In the Intel 8085 microprocessor interrupt system, which hardware interrupt line has the highest priority among all external interrupt inputs?

Introduction / Context:
This question tests understanding of the priority structure of hardware interrupts in the Intel 8085 microprocessor. When multiple interrupt requests occur at the same time, the CPU must decide which one to service first. The 8085 defines a fixed priority order for its hardware interrupt lines, and knowing which interrupt has the highest priority is important when designing real time systems, safety critical responses, or interrupt driven programs.

Given Data / Assumptions:

We are dealing specifically with the hardware interrupts of the 8085 microprocessor.

The hardware interrupt lines are TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR.

Each interrupt line has a defined priority level relative to the others.

Higher priority interrupts can preempt or be serviced before lower priority ones when multiple requests are pending.

We need to identify the single highest priority hardware interrupt.

Concept / Approach:
In the 8085, the hardware interrupts are arranged in a fixed priority chain. At the top of this chain is TRAP, which is both the highest priority interrupt and a non maskable interrupt. Non maskable means that it cannot be disabled by the usual interrupt enable or mask bits. Below TRAP, the priority order is RST 7.5, RST 6.5, RST 5.5, and finally INTR as the lowest priority. Because of its non maskable nature and highest priority, TRAP is commonly reserved for critical events like power failure or serious hardware faults.

Step-by-Step Solution:
Step 1: Recall the list of hardware interrupts in the 8085: TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR.Step 2: Remember the documented priority order where TRAP is at the top, followed by the RST interrupts, and INTR at the lowest level.Step 3: Note that TRAP is also non maskable, which reinforces its special status and high priority.Step 4: Compare this knowledge with the options, all of which list valid interrupt lines.Step 5: Select TRAP as the hardware interrupt line with the highest priority in the 8085.

Verification / Alternative check:
Standard pin diagrams and interrupt tables for the 8085 clearly mark TRAP as a non maskable interrupt. Documentation explains that TRAP cannot be masked by software and that it is intended for conditions that must not be ignored, such as power fail or emergency shutdown signals. This special status is consistent only with the highest possible priority. The other interrupts RST 7.5, RST 6.5, RST 5.5, and INTR are maskable and can be disabled or masked individually, which confirms that they must have lower priority than TRAP.

Why Other Options Are Wrong:
RST 7.5, RST 6.5, and RST 5.5 are vectored maskable interrupts with priority among themselves but all are lower than TRAP. INTR is a general purpose maskable interrupt and is in fact the lowest priority among the hardware interrupt lines, so it cannot be the correct answer. While RST 7.5 has the highest priority among the maskable interrupts, it still does not exceed TRAP. Therefore, only TRAP satisfies the requirement of being the highest priority hardware interrupt.

Common Pitfalls:
A common mistake is to focus only on the RST interrupts and forget about TRAP, especially if the learner has been working mainly with maskable interrupts in programming examples. Another pitfall is to confuse highest maskable priority with absolute highest priority. To avoid these errors, remember that TRAP stands alone as both non maskable and highest priority. When the exam asks for the highest priority interrupt in 8085, TRAP is the correct answer.

Final Answer:
The highest priority hardware interrupt in the Intel 8085 microprocessor is TRAP.