HelenOS sources

ISTATE_OFFSET_R21   53 kernel/arch/ppc32/include/arch/istate_struct.h #define ISTATE_OFFSET_R21       0x54
ISTATE_OFFSET_R21   53 uspace/lib/c/arch/ppc32/include/libarch/istate_struct.h #define ISTATE_OFFSET_R21       0x54
HelenOS homepage, sources at GitHub