| rax | 3 | read from 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 read | size_t ? |
| On return | rax |
| bytes read/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 |
| 21 | EISDIR | Is a directory |
| 22 | EINVAL | Invalid argument |