19. FILE_SET_HEAD

*------------------------------------------------------------------------------
* NAME          FILE_SET_HEAD
*------------------------------------------------------------------------------
* DEPENDENCY    FILE_GET_HEAD
* PURPOSE       To write a 64 bytes header for a file. (already open)
* DESCRIPTION   Writes a 64 byte file header for the open file whose ID is passed
*               in A0.L from the buffer (user supplied) whose address is passed
*               in A1.L. This buffer must be at least 64 bytes long !
* INPUTS :
*               A0.L = Channel ID
*               A1.L = Address of 64 byte buffer holding the (new) header
* OUTPUTS :
*               D0 = Error code
*               D1 = Size of header written from buffer
*               A0 = Channel id (preserved)
*               A1 = Address of buffer (preserved)
*               Z flag set if no errors, unset otherwise.
*------------------------------------------------------------------------------
file_set_head   movem.l d2-d3/a0-a1,-(a7) ; Save those working registers
                moveq   #FS_HEADS,d0    ; Get trap code
                bra     fgh_rest        ; Do rest via FILE_GET_HEAD