8. Interrupt Manager — RTEMS Classic API Guide 5.4-rc2 (21st August 2026) documentation

# 8. Interrupt Manager

## 8.1. Introduction

Any real-time executive must provide a mechanism for quick response to externally generated interrupts to satisfy the critical time constraints of the application. The interrupt manager provides this mechanism for RTEMS. This manager permits quick interrupt response times by providing the critical ability to alter task execution which allows a task to be preempted upon exit from an ISR. The interrupt manager includes the following directive:

- rtems_interrupt_catch - Establish an ISR

- rtems_interrupt_disable - Disable Interrupts

- rtems_interrupt_enable - Restore Interrupt Level

- rtems_interrupt_flash - Flash Interrupt

- rtems_interrupt_local_disable - Disable Interrupts on Current Processor

- rtems_interrupt_local_enable - Restore Interrupt Level on Current Processor

- rtems_interrupt_lock_initialize - Initialize an ISR Lock

- rtems_interrupt_lock_acquire - Acquire an ISR Lock

- rtems_interrupt_lock_release - Release an ISR Lock

- rtems_interrupt_lock_acquire_isr - Acquire an ISR Lock from ISR

- rtems_interrupt_lock_release_isr - Release an ISR Lock from ISR

- rtems_interrupt_is_in_progress - Is an ISR in Progress

## 8.2. Background

### 8.2.1. Processing an Interrupt

The interrupt manager allows the application to connect a function to a hardware interrupt vector. When an interrupt occurs, the processor will automatically vector to RTEMS. RTEMS saves and restores all registers which are not preserved by the normal C calling convention for the target processor and invokes the user’s ISR. The user’s ISR is responsible for processing the interrupt, clearing the interrupt if necessary, and device specific manipulation.

The `rtems_interrupt_catch` directive connects a procedure to an interrupt vector. The vector number is managed using the `rtems_vector_number` data type.

The interrupt service routine is assumed to abide by these conventions and have a prototype similar to the following:

    rtems_isr user_isr(
      rtems_vector_number vector
    );

The vector number argument is provided by RTEMS to allow the application to identify the interrupt source. This could be used to allow a single routine to service interrupts from multiple instances of the same device. For example, a single routine could service interrupts from multiple serial ports and use the vector number to identify which port requires servicing.

To minimize the masking of lower or equal priority level interrupts, the ISR should perform the minimum actions required to service the interrupt. Other non-essential actions should be handled by application tasks. Once the user’s ISR has completed, it returns control to the RTEMS interrupt manager which will perform task dispatching and restore the registers saved before the ISR was invoked.

The RTEMS interrupt manager guarantees that proper task scheduling and dispatching are performed at the conclusion of an ISR. A system call made by the ISR may have readied a task of higher priority than the interrupted task. Therefore, when the ISR completes, the postponed dispatch processing must be performed. No dispatch processing is performed as part of directives which have been invoked by an ISR.

Applications must adhere to the following rule if proper task scheduling and dispatching is to be performed:

Note

The interrupt manager must be used for all ISRs which may be interrupted by the highest priority ISR which invokes an RTEMS directive.

Consider a processor which allows a numerically low interrupt level to interrupt a numerically greater interrupt level. In this example, if an RTEMS directive is used in a level 4 ISR, then all ISRs which execute at levels 0 through 4 must use the interrupt manager.

Interrupts are nested whenever an interrupt occurs during the execution of another ISR. RTEMS supports efficient interrupt nesting by allowing the nested ISRs to terminate without performing any dispatch processing. Only when the outermost ISR terminates will the postponed dispatching occur.

### 8.2.2. RTEMS Interrupt Levels

Many processors support multiple interrupt levels or priorities. The exact number of interrupt levels is processor dependent. RTEMS internally supports 256 interrupt levels which are mapped to the processor’s interrupt levels. For specific information on the mapping between RTEMS and the target processor’s interrupt levels, refer to the Interrupt Processing chapter of the Applications Supplement document for a specific target processor.

### 8.2.3. Disabling of Interrupts by RTEMS

During the execution of directive calls, critical sections of code may be executed. When these sections are encountered, RTEMS disables all maskable interrupts before the execution of the section and restores them to the previous level upon completion of the section. RTEMS has been optimized to ensure that interrupts are disabled for a minimum length of time. The maximum length of time interrupts are disabled by RTEMS is processor dependent and is detailed in the Timing Specification chapter of the Applications Supplement document for a specific target processor.

Non-maskable interrupts (NMI) cannot be disabled, and ISRs which execute at this level MUST NEVER issue RTEMS system calls. If a directive is invoked, unpredictable results may occur due to the inability of RTEMS to protect its critical sections. However, ISRs that make no system calls may safely execute as non-maskable interrupts.

## 8.3. Operations

### 8.3.1. Establishing an ISR

The `rtems_interrupt_catch` directive establishes an ISR for the system. The address of the ISR and its associated CPU vector number are specified to this directive. This directive installs the RTEMS interrupt wrapper in the processor’s Interrupt Vector Table and the address of the user’s ISR in the RTEMS’ Vector Table. This directive returns the previous contents of the specified vector in the RTEMS’ Vector Table.

### 8.3.2. Directives Allowed from an ISR

Using the interrupt manager ensures that RTEMS knows when a directive is being called from an ISR. The ISR may then use system calls to synchronize itself with an application task. The synchronization may involve messages, events or signals being passed by the ISR to the desired task. Directives invoked by an ISR must operate only on objects which reside on the local node. The following is a list of RTEMS system calls that may be made from an ISR:

- Task Management Although it is acceptable to operate on the RTEMS_SELF task (e.g. the currently executing task), while in an ISR, this will refer to the interrupted task. Most of the time, it is an application implementation error to use RTEMS_SELF from an ISR.

  - rtems_task_suspend

  - rtems_task_resume

- Interrupt Management

  - rtems_interrupt_enable

  - rtems_interrupt_disable

  - rtems_interrupt_flash

  - rtems_interrupt_lock_acquire

  - rtems_interrupt_lock_release

  - rtems_interrupt_lock_acquire_isr

  - rtems_interrupt_lock_release_isr

  - rtems_interrupt_is_in_progress

  - rtems_interrupt_catch

- Clock Management

  - rtems_clock_set

  - rtems_clock_get_tod

  - rtems_clock_get_tod_timeval

  - rtems_clock_get_seconds_since_epoch

  - rtems_clock_get_ticks_per_second

  - rtems_clock_get_ticks_since_boot

  - rtems_clock_get_uptime

- Timer Management

  - rtems_timer_cancel

  - rtems_timer_reset

  - rtems_timer_fire_after

  - rtems_timer_fire_when

  - rtems_timer_server_fire_after

  - rtems_timer_server_fire_when

- Event Management

  - rtems_event_send

  - rtems_event_system_send

  - rtems_event_transient_send

- Semaphore Management

  - rtems_semaphore_release

- Message Management

  - rtems_message_queue_broadcast

  - rtems_message_queue_send

  - rtems_message_queue_urgent

- Signal Management

  - rtems_signal_send

- Dual-Ported Memory Management

  - rtems_port_external_to_internal

  - rtems_port_internal_to_external

- IO Management The following services are safe to call from an ISR if and only if the device driver service invoked is also safe. The IO Manager itself is safe but the invoked driver entry point may or may not be.

  - rtems_io_initialize

  - rtems_io_open

  - rtems_io_close

  - rtems_io_read

  - rtems_io_write

  - rtems_io_control

- Fatal Error Management

  - rtems_fatal

  - rtems_fatal_error_occurred

- Multiprocessing

  - rtems_multiprocessing_announce

## 8.4. Directives

This section details the interrupt manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

### 8.4.1. INTERRUPT_CATCH - Establish an ISR

CALLING SEQUENCE:

      rtems_status_code rtems_interrupt_catch(
        rtems_isr_entry      new_isr_handler,
        rtems_vector_number  vector,
        rtems_isr_entry     *old_isr_handler
      );

DIRECTIVE STATUS CODES:

  `RTEMS_SUCCESSFUL` | ISR established successfully
  `RTEMS_INVALID_NUMBER` | illegal vector number
  `RTEMS_INVALID_ADDRESS` | illegal ISR entry point or invalid `old_isr_handler`

DESCRIPTION:

  This directive establishes an interrupt service routine (ISR) for the specified interrupt vector number. The `new_isr_handler` parameter specifies the entry point of the ISR. The entry point of the previous ISR for the specified vector is returned in `old_isr_handler`.

  To release an interrupt vector, pass the old handler’s address obtained when the vector was first capture.

NOTES:

  This directive will not cause the calling task to be preempted.

### 8.4.2. INTERRUPT_DISABLE - Disable Interrupts

CALLING SEQUENCE:

      void rtems_interrupt_disable(
        rtems_interrupt_level level
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive disables all maskable interrupts and returns the previous interrupt level in `level`.

NOTES:

  A later invocation of the `rtems_interrupt_enable` directive should be used to restore the interrupt level.

  This directive is implemented as a macro which sets the `level` parameter.

  This directive will not cause the calling task to be preempted.

  This directive is only available in uniprocessor configurations. The directive `rtems_interrupt_local_disable` is available in all configurations.

      void critical_section( void )
      {
        rtems_interrupt_level level;

        /*
         * Please note that the rtems_interrupt_disable() is a macro.  The
         * previous interrupt level (before the maskable interrupts are
         * disabled) is returned here in the level macro parameter.  This
         * would be wrong:
         *
         * rtems_interrupt_disable( &level );
         */
        rtems_interrupt_disable( level );

        /* Critical section, maskable interrupts are disabled */

        {
          rtems_interrupt_level level2;

          rtems_interrupt_disable( level2 );

          /* Nested critical section */

          rtems_interrupt_enable( level2 );
        }

        /* Maskable interrupts are still disabled */

        rtems_interrupt_enable( level );
      }

### 8.4.3. INTERRUPT_ENABLE - Restore Interrupt Level

CALLING SEQUENCE:

      void rtems_interrupt_enable(
        rtems_interrupt_level level
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive restores the interrupt level specified by `level`.

NOTES:

  The `level` parameter value must be obtained by a previous call to `rtems_interrupt_disable` or `rtems_interrupt_flash`. Using an otherwise obtained value is undefined behaviour.

  This directive is unsuitable to enable particular interrupt sources, for example in an interrupt controller.

  This directive will not cause the calling task to be preempted.

  This directive is only available in uniprocessor configurations. The directive `rtems_interrupt_local_enable` is available in all configurations.

### 8.4.4. INTERRUPT_FLASH - Flash Interrupts

CALLING SEQUENCE:

      void rtems_interrupt_flash(
        rtems_interrupt_level level
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive is functionally equivalent to a `rtems_interrupt_enable( level )` immediately followed by a `rtems_interrupt_disable( level )`. On some architectures it is possible to provide an optimized implementation for this sequence.

NOTES:

  The `level` parameter value must be obtained by a previous call to `rtems_interrupt_disable` or `rtems_interrupt_flash`. Using an otherwise obtained value is undefined behaviour.

  This directive will not cause the calling task to be preempted.

  This directive is only available in uniprocessor configurations. The directives `rtems_interrupt_local_disable` and `rtems_interrupt_local_enable` are available in all configurations.

  Historically, the interrupt flash directive was heavily used in the operating system implementation. However, this is no longer the case. The interrupt flash directive is provided for backward compatibility reasons.

### 8.4.5. INTERRUPT_LOCAL_DISABLE - Disable Interrupts on Current Processor

CALLING SEQUENCE:

      void rtems_interrupt_local_disable(
        rtems_interrupt_level level
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive disables all maskable interrupts on the current processor and returns the previous interrupt level in `level`.

NOTES:

  A later invocation of the `rtems_interrupt_local_enable` directive should be used to restore the interrupt level.

  This directive is implemented as a macro which sets the `level` parameter.

  This directive will not cause the calling task to be preempted.

  In SMP configurations, this will not ensure system wide mutual exclusion. Use interrupt locks instead.

      void local_critical_section( void )
      {
        rtems_interrupt_level level;

        /*
         * Please note that the rtems_interrupt_local_disable() is a macro.
         * The previous interrupt level (before the maskable interrupts are
         * disabled) is returned here in the level macro parameter.  This
         * would be wrong:
         *
         * rtems_interrupt_local_disable( &level );
         */
        rtems_interrupt_local_disable( level );

        /*
         * Local critical section, maskable interrupts on the current
         * processor are disabled.
         */

        {
          rtems_interrupt_level level2;

          rtems_interrupt_local_disable( level2 );

          /* Nested local critical section */

          rtems_interrupt_local_enable( level2 );
        }

        /* Maskable interrupts are still disabled */

        rtems_interrupt_local_enable( level );
      }

### 8.4.6. INTERRUPT_LOCAL_ENABLE - Restore Interrupt Level on Current Processor

CALLING SEQUENCE:

      void rtems_interrupt_local_enable(
        rtems_interrupt_level level
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive restores the interrupt level specified by `level` on the current processor.

NOTES:

  The `level` parameter value must be obtained by a previous call to `rtems_interrupt_local_disable`. Using an otherwise obtained value is undefined behaviour.

  This directive is unsuitable to enable particular interrupt sources, for example in an interrupt controller.

  This directive will not cause the calling task to be preempted.

### 8.4.7. INTERRUPT_LOCK_INITIALIZE - Initialize an ISR Lock

CALLING SEQUENCE:

      void rtems_interrupt_lock_initialize(
        rtems_interrupt_lock *lock,
        const char           *name
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  Initializes an interrupt lock. The name must be persistent throughout the lifetime of the lock.

NOTES:

  Concurrent initialization leads to unpredictable results.

### 8.4.8. INTERRUPT_LOCK_ACQUIRE - Acquire an ISR Lock

CALLING SEQUENCE:

      void rtems_interrupt_lock_acquire(
        rtems_interrupt_lock         *lock,
        rtems_interrupt_lock_context *lock_context
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  Maskable interrupts will be disabled. In SMP configurations, this directive acquires an SMP lock.

NOTES:

  A separate lock context must be provided for each acquire/release pair, for example an automatic variable.

  An attempt to recursively acquire the lock may result in an infinite loop with maskable interrupts disabled.

  This directive will not cause the calling thread to be preempted. This directive can be used in thread and interrupt context.

### 8.4.9. INTERRUPT_LOCK_RELEASE - Release an ISR Lock

CALLING SEQUENCE:

      void rtems_interrupt_lock_release(
        rtems_interrupt_lock         *lock,
        rtems_interrupt_lock_context *lock_context
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  The interrupt level will be restored. In SMP configurations, this directive releases an SMP lock.

NOTES:

  The lock context must be the one used to acquire the lock, otherwise the result is unpredictable.

  This directive will not cause the calling thread to be preempted. This directive can be used in thread and interrupt context.

### 8.4.10. INTERRUPT_LOCK_ACQUIRE_ISR - Acquire an ISR Lock from ISR

CALLING SEQUENCE:

      void rtems_interrupt_lock_acquire_isr(
        rtems_interrupt_lock         *lock,
        rtems_interrupt_lock_context *lock_context
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  The interrupt level will remain unchanged. In SMP configurations, this directive acquires an SMP lock.

NOTES:

  A separate lock context must be provided for each acquire/release pair, for example an automatic variable.

  An attempt to recursively acquire the lock may result in an infinite loop.

  This directive is intended for device drivers and should be called from the corresponding interrupt service routine.

  In case the corresponding interrupt service routine can be interrupted by higher priority interrupts and these interrupts enter the critical section protected by this lock, then the result is unpredictable.

### 8.4.11. INTERRUPT_LOCK_RELEASE_ISR - Release an ISR Lock from ISR

CALLING SEQUENCE:

      void rtems_interrupt_lock_release_isr(
        rtems_interrupt_lock         *lock,
        rtems_interrupt_lock_context *lock_context
      );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  The interrupt level will remain unchanged. In SMP configurations, this directive releases an SMP lock.

NOTES:

  The lock context must be the one used to acquire the lock, otherwise the result is unpredictable.

  This directive is intended for device drivers and should be called from the corresponding interrupt service routine.

### 8.4.12. INTERRUPT_IS_IN_PROGRESS - Is an ISR in Progress

CALLING SEQUENCE:

      bool rtems_interrupt_is_in_progress( void );

DIRECTIVE STATUS CODES:

  NONE

DESCRIPTION:

  This directive returns `TRUE` if the processor is currently servicing an interrupt and `FALSE` otherwise. A return value of `TRUE` indicates that the caller is an interrupt service routine, NOT a task. The directives available to an interrupt service routine are restricted.

NOTES:

  This directive will not cause the calling task to be preempted.