stack pointer usage

[Roy Pollock]

The stack pointer should be saved and restored in the _CPU_Context_switch
routine along with the other non-temporary registers.

So the stack pointer used by the task is stored in the tasks context block
when the task is not executing, and is identical to the a7 stack pointer
when the task is executing.  (the contex block is structure
Context_Control defined in the file cpu.h.  It is, IIRC, stored on the
heap).

My knowledge of m68k assembly is pretty basic, but here's how it looks
like it works.

RTEMS calls _CPU_Context_switch(from,to)
where from and to are the threads to switch from and to
at this point a7 is the stack pointer in the from task's context.
Then it goes to the _CPU_Context_switch routine:

moval a7@(RUNCONTEXT_ARG),a0

which takes the first argument (from) off the stack and puts it in
register a0.

movw sr,d1

saves the status register.

movml d1-d7/a2-a7,a0@
saves all the registers, including the stack pointer, to the memory
address pointed at by from.

moval a7@(HEIRCONTEXT_ARG),a0
moves the second argument, two, from the current stack into a0.
This is the address of the context block of the thread about to execute

movml a0@,d1-d7/a2-a7
This restores _all_ the registers, including the stack pointer.
The stack pointer has no been changed in the middle of the assembly
language routine.  Then it restores the status register and returns.


Hope this answers your question,

Roy A. Pollock

On Fri, 7 Jul 2000, Ahmet Akkas wrote:

> 
> Hi everybody,
> 
> In RTEMS, each task has its own stack. And I am just wondering
> how stack pointer in the target architecture is set by task's
> stack. For example, in m68k architecture a7 is the user stack
> pointer. In the _CPU_Context_switch and _ISR_Handler assembly
> routines, a7 user stack pointer is used and I did not see any
> place where a7 gets the task's stack address. I am not OS person,
> so I may have wrong interpretation of the RTEMS. But if somebody 
> gives me how RTEMS deals with stack pointer (interaction between
> the stack pointer used in the architecture and task's stack), 
> I really appreciate it.
> 
> Thanks a lot.
> 
> Ahmet Akkas
>