[ECOS] ref to previous question-'calling path questions, thanks'

Bart Veer bartv@ecoscentric.com
Sun Oct 1 15:59:00 GMT 2006

>>>>> " " == Qiang Huang <jameshq@hotmail.com> writes:

     > Hi eCOS experts:
     >    Maybe my previous question 
     > (http://ecos.sourceware.org/ml/ecos-discuss/2006-09/msg00231.html) is a 
     > little confusing, what it really means is the followings:

     > (ARM target)
     > Two thread #1 and #2 exist in system

     > (1) Thread #1 runs first
     > (2) Thread #1 issues SWI instruction
     > (3) Enter SWI ISR and run on thread #1 stack (svc mode)
     > (4) At the time IRQ occurs and preempts SWI ISR
     > (5) Switch SVC and run on thread #1 stack (svc mode)
     > (6) Eventually comes to interrupt_end() and unlock_inner(), which will cause 
     > a context switching
     > (7) Thread #2 context is loadded and return with thread #2 running

     > Here seems SWI is preempted by thread #2? Is this correct?
     > (shouldn't all ISR be finished before any thread has the chance to run?)

     > Hope someone can give me some hint, that would be very
     > thankful.

eCos does not use SWI's. The main purpose of the SWI instruction is to
provide a controlled mechanism for switching between user mode and
supervisor mode, but eCos applications always run in supervisor mode
so there is no need for a switch. If for some obscure reason an
application does execute a SWI instruction the resulting system
behaviour is basically undefined.


Bart Veer                       eCos Configuration Architect
http://www.ecoscentric.com/     The eCos and RedBoot experts

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