RTEMS 4.5.0 On-Line Library

Task Context Management Initializing a Context

RTEMS Porting Guide

6.3.2: Initializing a Context

The _CPU_Context_Initialize routine initializes the context to a state suitable for starting a task after a context restore operation. Generally, this involves:

setting a starting address,
preparing the stack,
preparing the stack and frame pointers,
setting the proper interrupt level in the context, and
initializing the floating point context

This routine generally does not set any unnecessary register in the context. The state of the "general data" registers is undefined at task start time. The _CPU_Context_initialize routine is prototyped as follows:

void _CPU_Context_Initialize(
    Context_Control *_the_context,
    void            *_stack_base,
    unsigned32       _size,
    unsigned32       _isr,
    void            *_entry_point,
    unsigned32       _is_fp
);

The is_fp parameter is TRUE if the thread is to be a floating point thread. This is typically only used on CPUs where the FPU may be easily disabled by software such as on the SPARC where the PSR contains an enable FPU bit. The use of an FPU enable bit allows RTEMS to ensure that a non-floating point task is unable to access the FPU. This guarantees that a deferred floating point context switch is safe.

The _stack_base parameter is the base address of the memory area allocated for use as the task stack. It is critical to understand that _stack_base may not be the starting stack pointer for this task. On CPU families where the stack grows from high addresses to lower ones, (i.e. CPU_STACK_GROWS_UP is FALSE) the starting stack point will be near the end of the stack memory area or close to _stack_base + _size. Even on CPU families where the stack grows from low to higher addresses, there may be some required outermost stack frame that must be put at the address _stack_base.

The _size parameter is the requested size in bytes of the stack for this task. It is assumed that the memory area _stack_base is of this size.

XXX explain other parameters and check prototype

RTEMS Porting Guide