What is the function of the INTA’ signal in the 8085 microprocessor?
To acknowledge an interrupt request
The 8085 uses a vectored/non-vectored interrupt handshake, and the INTA' (Interrupt Acknowledge) pin is the microprocessor's active-LOW reply to the general-purpose INTR interrupt input. It is an output signal, and its whole purpose is to tell the interrupting device, "I have seen your request and I am ready to service it — now put your instruction on the data bus."
The complete sequence works like this:
- An external device drives the INTR pin HIGH to request service.
- The 8085 finishes executing the current instruction (it never interrupts mid-instruction).
- It samples INTR during the last clock state; if interrupts are enabled (a prior EI was executed and the interrupt-enable flip-flop is set), it accepts the request.
- The processor issues INTA' = LOW to acknowledge the interrupt request. This is exactly the function asked for.
- The device, on seeing INTA', places an RST opcode (or a CALL) on the data bus; the CPU pushes the program counter onto the stack and branches to the interrupt service routine.
Why the other choices are wrong: enabling and disabling interrupts is done in software by the EI and DI instructions (and by the interrupt-enable flip-flop), not by any pin called INTA'. Resetting the processor is the job of the separate RESET IN' input, which clears the program counter and internal registers — a completely different operation. Thus INTA' neither enables, disables, nor resets; it strictly acknowledges an interrupt request.
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