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Interrupts In 8085

what is Interrupt?

Interrupt is a mechanism by which an I/O or an instruction can suspend the normal execution of processor and get itself serviced. Generally, a particular task is assigned to that interrupt signal. In the microprocessor based system the interrupts are used for data transfer between the peripheral devices and the microprocessor.

Interrupt Service Routine(ISR)

A small program or a routine that when executed services the corresponding interrupting source is called as an ISR.

Maskable/Non-Maskable Interrupt

An interrupt that can be disabled by writing some instruction is known as Maskable Interrupt otherwise it is called Non-Maskable Interrupt.

There are 6 pins available in 8085 for interrupt:

  1. TRAP
  2. RST 7.5
  3. RST6.5
  4. RST5.5
  5. INTR
  6. INTA


Execution of Interrupts

When there is an interrupt requests to the Microprocessor then after accepting the interrupts Microprocessor send the INTA (active low) signal to the peripheral. The vectored address of particular interrupt is stored in program counter. The processor executes an interrupt service routine (ISR) addressed in program counter.

There are two types of interrupts used in 8085 Microprocessor:
  1. Hardware Interrupts
  2. Software Interrupts


Software Interrupts

A software interrupts is a particular instructions that can be inserted into the desired location in the rpogram. There are eight Software interrupts in 8085 Microprocessor. From RST0 to RST7.
  1. RST0
  2. RST1
  3. RST2
  4. RST3
  5. RST4
  6. RST5
  7. RST6
  8. RST7
They allow the microprocessor to transfer program control from the main program to the subroutine program. After completing the subroutine program, the program control returns back to the main program.
We can calculate the vector address of these interrupts using the formula given below: 
Vector Address = Interrupt Number * 8

For Example:
RST2:        vector address=2*8    =  16
RST1:        vector address=1*8    =  08
RST3:        vector address=3*8    =  24
Vector address table for the software interrupts:
Interrupt Vector Address
RST0
RST1		 0000H
0008H
RST2
RST3		 0010H
0018H
RST4
RST5		 0020H
0028H
RST6
RST7		 0030H
0038H


Hardware Interrupt

As i have already discussed that there are 6 interrupt pins in the microprocessor used as Hardware Interrrupts given below:
  1. TRAP
  2. RST7.5
  3. RST6.5
  4. RST5.5
  5. INTR


Note:

INTA is not an interrupt. INTA is used by the Microprocessor for sending the acknowledgement. TRAP has highest priority and RST7.5 has second highest priority and so on.

The Vector address of these interrupts are given below:
Interrupt Vector Address
RST7.5 003CH
RST6.5 0034H
RST5.5 002CH
TRAP 0024H


TRAP

It is non maskable edge and level triggered interrupt. TRAP has the highest priority and vectores interrupt. Edge and level triggered means that the TRAP must go high and remain high until it is acknowledged. In case of sudden power failure, it executes a ISR and send the data from main memory to backup memory.
As we know that TRAP can not be masked but it can be delayed using HOLD signal. This interrupt transfers the microprocessor's control to location 0024H.
TRAP interrupts can only be masked by reseting the microprocessor. There is no other way to mask it.

RST7.5

It has the second highest priority. It is maskable and edge level triggered interrupt. The vector address of this interrupt is 003CH. Edge sensitive means input goes high and no need to maintain high state until it is recognized.
It can also be reset or masked by reseting microprocessor. It can also be resetted by DI instruction.

RST6.5 and RST5.5

These are level triggered and maskable interrupts. When RST6.5 pin is at logic 1, INTE flip-flop is set. RST 6.5 has third highest priority and RST 5.5 has fourth highest priority.
It can be masked by giving DI and SIM instructions or by reseting microprocessor.

INTR

It is level triggered and maskable interrupt. The following sequence of events occurs when INTR signal goes high:
  1. The 8085 checks the status of INTR signal during execution of each instruction.
  2. If INTR signal is high, then 8085 complete its current instruction and sends active low interrupt acknowledge signal, if the interrupt is enabled.
  3. On receiving the instruction, the 8085 save the address of next instruction on stack and execute received instruction.
It has the lowest priority. It can be disabled by reseting the microprocessor or by DI and SIM instruction.


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