In the [8086](https://www.ampheo.com/search/8086) [microprocessor](https://www.ampheo.com/c/microprocessors), the interrupt with the lowest priority is the single-step interrupt, generated internally when the Trap Flag (TF) is set.

Here's a detailed breakdown of the 8086's interrupt priority hierarchy and why the single-step interrupt sits at the bottom.


**The Official 8086 Interrupt Priority Order**
From highest to lowest priority, the hierarchy is:

1. Internal Hardware Interrupts & Software INT instructions: Divide Error (INT 0), Single-Step (INT 1), INTO (INT 4), INT *n* (e.g., INT 21h for DOS services).

Note: This is a broad category. The INT n instruction, while software-triggered, has the highest priority level within the microprocessor's internal logic.

2. Non-Maskable Interrupt (NMI): A hardware interrupt for critical events like power failure or memory parity errors. It cannot be disabled by software.
3. Maskable Interrupt (INTR): The common hardware interrupt pin, which can be disabled by clearing the Interrupt Flag (IF) with the CLI instruction. This is how I/O devices (like keyboard, timer, disk) request service.
4. Single-Step Interrupt (Generated by Trap Flag): The lowest priority interrupt.

**Why is the Single-Step Interrupt the Lowest?**
The reason is deeply tied to its purpose: debugging.

* Function: When the Trap Flag (TF) is set to 1, the 8086 automatically generates an internal interrupt (type 1) after the execution of every single instruction. This allows a debugger (like DEBUG.COM in DOS) to take control after each instruction, display the state of registers and memory, and let the programmer step through code.
* The Priority Logic: If the single-step interrupt had a high priority, it would disrupt the normal handling of other critical interrupts.

* Imagine you are single-stepping through a program that communicates with a hard disk via the INTR pin.
* If a disk INTR occurs, the 8086 must service it to avoid losing data. If the single-step interrupt had higher priority, the CPU would finish the current instruction, service the single-step interrupt (showing the debugger), and then service the disk. This delay could be catastrophic.
* By giving it the lowest priority, the 8086 ensures that all other important interrupts (NMI for critical errors, INTR for I/O) are serviced before the debugger gets control. This makes the system stable and responsive even during debugging.

In essence, the debugger (single-step) should observe the system, including its interrupt activity, without interfering with it.

**Visual Summary of the Interrupt Priority**
The following chart illustrates the hierarchy of [8086](https://www.ampheoelec.de/search/8086) interrupts, from the highest-priority internal events to the lowest-priority single-step debugger:

**Key Takeaways**
* Lowest Priority: Single-Step Interrupt (via Trap Flag).
* Reason: To ensure debugging does not interfere with the timely servicing of critical hardware events and I/O.
* Practical Use: This priority scheme is what allows developers to reliably debug complex, interrupt-driven software on the 8086.