In microprocessor systems such as the Intel 8085, which statement best describes software interrupts and how they are generated?

Introduction / Context:
This question deals with the concept of software interrupts in microprocessor based systems, with particular reference to the Intel 8085. Interrupts are mechanisms that temporarily halt normal program flow so that the processor can service an event or request. While hardware interrupts are triggered by external signals on interrupt pins, software interrupts are initiated by instructions embedded in the program itself. Understanding this distinction is essential for assembly language programming and interrupt driven design.

Given Data / Assumptions:

We are considering software interrupts rather than hardware interrupts.

The context is microprocessors such as the Intel 8085.

The 8085 provides RST type instructions that act as software interrupt instructions.

Software interrupts are generated internally by executing an instruction, not by an external pin.

A software interrupt typically transfers program control to a predefined interrupt service routine address.

Concept / Approach:
In the 8085, software interrupts are implemented using RST instructions such as RST 0, RST 1, up to RST 7. When the microprocessor executes one of these instructions, it behaves as if an interrupt with a predefined vector address has occurred. The current program counter is saved on the stack, and control jumps to a fixed memory location where the service routine resides. Because the programmer places the RST instruction in code, the interrupt is said to be generated by software rather than by hardware. This mechanism is often used to implement system calls, debug services, or frequently used subroutines.

Step-by-Step Solution:
Step 1: Recall that software interrupts are triggered by special instructions that the program executes.Step 2: In the 8085, identify RST instructions as examples of software interrupt instructions.Step 3: Recognize that executing such an instruction causes the CPU to push the current program counter on the stack and jump to a predefined interrupt vector address.Step 4: Compare this description with the option statements provided in the question.Step 5: Select the option that explicitly states that software interrupts are generated by executing specific program instructions that cause control to jump to predefined service routines.

Verification / Alternative check:
Standard microprocessor references describe software interrupts as program controlled interrupts. In the 8085, each RST n instruction corresponds to a particular address such as 0038H, 0040H, and so on. Execution of the instruction triggers an internal process similar to external interrupt handling but without any external signal. This consistent description matches the correct option and differs clearly from explanations of hardware interrupts, which are tied to pins like TRAP or INTR. Therefore the chosen option accurately captures the essence of software interrupts.

Why Other Options Are Wrong:
Option A describes hardware interrupts, which are generated by external devices through interrupt pins, not software interrupts. Option B refers to noise related phenomena that engineers reduce with decoupling and is not an interrupt type. Option D claims that software interrupts are always non maskable, which is not generally true and does not define them correctly. Option E suggests that interrupts are generated automatically on stack overflow, which is not how standard software interrupts work in the 8085. Only option C explains that software interrupts are created by executing special instructions that transfer control to service routines.

Common Pitfalls:
A common confusion is to treat software and hardware interrupts as completely independent mechanisms, when in fact software interrupts reuse much of the same internal hardware used by external interrupts. Another pitfall is to assume that any unusual event such as stack overflow must be a type of software interrupt, which is not accurate in the 8085 context. To answer exam questions correctly, focus on the source of the interrupt request: external hardware for hardware interrupts and program instructions for software interrupts.

Final Answer:
Software interrupts are interrupts that are generated by executing specific instructions in the program code, which cause control to jump to predefined service routines.