Function 28h Random Block Write

mov cx, cRecords ;number of records to write

mov dx, seg FileFCB

mov ds, dx

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

mov ah, 28h ;Random Block Write

int 21h

cmp al, 0 ;zero means success

jne error_handler

Random Block Write (Function 28h) writes the data at the current disk transfer address (DTA) to one or more records in the file identified by the file control block (FCB).

This function has been superseded by Write File or Device (Function 40h) and Move File Pointer (Function 42h).

Parameters

cRecords

Specifies the number of records to write.

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 first 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.

If the function returns 00h or 01h, the CX register contains the number of records actually written.

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 records to the disk. The block and record fields are incremented after a successful write operation; successive calls to this function write sequential groups of records to 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.