14. Ada95 Interrupt Support

14.1. Introduction

This chapter describes what is required to enable Ada interrupt and error exception handling when using GNAT over RTEMS.

The GNAT Ada95 interrupt support RTEMS was developed by Jiri Gaisler <jgais@ws.estec.esa.nl> who also wrote this chapter.

14.2. Mapping Interrupts to POSIX Signals

In Ada95, interrupts can be attached with the interrupt_attach pragma. For most systems, the gnat run-time will use POSIX signal to implement the interrupt handling, mapping one signal per interrupt. For interrupts to be propagated to the attached Ada handler, the corresponding signal must be raised when the interrupt occurs.

The same mechanism is used to generate Ada error exceptions. Three error exceptions are defined: program, constraint and storage error. These are generated by raising the predefined signals: SIGILL, SIGFPE and SIGSEGV. These signals should be raised when a spurious or erroneous trap occurs.

To enable gnat interrupt and error exception support for a particular BSP, the following has to be done:

  • Write an interrupt/trap handler that will raise the corresponding signal depending on the interrupt/trap number.
  • Install the interrupt handler for all interrupts/traps that will be handled by gnat (including spurious).
  • At startup, gnat calls __gnat_install_handler(). The BSP must provide this function which installs the interrupt/trap handlers.

Which CPU-interrupt will generate which signal is implementation defined. There are 32 POSIX signals (1 - 32), and all except the three error signals (SIGILL, SIGFPE and SIGSEGV) can be used. I would suggest to use the upper 16 (17 - 32) which do not have an assigned POSIX name.

Note that the pragma interrupt_attach will only bind a signal to a particular Ada handler - it will not unmask the interrupt or do any other things to enable it. This have to be done separately, typically by writing various device register.

14.3. Example Ada95 Interrupt Program

An example program (irq_test) is included in the Ada examples package to show how interrupts can be handled in Ada95. Note that generation of the test interrupt (irqforce.c) is BSP specific and must be edited.


The irq_test example was written for the SPARC/ERC32 BSP.

14.4. Version Requirements

With RTEMS 4.0, a patch was required to psignal.c in RTEMS sources (to correct a bug associated to the default action of signals 15-32). The SPARC/ERC32 RTEMS BSP includes the``gnatsupp`` subdirectory that can be used as an example for other BSPs.

With GNAT 3.11p, a patch is required for a-init.c to invoke the BSP specific routine that installs the exception handlers.