| rax | 4 | write to a file descriptor | 8 bit |
| rbx | fd | a file descriptor | 16 bit |
| rcx | *buf | Pointer to a buffer | count bytes |
| rdx | count | number of bytes to write | 32 bit |
| On return | rax |
| bytes written/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 |