Introduction

Interrupt is one of the most common methods for an external device to communicate with the processor. Figure below shows a general concept of the interrupt in our day-to-day activity. Suppose you are enjoying listening to music, if now your phone bell rings, you stop or pauses your music and attends the phone, after conversation is completed, you restart your paused audio and continue listening the music.

You do not pause your audio every minute to check if phone bell is ringing, which would be an inefficient and slow method.  Your computing device behaves a similar way.

Interrupt in 8085:

We can define interrupt as a process or techniques using which an external slow speed device seek the attention of the microprocessor to informs that it is ready for communication with it. This process of interrupt is generally initiated by an asynchronous in nature. The response to this signal is controlled by the processor wherein it may be rejected or accepted depending on the priority of the interrupt.

The figure 3.2 shows the complete steps in interrupt process, these are:

• Processor executing main program
• The peripheral request for service in the form of an interrupt
• After completing the current task, the processor suspends the program in execution and saves the return addresses
• Load the PC with the address of ISR and jumps to execute the ISR
• Execution continues till an end of interrupt instruction is encountered
• The return address from stack is restored in PC
• The execution of main program continues

There are two ways a CPU can interact with the external devices that are connected to it. One method is polling and the second interrupt.

Polling: In this method the processor will be checking at regular intervals if any of the external device needs service. With this method the processor will be wasting most of its time checking the device instead of doing some useful tasks.

Interrupt: The processor can continue it productive task. Whenever any device wishes to communicate, it will do so by requesting the attention of the CPU. The request may go unnoticed if the processor is executing some higher priority tasks or may give its attention (processing time) and requested service provided to the device. When the processor listens to the interrupt it suspends its current job and takes care of interrupting device. Once the required service is given to the device, the processor goes back to handle the previously executing task.

Types of interrupts in 8085

There are two types of interrupts in 8085 microprocessor, these are hardware interrupt and the software interrupts.

Software Interrupts:

Software interrupts are the interrupts instructions inserted in the main program and when these instructions are encountered a software interrupt take place. There are 8 software interrupt instructions. these are:

These software interrupt instructions are like a CALL instructions but call goes to the above default ISR addresses. Software instructions do not have any priority as the are inserted in the program by programmer.

Hardware interrupts:

hardware interrupts are as a result of the request from the peripherals. These are externally initiated signals. 8085 supports the following 5 types of hardware interrupts:

• Hardware Interrupt	Vector address (IN Hex)	Type of hardware interrupt	Priority Level
  TRAP	0024	Non-Maskable	1
  RST 7.5	003C	Maskable	2
  RST 6.5	0034	Maskable	3
  RST 5.5	002C	Maskable	4
  INTR	Decided by H/W	Maskable	5

Hardware interrupts are further classified as:

1. Maskable and non-maskable interrupts
2. Vectored and non-vectored interrupts.

1. Difference between the maskable and non-maskable interrupts

The table below shows the difference between the maskable and non-maskable interrupts.

2. Difference between Vectored and Non-vectored interrupt

Then these interrupts can be further classified as vectored and non-vectored interrupts. Vectored interrupts are those interrupt whose vector address is known to 8085 whereas non-vectored interrupts are those whose ISR address is to be supplied by interrupting device. The difference between the vectored and non-vectored interrupts is given below:

Recognition of Valid Interrupt:

The interrupt are sensed by the 8085 one cycle before the end of execution of each instruction. Longest instruction for an 8085 microprocessor take 18 clock periods. So an interrupt must be applied for at least 17.5 clock period. This will decide the minimum pulse width for the interrupt signal. Maximum duration of the interrupt is decided by the execution of the EI instruction. Also it important to note that the interrupt signal must be removed before the EI instruction is executed so that the interrupt is not recognized again.

Interrupt Acknowledgement(INTA’)

Pin number 11 on 8085 is the interrupt acknowledgement. In response to the valid INTR interrupt sent by the external device, 8085 generates an active low on INTA’ pin. When the interrupting device receives this active lo signal, the peripheral device will place the machine code of the RST instruction to be executed on the data bus. The processor after issuing the INTA’ reads the code os RST instruction from data bus and places it in the instruction register which is then decoded and executed.