Function 22h Random Write

mov dx, seg FileFCB

mov ds, dx

mov dx, offset FileFCB ;ds:dx points to FCB

mov ah, 22h ;Random Write

int 21h

cmp al, 0 ;zero means success

jne error_handler

Random Write (Function 22h) writes the data at the current disk transfer address (DTA) to a record in the file identified by the file control block (FCB).

This function has been superseded by Write File or Device (Function 40h).

Parameter

FileFCB

Points to an FCB structure that identifies an open file. The structure must have been previously filled by using Open File with FCB (Function 0Fh) or Create File with FCB (Function 16h). Also, the fcbRandomRecNo field must specify the record to write. The FCB structure has the following form:

FCB STRUC

fcbDriveID db ? ;drive no. (0=default, 1=A, etc.)

fcbFileName db '????????' ;filename

fcbExtent db '???' ;file extension

fcbCurBlockNo dw ? ;current block number

fcbRecSize dw ? ;record size

fcbFileSize db 4 dup (?) ;size of file in bytes

fcbFileDate dw ? ;date file last modified

fcbFileTime dw ? ;time file last modified

fcbReserved db 8 dup (?) ;reserved

fcbCurRecNo db ? ;current record number

fcbRandomRecNo db 4 dup (?) ;random record number

FCB ENDS

For a full description of the FCB structure, see Chapter 3, “File System.”

Return Value

If the function is successful, the AL register contains 00h. Otherwise, the AL register contains 01h if the disk is full or 02h if the DTA overlapped a segment boundary. In either case, the write operation is canceled.

Comments

MS-DOS updates the fcbCurBlockNo and fcbCurRecNo fields in the FCB structure to agree with the fcbRandomRecNo field before it attempts to write the record to the disk. No record numbers are incremented; successive calls to this function write to the same record in the file.

This function can be used to write files on a network drive but only if the network has granted write (or similar) access to the given file or drive.