a0                303 kernel/proc.c    np->trapframe->a0 = 0;
a0                 58 kernel/proc.h    /* 112 */ uint64 a0;
a0                 39 kernel/syscall.c     return p->trapframe->a0;
a0                141 kernel/syscall.c     p->trapframe->a0 = syscalls[num]();
a0                145 kernel/syscall.c     p->trapframe->a0 = -1;