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Scheduling
and Timeslice •
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Context •
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API
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Assembly
Functions •
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ASSEMBLY
FUNCTIONS
These functions need
to be modified when porting µexec or if you choose
to use a different timer counter instead of TOC4 (Timer
Output Compare 4) under ’HC11. They don’t
take any arguments or return any value, which should
make them easy to port. In total, there are less than
50 assembly source lines, 10 of which deal with the
timer registers and can actually be written in C.
UEXC_SystemInterrupt
is the timer interrupt handler. After storing the
stack pointer to the global variable uexc_current_sp,
it calls a function to see if the current task’s
timeslice has run out.
If it has, then a new
task is scheduled and control transfers to the new
task. If the timeslice has not run out, handler returns
via ReturnFromInterrupt.
You must arrange for
this handler to be invoked for the appropriate timer
interrupt. In most cases, this means putting the address
of this function in the interrupt vector table. If
you are using a debug monitor, typically the monitor
provides a RAM-based pseudo-vector table.
The function UEXC_SavregsAndResched
is called by UEXC_Defer to create a fake interrupt
stack.
UEXC_StartNewTask is
called to run a new task. The stack is set up so that
if the task function ever returns, then an internal
function, UEXC_KillSelf, is called. UEXC_KillSelf
is equivalent to UEXC_KillTask (current_task->tid).
The UEXC_Resume is a
function that gets the stack pointer from uexc_ current_sp
and does a ReturnFromInterrupt.
The function UEXC_StartTimer
can be written in C, but we put it in the set of assembler
functions because it is microcontroller specific.
It needs to set the timer registers to enable the
timer interrupt at INTERRUPT_CYCLES later.
TOC4 is used in the supplied
code. Typically, a TOC function can be associated
with an output pin, but in this case, the output pin
function is disabled.
The value given by INTERRUPT_
CYCLES is the system tick and must be adjusted for
different system clock speeds. As supplied, it is
defined to be 2000 (cycles).
UEXC_RestartTimer can
also be written in C. It simply reenables the timer
interrupt at INTERRUPT_CYCLES later.
