Data transfer between the CPU and the peripherals is initiated by the CPU. But the CPU cannot start the transfer unless the peripheral is ready to communicate with the CPU. When a device is ready to communicate with the CPU, it generates an interrupt signal. A number of input-output devices are attached to the computer and each device is able to generate an interrupt request.
The main job of the interrupt system is to identify the source of the interrupt. There is also a possibility that several devices will request simultaneously for CPU communication. Then, the interrupt system has to decide which device is to be serviced first.
A priority interrupt is a system which decides the priority at which various devices, which generates the interrupt signal at the same time, will be serviced by the CPU. The system has authority to decide which conditions are allowed to interrupt the CPU, while some other interrupt is being serviced. Generally, devices with high speed transfer such as magnetic disks are given high priority and slow devices such as keyboards are given low priority.
When two or more devices interrupt the computer simultaneously, the computer services the device with the higher priority first.
Following are some different types of interrupts:
When the signal for the processor is from an external device or hardware then this interrupts is known as hardware interrupt.
Let us consider an example: when we press any key on our keyboard to do some action, then this pressing of the key will generate an interrupt signal for the processor to perform certain action. Such an interrupt can be of two types:
The hardware interrupts which can be delayed when a much high priority interrupt has occurred at the same time.
The hardware interrupts which cannot be delayed and should be processed by the processor immediately.
The interrupt that is caused by any internal system of the computer system is known as a software interrupt. It can also be of two types:
The interrupts that are caused by software instructions are called normal software interrupts.
Unplanned interrupts which are produced during the execution of some program are called exceptions, such as division by zero.
This way of deciding the interrupt priority consists of serial connection of all the devices which generates an interrupt signal. The device with the highest priority is placed at the first position followed by lower priority devices and the device which has lowest priority among all is placed at the last in the chain.
In daisy chaining system all the devices are connected in a serial form. The interrupt line request is common to all devices. If any device has interrupt signal in low level state then interrupt line goes to low level state and enables the interrupt input in the CPU. When there is no interrupt the interrupt line stays in high level state. The CPU respond to the interrupt by enabling the interrupt acknowledge line. This signal is received by the device 1 at its PI input. The acknowledge signal passes to next device through PO output only if device 1 is not requesting an interrupt.
The following figure shows the block diagram for daisy chaining priority system.