why

#define lv2syscall0(syscall)                                                     \
        register u64 p1 asm("3");                                                \
        register u64 p2 asm("4");                                                \
        register u64 p3 asm("5");                                                \
        register u64 p4 asm("6");                                                \
        register u64 p5 asm("7");                                                \
        register u64 p6 asm("8");                                                \
        register u64 p7 asm("9");                                                \
        register u64 p8 asm("10");                                               \
        register u64 scn asm("11") = (syscall);                                  \
        __asm__ __volatile__("sc"                                                \
                             : "=r"(p1), "=r"(p2), "=r"(p3), "=r"(p4),           \
                               "=r"(p5), "=r"(p6), "=r"(p7), "=r"(p8), "=r"(scn) \
                             : "r"(p1), "r"(p2), "r"(p3), "r"(p4),               \
                               "r"(p5), "r"(p6), "r"(p7), "r"(p8), "r"(scn)      \
                             : "r0","r12","lr","ctr","xer","cr0","cr1","cr5","cr6","cr7","memory")