Interrupt Service Routine (ISR) and Execution in 8086 Microprocessor: Detailed Steps | 8086

Summary

This video tutorial explores the execution of interrupts in the 8086 microprocessor, detailing the step-by-step process that occurs when an interrupt is triggered during program execution. It covers the importance of the Interrupt Vector Table, the necessary steps to save the program's state, and the distinction between handling interrupts and normal subroutines. Key operations such as pushing and popping the Flag Register, Code Segment, and Instruction Pointer are emphasized. Viewers will gain a clear understanding of how to manage interrupts effectively, ensuring program integrity while responding to external events.

Takeaways

• 😀 The 8086 microprocessor can handle 256 interrupts, each with a defined vector address in the Interrupt Vector Table (IVT).
• 📄 During the execution of a main program, if an interrupt occurs, the 8086 must complete the current instruction before servicing the interrupt.
• 🔍 The status of the program is stored in the flag register, which is essential for resuming the main program after handling the interrupt.
• 🔗 The next instruction's physical address, determined by the code segment (CS) and instruction pointer (IP), is stored on the stack to allow for proper resumption of the main program.
• 🔄 The 8086 executes a 'push flag' operation to save the current status of the program before servicing the interrupt.
• 🚫 The Interrupt Flag (IF) and Trap Flag (TF) are cleared to prevent any additional interrupts from being processed during the interrupt service.
• 💾 The physical address of the main program is saved on the stack by pushing CS and IP values, with CS pushed first as it holds the higher address.
• 📡 The interrupt service routine (ISR) is executed by retrieving its address from the IVT, allowing the processor to respond to the interrupt.
• 🔁 After the ISR is completed, the processor uses the 'IRET' instruction to return to the main program, restoring the flag register and program status.
• ⚙️ Unlike normal subroutine calls, interrupt service routines require pushing and popping the flag register, as well as managing different segments for the main program and ISR.

Q & A

What is the primary focus of the video?

The video focuses on explaining the execution of interrupts in the 8086 microprocessor, detailing the steps involved in handling an interrupt during program execution.

What happens when an interrupt occurs while a main program is executing?

When an interrupt occurs, the 8086 microprocessor must complete the current instruction and then handle the interrupt by executing the corresponding interrupt service routine.

What are the key components involved in identifying the location of the main program?

The key components include the Code Segment Register (CS), which holds the higher address of the main program, and the Instruction Pointer (IP), which holds the lower address.

How does the 8086 processor manage the current program's status before servicing an interrupt?

Before servicing an interrupt, the 8086 saves the current status of the program, including the contents of the flag register and the physical address of the next instruction, onto the stack.

What does the instruction 'PUSH FLAG' do during interrupt handling?

The 'PUSH FLAG' instruction saves the status of the flag register onto the stack, preserving the current state of the program before the interrupt is serviced.

Why are the Interrupt Flag (IF) and Trap Flag (TF) cleared during interrupt handling?

The IF and TF are cleared to disable further interrupts and prevent them from being processed during the execution of the current interrupt service routine, ensuring it runs without interruption.

What is the role of the Interrupt Vector Table (IVT)?

The Interrupt Vector Table (IVT) stores the addresses of all possible interrupts (0 to 255), allowing the 8086 to retrieve the correct address for the interrupt service routine to execute.

What is the significance of the instruction 'IRET' in the context of interrupts?

'IRET' stands for 'Interrupt Return' and is used to return control to the main program after completing the interrupt service routine, restoring the program's previous state.

How does the 8086 differentiate between interrupt subroutines and normal subroutines?

The 8086 differentiates them by requiring the saving and restoring of the flag register in interrupt subroutines, while normal subroutines do not need to preserve the flag state.

What is meant by an 'inter-segment branch' in the context of interrupt execution?

An 'inter-segment branch' occurs when the interrupt service routine is located in a different segment than the main program, necessitating the saving and restoring of segment registers.