i am a beginner in this subject and i started reading the TRM from the infocenter
one thing confused me
Interrupt grouping in this link it is written as
"Only the group priority determines preemption of interrupt exceptions. When the processor is executing an interrupt exception handler, another interrupt with the same group priority as the interrupt being handled does not preempt the handler, If multiple pending interrupts have the same group priority, the subpriority field determines the order in which they are processed. If multiple pending interrupts have the same group priority and subpriority, the interrupt with the lowest IRQ number is processed first."
"Only the group priority determines preemption of interrupt exceptions. When the processor is executing an interrupt exception handler, another interrupt with the same group priority as the interrupt being handled does not preempt the handler,
If multiple pending interrupts have the same group priority, the subpriority field determines the order in which they are processed. If multiple pending interrupts have the same group priority and subpriority, the interrupt with the lowest IRQ number is processed first."
and in this link ARM Exception entry and return
there is one more contradictory statement for the above statement (i feel)
"When the processor is executing an exception handler, an exception can preempt the exception handler if its priority is higher than the priority of the exception being handled. See Interrupt priority grouping for more information about preemption by an interrupt. "
in the first link why did they write like that if its PRIGROUP is 011 and if one interrupt sub group number is 2 and it's handler is currently running mean while if the interrupt with subgroup 1 of same priority group occurs then why doesnot it preempt the 2 and go to 1's handler? why does it preempt only when multiple interrupts of same group priority are present ?
Only the group priority determines preemption of interrupt exceptions. When the processor is executing an interrupt exception handler, another interrupt with the same group priority as the interrupt being handled does not preempt the handler,
If multiple pending interrupts have the same group priority, the subpriority field determines the order in which they are processed. If multiple pending interrupts have the same group priority and subpriority, the interrupt with the lowest IRQ number is processed first. "
In this in the second line of what still confusing me
In the ending of the first line they told that IF ANOTHER INTERRUPT OF SAME PRIORITY GROUP OCCURS THEN IT DOES NOT PREEMPT.
in the starting of the second line they again told that IF MULTIPLE PENDING INTERIORS HAVE THE SAME GROUP THE SUB PRIORITY DETERMINES ........(SO ON )
WHY DOESN'T THE SUB PRIORITY COMESINTO PLAY IF THERE IS ONLY ONE PENDING INTERRUPT OF THE SAME GROUP priority.
Thank you sir for your reply
Hi srikar ,
I am afraid you have some misunderstandings.
The second line dones not at all refer to the pre-emption of interrupts.
The several pending interrupts of the same priority (and the same subpriprity) will be served sequentially (without any pre-emption) according to the interrupt number.
If the number of pending interrupts was one, the service order was already determined (in only one order) among the same priority group.
Best regards,
Yasuhiko Koumoto.