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:
Name | Directive Description |
interrupt_catch | Establish an ISR |
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 interrupt_catch directive connects a procedure to an interrupt vector. 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:
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.
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 C Applications Supplement document for a specific target processor.
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 insure 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 C 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.
The 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.
Using the interrupt manager insures 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
task_get_note, task_set_note, task_suspend, task_resume
Clock Management
clock_get, clock_tick
Message, Event, and Signal Management
message_queue_send, message_queue_urgent
event_send
signal_send
Semaphore Management
semaphore_release
Dual-Ported Memory Management
port_external_to_internal, port_internal_to_external
IO Management
io_initialize, io_open, io_close, io_read, io_write, io_control
Fatal Error Management
fatal_error_occurred
Multiprocessing
multiprocessing_announce
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.
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:
SUCCESSFUL ISR established successfully INVALID_NUMBER illegal vector number INVALID_ADDRESS illegal ISR entry point
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.
NOTES:
This directive will not cause the calling task to be preempted.