Writes one or more sequential records from memory to a file, starting at a designated file location.
Call with:
AH = 28H
CX = number of records to write
DS:DX = segment:offset of previously opened file control block
Returns:
AL = 00H if all requested records written
01H if disk full
02H if segment wrap
CX = actual number of records written
Notes:
The records are written (logically, not necessarily physically) to disk from memory at the current disk transfer area address, specified by the most recent call to Int 21H Function 1AH. If the size and location of the buffer are such that a segment overflow or wraparound would occur, the function fails with a return code of 02H.
The file location of the data to be written is determined by the combination of the relative-record field (offset 21H) and the record-size field (offset 0EH) of the FCB. The default record size is 128 bytes.
After the disk transfer is performed, the current block (offset 0CH), current record (offset 20H), and relative-record (offset 21H) fields of the FCB are updated to point to the next record in the file.
If this function is called with CX = 0, no data is written to the disk but the file is extended or truncated to the length specified by combination of the record-size (offset 0EH) and the relative-record (offset 21H) fields of the FCB.
Compare with Int 21H Function 22H, which transfers only one record per function call and does not update the FCB relative-record field.
[3.0+] If the program is running on a network, the user must have Write access rights to the directory containing the file to be written.
Example:
Write four 1024-byte records, starting at record number 8, to disk from the buffer named buff, using the file control block myfcb.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
buff db 4096 dup (?) ; buffer for data
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; set DTA address
mov ah,1ah ; function number
mov dx,seg buff ; address of buffer
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
; set relative-record number
mov word ptr myfcb+21h,8
mov word ptr myfcb+23h,0
; set record size
mov word ptr myfcb+0eh,1024
; write the records
mov ah,28h ; function number
mov cx,4 ; number of records
mov dx,offset myfcb ; address of FCB
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if write error
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