In computer systems, especially in microprocessors like the 8085, the processor is constantly working on instructions. But sometimes, an event occurs that demands immediate attention, like a device sending a signal or a program requesting a specific service.
This urgent “attention request” to the CPU is called an interrupt. An interrupt is like someone tapping you on the shoulder while you’re busy to tell you something important. You stop, deal with the new issue, and then return to your work.
In this blog, you’ll learn what are hardware and software interrupts mean, the difference between hardware and software interrupts, the hardware interrupt vs software interrupt concept in 8085, examples of software and hardware interrupts, and how interrupts are enabled, disabled, and prioritised
What Are Hardware and Software Interrupts?
Before we compare the difference between hardware and software interrupts, let’s first understand them individually.
Hardware Interrupt
A hardware interrupt is triggered by an external device to get the CPU’s attention. Think of it like your phone vibrating when a call comes in, that’s an external signal making you stop and attend to it.
In computers or microprocessors, devices like keyboards, printers, or timers send a signal to the CPU through specific interrupt pins to request processing.
For example:
- In an 8085 microprocessor, hardware interrupts come through pins like INTR, RST7.5, RST6.5, RST5.5, and TRAP.
Software Interrupt
A software interrupt is triggered by a program or instruction inside the CPU itself.
It’s like you remembering mid-task that you need to check something else and temporarily pausing what you’re doing.
In microprocessors, software interrupts are initiated by specific instructions written in the program.
For example:
- In an 8085, software interrupts include RST0 to RST7 instructions.
Hardware Interrupt vs Software Interrupt: The Core Idea
The difference between hardware and software interrupts can be summed up as:
- A hardware interrupt comes from outside the CPU (hardware device).
- A software interrupt comes from inside the CPU (software/program).
Hardware and Software Interrupts in 8085
In an 8085 microprocessor:
Hardware interrupts:
- TRAP: Non-maskable, highest priority, used for emergency tasks.
- RST7.5: Edge-triggered, maskable, high priority.
- RST6.5: Level-triggered, maskable.
- RST5.5: Level-triggered, maskable.
- INTR: Lowest priority, general purpose.
Software interrupts:
- RST0 to RST7: Triggered via program instructions for specific subroutines.
Enabling and Disabling Interrupts
Enabling interrupts means allowing the CPU to respond to incoming interrupt signals.
Disabling interrupts means telling the CPU to temporarily ignore interrupt requests.
In 8085:
- EI instruction enables interrupts.
- DI instruction disables interrupts.
Example: During a sensitive calculation, interrupts might be disabled to prevent errors. Once finished, interrupts are enabled again.
Compare Between Hardware and Software Interrupts
To make the difference between hardware and software interrupts clearer, let’s break it down into a side-by-side comparison.
| Feature | Hardware Interrupt | Software Interrupt |
| Source | External device | Internal program |
| Example in 8085 | TRAP, RST7.5 | RST 0–RST 7 |
| Priority | Fixed hardware priority | Determined by program |
| Trigger | Signal from device | Instruction execution |
| Masking | Some maskable, some not | Maskable |
| Speed | Depends on signal recognition | Immediate |
As you can see, the difference between hardware interrupt and software interrupt, types of interrupts serve the same purpose of pausing current tasks to handle important events, but their source, trigger method, and handling speed set them apart.
Examples of Software and Hardware Interrupts
Hardware Interrupt Examples:
- Keyboard key press.
- Printer ready signal.
- Mouse click.
- Network card receiving data.
Software Interrupt Examples:
- Debugging breakpoints.
- System calls to the operating system.
- RST instruction in the 8085 program.
- Triggering error messages.
Hardware Interrupt vs Software Interrupt: Practical Scenario
Imagine a medical monitoring system using an 8085:
- Hardware interrupt: A heart-rate sensor detects a dangerous reading and sends a signal via TRAP for emergency handling.
- Software interrupt: A program executes an RST instruction to log readings into memory at set intervals.
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Conclusion
In essence, the difference between hardware and software interrupts lies in their source and trigger method:
- Hardware interrupts originate from external devices and are activated by electrical signals.
- Software interrupts originate from within the CPU and are triggered by program instructions.
While hardware interrupts handle unpredictable, urgent events, software interrupts manage planned or routine tasks. Together, they enable a microprocessor, such as the 8085, to be both reactive to the outside world and efficient in executing internal processes.
Frequently Asked Questions (FAQs)
Ans. Enabling interrupts means allowing the CPU to respond to interrupt requests. Disabling interrupts means telling the CPU to ignore them temporarily. In the 8085, EI enables and DI disables maskable interrupts.
Ans. The hardware interrupts of 8085 are TRAP, RST7.5, RST6.5, RST5.5, and INTR. TRAP has the highest priority and is non-maskable, while the rest are maskable with varying priorities.
Ans. Yes, maskable hardware interrupts and software interrupts can be disabled using DI. Non-maskable interrupts like TRAP cannot be disabled.
