| rax | 5 | open and possibly create a file or device | 8 bit |
| rbx | *pathname | Pointer to direktory/name | some bytes |
| rcx | flags | flags | 32 bit |
| rdx | mode | mode_t | 16 bit |
| On return | eax |
| file number/errorcode |
| Errorcode | c++ name | Description |
| 4 | EINTR | Interrupted system call |
| 5 | EIO | I/O error |
| 9 | EBADF | Bad file number |
| 11 | EAGAIN | Try again |
| 14 | EFAULT | Bad address |
| 22 | EINVAL | Invalid argument |
| 27 | EFBIG | File too large |
| 28 | ENOSPC | No space left on device |
| 32 | EPIPE | Broken pipe |