RTEMS Porting Guide
The _CPU_Context_Fp_start macro is used in the XXX routine and allows the initial pointer into a floating point context area (used to save the floating point context) to be at an arbitrary place in the floating point context area. This is necessary because some FP units are designed to have their context saved as a stack which grows into lower addresses. Other FP units can be saved by simply moving registers into offsets from the base of the context area. Finally some FP units provide a "dump context" instruction which could fill in from high to low or low to high based on the whim of the CPU designers. Regardless, the address at which that floating point context area pointer should start within the actual floating point context area varies between ports and this macro provides a clean way of addressing this.
This is a common implementation of the _CPU_Context_Fp_start routine which is suitable for many processors. In particular, RISC processors tend to use this implementation since the floating point context is saved as a sequence of store operations.
#define _CPU_Context_Fp_start( _base, _offset ) \ ( (void *) _Addresses_Add_offset( (_base), (_offset) ) )
In contrast, the m68k treats the floating point context area as a stack which grows downward in memory. Thus the following implementation of _CPU_Context_Fp_start is used in that port:
XXX insert m68k version here
RTEMS Porting Guide
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