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-><A
-NAME="HAL-DEFAULT-INTERRUPT-HANDLING">Default Interrupt Handling</H1
-><P
->Most asynchronous external interrupt vectors will point to a default
-interrupt VSR which decodes the actual interrupt being delivered from
-the interrupt controller and invokes the appropriate ISR.</P
-><P
->The default interrupt VSR has a number of responsibilities if it is
-going to interact with the Kernel cleanly and allow interrupts to
-cause thread preemption.</P
-><P
->To support this VSR an ISR vector table is needed. For each valid
-vector three pointers need to be stored: the ISR, its data pointer and
-an opaque (to the HAL) interrupt object pointer needed by the
-kernel. It is implementation defined whether these are stored in a
-single table of triples, or in three separate tables.</P
-><P
->The VSR follows the following approximate plan:</P
-><P
-></P
-><OL
-TYPE="1"
-><LI
-><P
-> Save the CPU state. In non-debug configurations, it may be
- possible to get away with saving less than the entire machine
- state. The option
- <TT
-CLASS="LITERAL"
->CYGDBG_HAL_COMMON_INTERRUPTS_SAVE_MINIMUM_CONTEXT</TT
->
- is supported in some targets to do this.
- </P
-></LI
-><LI
-><P
-> Increment the kernel scheduler lock. This is a static member of
- the Cyg_Scheduler class, however it has also been aliased to
- <TT
-CLASS="LITERAL"
->cyg_scheduler_sched_lock</TT
-> so that it can be
- accessed from assembly code.
- </P
-></LI
-><LI
-><P
-> (Optional) Switch to an interrupt stack if not already running on
- it. This allows nested interrupts to be delivered without needing
- every thread to have a stack large enough to take the maximum
- possible nesting. It is implementation defined how to detect
- whether this is a nested interrupt but there are two basic
- techniques. The first is to inspect the stack pointer and switch
- only if it is not currently within the interrupt stack range; the
- second is to maintain a counter of the interrupt nesting level and
- switch only if it is zero. The option
- <TT
-CLASS="LITERAL"
->CYGIMP_HAL_COMMON_INTERRUPTS_USE_INTERRUPT_STACK</TT
->
- controls whether this happens.
- </P
-></LI
-><LI
-><P
-> Decode the actual external interrupt being delivered from
- the interrupt controller. This will yield the ISR vector
- number. The code to do this usually needs to come from the
- variant or platform HAL, so is usually present in the form of a
- macro or procedure callout.
- </P
-></LI
-><LI
-><P
-> (Optional) Re-enable interrupts to permit nesting. At this point
- we can potentially allow higher priority interrupts to occur. It
- depends on the interrupt architecture of the CPU and platform
- whether more interrupts will occur at this point, or whether they
- will only be delivered after the current interrupt has been
- acknowledged (by a call to
- <TT
-CLASS="FUNCTION"
->HAL_INTERRUPT_ACKNOWLEDGE()</TT
-> in the ISR).
- </P
-></LI
-><LI
-><P
-> Using the ISR vector number as an index, retrieve the
- ISR pointer and its data pointer from the ISR vector table.
- </P
-></LI
-><LI
-><P
-> Construct a C call stack frame. This may involve making stack
- space for call frames, and arguments, and initializing the back
- pointers to halt a GDB backtrace operation.
- </P
-></LI
-><LI
-><P
-> Call the ISR, passing the vector number and data pointer. The
- vector number and a pointer to the saved state should be preserved
- across this call, preferably by storing them in registers that are
- defined to be callee-saved by the calling conventions.
- </P
-></LI
-><LI
-><P
-> If this is an un-nested interrupt and a separate interrupt
- stack is being used, switch back to the interrupted thread's
- own stack.
- </P
-></LI
-><LI
-><P
-> Use the saved ISR vector number to get the interrupt object
- pointer from the ISR vector table.
- </P
-></LI
-><LI
-><P
-> Call <TT
-CLASS="FUNCTION"
->interrupt_end()</TT
-> passing it the return
- value from the ISR, the interrupt object pointer and a pointer to
- the saved CPU state. This function is implemented by the Kernel
- and is responsible for finishing off the interrupt
- handling. Specifically, it may post a DSR depending on the ISR
- return value, and will decrement the scheduler lock. If the lock
- is zeroed by this operation then any posted DSRs may be called and
- may in turn result in a thread context switch.
- </P
-></LI
-><LI
-><P
-> The return from <TT
-CLASS="FUNCTION"
->interrupt_end()</TT
-> may occur
- some time after the call. Many other threads may have executed in
- the meantime. So here all we may do is restore the machine state
- and resume execution of the interrupted thread. Depending on the
- architecture, it may be necessary to disable interrupts again for
- part of this.
- </P
-></LI
-></OL
-><P
->The detailed order of these steps may vary slightly depending on the
-architecture, in particular where interrupts are enabled and disabled.</P
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