RTEMS Ada User's Guide
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 Ada 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:
procedure User_ISR ( vector : in RTEMS.Vector_Number );
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:
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.
RTEMS Ada User's Guide
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