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This clause specifies the language-defined model for hardware interrupts
in addition to mechanisms for handling interrupts.
An interrupt represents a class of events that are detected by the
hardware or the system software. Interrupts are said to occur. An
occurrence of an interrupt is separable into generation and delivery.
Generation of an interrupt is the event in the underlying hardware or
system that makes the interrupt available to the program. Delivery is
the action that invokes part of the program as response to the interrupt
occurrence. Between generation and delivery, the interrupt occurrence
(or interrupt) is pending. Some or all interrupts may be blocked. When
an interrupt is blocked, all occurrences of that interrupt are prevented
from being delivered. Certain interrupts are reserved. The set of
reserved interrupts is implementation defined. A reserved interrupt is
either an interrupt for which user-defined handlers are not supported,
or one which already has an attached handler by some other
implementation-defined means. Program units can be connected to
non-reserved interrupts. While connected, the program unit is said to be
attached to that interrupt. The execution of that program unit, the
interrupt handler, is invoked upon delivery of the interrupt occurrence.
While a handler is attached to an interrupt, it is called once for each
delivered occurrence of that interrupt. While the handler executes, the
corresponding interrupt is blocked.
While an interrupt is blocked, all occurrences of that interrupt are
prevented from being delivered. Whether such occurrences remain pending
or are lost is implementation defined.
Each interrupt has a default treatment which determines the system's
response to an occurrence of that interrupt when no user-defined handler
is attached. The set of possible default treatments is implementation
defined, as is the method (if one exists) for configuring the default
treatments for interrupts.
An interrupt is delivered to the handler (or default treatment) that is
in effect for that interrupt at the time of delivery.
An exception propagated from a handler that is invoked by an interrupt
has no effect.
If the Ceiling_Locking policy, See section D.3 Priority Ceiling Locking, is in effect, the interrupt
handler executes with the active priority that is the ceiling priority
of the corresponding protected object.
The implementation shall provide a mechanism to determine the minimum
stack space that is needed for each interrupt handler and to reserve
that space for the execution of the handler. This space should
accommodate nested invocations of the handler where the system permits
If the hardware or the underlying system holds pending interrupt
occurrences, the implementation shall provide for later delivery of
these occurrences to the program.
If the Ceiling_Locking policy is not in effect, the implementation shall
provide means for the application to specify whether interrupts are to
be blocked during protected actions.
The implementation shall document the following items:
For each interrupt, which interrupts are blocked from delivery when a
handler attached to that interrupt executes (either as a result of an
interrupt delivery or of an ordinary call on a procedure of the
corresponding protected object).
Any interrupts that cannot be blocked, and the effect of attaching
handlers to such interrupts, if this is permitted.
Which run-time stack an interrupt handler uses when it executes as a
result of an interrupt delivery; if this is configurable, what is the
mechanism to do so; how to specify how much space to reserve on that
Any implementation- or hardware-specific activity that happens before a
user-defined interrupt handler gets control (e.g., reading device
registers, acknowledging devices).
Any timing or other limitations imposed on the execution of interrupt
The state (blocked/unblocked) of the non-reserved interrupts when the
program starts; if some interrupts are unblocked, what is the mechanism
a program can use to protect itself before it can attach the
Whether the interrupted task is allowed to resume execution before the
interrupt handler returns.
The treatment of interrupt occurrences that are generated while the
interrupt is blocked; i.e., whether one or more occurrences are held for
later delivery, or all are lost.
Whether predefined or implementation-defined exceptions are raised as a
result of the occurrence of any interrupt, and the mapping between the
machine interrupts (or traps) and the predefined exceptions.
On a multi-processor, the rules governing the delivery of an interrupt
to a particular processor.
If the underlying system or hardware does not allow interrupts to be
blocked, then no blocking is required as part of the execution of
subprograms of a protected object whose one of its subprograms is an
In a multi-processor with more than one interrupt subsystem, it is
implementation defined whether (and how) interrupt sources from separate
subsystems share the same Interrupt_ID type, See section C.3.2 The Package Interrupts. In
particular, the meaning of a blocked or pending interrupt may then be
applicable to one processor only.
Implementations are allowed to impose timing or other limitations on the
execution of interrupt handlers.
Other forms of handlers are allowed to be supported, in which case, the
rules of this subclause should be adhered to.
The active priority of the execution of an interrupt handler is allowed
to vary from one occurrence of the same interrupt to another.
If the Ceiling_Locking policy is not in effect, the implementation
should provide means for the application to specify which interrupts are
to be blocked during protected actions, if the underlying system allows
for a finer-grain control of interrupt blocking.
(1) The default treatment for an interrupt can be to keep the interrupt
pending or to deliver it to an implementation-defined handler. Examples
of actions that an implementation-defined handler is allowed to perform
include aborting the partition, ignoring (i.e., discarding occurrences
of) the interrupt, or queuing one or more occurrences of the interrupt
for possible later delivery when a user-defined handler is attached to
(2) It is a bounded error to call Task_Identification.Current_Task,
See section C.7.1 The Package Task_Identification, from an interrupt handler.
(3) The rule that an exception propagated from an interrupt handler has
no effect is modeled after the rule about exceptions propagated out of
- C.3.1: Protected Procedure Handlers
- C.3.2: The Package Interrupts
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