ustack             15 exec.c           uint argc, sz, sp, ustack[3+MAXARG+1];
ustack             71 exec.c             ustack[3+argc] = sp;
ustack             73 exec.c           ustack[3+argc] = 0;
ustack             75 exec.c           ustack[0] = 0xffffffff;  // fake return PC
ustack             76 exec.c           ustack[1] = argc;
ustack             77 exec.c           ustack[2] = sp - (argc+1)*4;  // argv pointer
ustack             80 exec.c           if(copyout(pgdir, sp, ustack, (3+argc+1)*4) < 0)