| rax | 54 | input/output control | 8 bit |
| rbx | fd | a file descriptor | 16 bit |
| rcx | request | a ioctl_subrutine number | 16 bit |
| rdx | *argp | pointer to an array | some bytes (depending on the request) |
| rsi | *arg | also a pointer to an array | some bytes (depending on the request) |
| On return | rax |
| 0/errorcode |
| Errorcode | c++ name | Description |
| 9 | EBADF | Bad file number |
| 14 | EFAULT | Bad address |
| 22 | EINVAL | Invalid argument |
| 25 | ENOTTY | Not a typewriter |