_REG_PC
gr[_REG_PC] = (__greg_t)_ctx_start;
gr[_REG_PC] = (__greg_t)_ctx_start;
gr[_REG_PC] = r->r_pc;
r->r_pc = gr[_REG_PC];
gr[_REG_PC] = tf->tf_pc;
tf->tf_pc = gr[_REG_PC];