit is itself indicating the operation performed by it. This register unit stores the interrupts which are currently being executed by the processor. The priority resolver sets each bit of ISR and after getting interrupt word command by the processor, the bits get reset.

As the processor holds the ability to directly read the status of in-service register. Interrupt mask register: This register unit holds the masking bit of those interrupts which are to be masked. Through operation command word OCW the processor sends the required information and programs the interrupt mask register. The unit allows the comparison of IDs of different s cascaded together. It permits the operation of the system in two modes: master mode and slave mode.

In the master mode of operation, it acts as a cascaded buffer. Whereas in slave mode, this unit acts as a comparator. Among the various cascaded , one directly handles the interrupts by forming a connection with the processor and it is known to be master While the other s that interrupts the master are known as slave Each of the s can be separately programmed as all of them holds a specific address. For the slave devices, these pins act as input pins while for a master device these acts as output pins.

In this way, a programmable interrupt controller operates. You Might Also Like:.

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8259A Interrupt Controller

Draw the pin diagram of PIC The following shows the pin details of PIC Draw the functional block diagram of PIC The following shows the functional block diagram of The block schematic of the interconnections is shown below: Fig.


Intel 8259

There are 5 hardware interrupts and 2 hardware interrupts in and respectively. But by connecting with CPU, we can increase the interrupt handling capability. For example, Interfacing of and increases the interrupt handling capability of microprocessor from 5 to 8 interrupt levels. It can be programmed either in level triggered or in edge triggered interrupt level.


8259 Programmable Interrupt Controller

Programming an in conjunction with DOS and Microsoft Windows has introduced a number of confusing issues for the sake of backwards compatibility, which extends as far back as the original PC introduced in The first issue is more or less the root of the second issue. DOS device drivers are expected to send a non-specific EOI to the s when they finish servicing their device. This was done despite the first 32 INTINT1F interrupt vectors being reserved by the processor for internal exceptions this was ignored for the design of the PC for some reason.


8259 PIC Microprocessor


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