C.3 Interrupt Support
This subclause specifies the language-defined model
for hardware interrupts in addition to mechanisms for handling interrupts.
Dynamic Semantics
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 nonreserved 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
D.3)
is in effect, the interrupt handler executes with the active priority
that is the ceiling priority of the corresponding protected object.
Implementation Requirements
The implementation shall provide a mechanism to determine
the minimum stack space that is necessary 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 this.
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.
Documentation Requirements
The implementation
shall document the following items:
1.
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).
2.
Any interrupts that cannot be blocked, and the effect of attaching handlers
to such interrupts, if this is permitted.
3.
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
stack.
4.
Any implementation- or hardware-specific activity that happens before
a user-defined interrupt handler gets control (for example, reading device
registers, acknowledging devices).
5.
Any timing or other limitations imposed on the execution of interrupt
handlers.
6.
The state (blocked/unblocked) of the nonreserved 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 corresponding
handlers.
7.
Whether the interrupted task is allowed to resume execution before the
interrupt handler returns.
8.
The treatment of interrupt occurrences that are generated while the interrupt
is blocked; that is, whether one or more occurrences are held for later
delivery, or all are lost.
9.
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.
10.
On a multi-processor, the rules governing the delivery of an interrupt
to a particular processor.
Implementation Permissions
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 for which one of its subprograms
is an interrupt handler.
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
C.3.2).
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.
Implementation Advice
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 finer-grained control of interrupt blocking.
NOTE 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 can perform include aborting the partition, ignoring (that is,
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 that interrupt.
NOTE 2 It is a bounded error to call
Task_Identification.Current_Task (see
C.7.1)
from an interrupt handler.
NOTE 3 The rule that an exception
propagated from an interrupt handler has no effect is modeled after the
rule about exceptions propagated out of task bodies.
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