# What is config KERNEL INTERRUPT PRIORITY?

**URL:** <https://forums.freertos.org/t/what-is-config-kernel-interrupt-priority/11501>\
**Category:** Kernel\
**Created:** [January 10, 2021, 10:59am UTC](https://forums.freertos.org/t/what-is-config-kernel-interrupt-priority/11501 "2021-01-10T10:59:19Z")\
**Posts on this page:** 1\
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**Author:** ![aggarg](https://d3iykfbgfh8zhg.cloudfront.net/user_avatar/forums.freertos.org/aggarg/32/4245_2.png) [@aggarg](https://forums.freertos.org/u/aggarg)\
**Post date:** [January 19, 2021, 9:10pm UTC](https://forums.freertos.org/t/what-is-config-kernel-interrupt-priority/11501/8 "2021-01-19T21:10:36Z")

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> Why is it that “Interrupts that call API functions must also execute at this priority”? Is there an example? What is the treatment different between interrupts which **call** , and **don’t call** , API functions?

For the discussion below, lets make the following assumptions:

1. Numerically higher priority interrupts have logically higher priority (i.e. priority 2 interrupt can preempt priority 1 interrupt handler).
2. The port implements `configKERNEL_INTERRUPT_PRIORITY` and `configMAX_SYSCALL_INTERRUPT_PRIORITY`.

FreeRTOS critical sections disable interrupts up to `configMAX_SYSCALL_INTERRUPT_PRIORITY`. Critical sections are the sections of code which manipulate FreeRTOS internal data structures and therefore, any other FreeRTOS API **must not** be called when the code is in a critical section (otherwise FreeRTOS internal data structures may get corrupted).

Lets say you have 5 interrupts and `configMAX_SYSCALL_INTERRUPT_PRIORITY` is 3.

```auto
                                         ^ +------------+
                                         | | IRQ1 |
                                         | | |
                                         | +------------+
        Blocked during critical section | | IRQ2 |
                                         | | |
                                         | +------------+
                                         | | IRQ3 |<------- configMAX_SYSCALL_INTERRUPT_PRIORITY
                                         v | |
                                        ---+------------+
                                         ^ | IRQ4 |
                                         | | |
    Not blocked during critical section | +------------+
                                         | | IRQ5 |
                                         | | |
                                         v +------------+

```

When FreeRTOS kernel is in a critical section, IRQ1, IRQ2 and IRQ3 are blocked but IRQ4 and IRQ5 are not. So when the code is in a critical section:

- If IRQ1, IRQ2 or IRQ3 happens, the corresponding ISR will not be executed until the code exits the critical section.
- If IRQ4, or IRQ5 happens, the corresponding ISR will be executed immediately and the critical section will be interrupted.

```auto
           Critical Critical
            Section IRQ3 Section ISR3
             Starts active Ends starts
                  ^ ^ ^ ^
                  | | | |
                  | | | |
                  | | | |
          --------+--------------+------------+-+----------------------
                            ---- Increasing time ---->

                            IRQ4 ISR4 ISR4
                           active starts ends
                                ^ ^ ^
           Critical | | | Critical
            Section | | | Section
             Starts | | | Resumes
                  ^ | | | ^
                  | | | | |
                  | | | | |
                  | | | | |
          --------+-------------+-+-------+-+-----------------------
                            ---- Increasing time ---->

```

If ISR for IRQ1, IRQ2 or IRQ3 calls a FreeRTOS API, it will be okay as it will not interrupt a critical section. But if ISR for IRQ4 or IRQ5 calls a FreeRTOS API, that API may be called at a time when the code is in a critical section which will lead to data corruption.

Hope it helps.

Thanks.

---

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