| rax | 123 | modify this LDT (local decriptor table) | 8 bit |
| rbx | func | function 0 = read function 1 = write | 32 bit |
| rcx | *ptr | pointer to store the LDT data | 32 bit |
| rdx | bytecount | count bytes read or written | LDT in bytes |
| On return | rax |
| read | bytecount LDT |
| write | 0 |
| or | errorcode |
| Errorcode | c++ name | Description |
| 14 | EFAULT | Bad address |
| 22 | EINVAL | Invalid argument |
| 38 | ENOSYS | func is neither 0 nor 1 |