Int 21H, AH=40H

2 and higher

“WRITE” - WRITE TO FILE OR DEVICE

      AH = 40h
      BX = file handle
      CX = number of bytes to write
      DS:DX -> data to write

CF clear if successful

          AX = number of bytes actually written
      CF set on error
          AX = error code (05h,06h) (see #01680 at AH=59h/BX=0000h)

if CX is zero, no data is written, and the file is truncated or extended to the current position

data is written beginning at the current file position, and the file position is updated after a successful write

for FAT32 drives, the file must have been opened with AX=6C00h with the “extended size” flag in order to expand the file beyond 2GB; otherwise the write will fail with error code 0005h (access denied)

the usual cause for AX < CX on return is a full disk

BUG: a write of zero bytes will appear to succeed when it actually failed if the write is extending the file and there is not enough disk space for the expanded file (DOS 5.0-6.0); one should therefore check whether the file was in fact extended by seeking to 0 bytes from the end of the file (INT 21/AX=4202h/CX=0000h/DX=0000h)

under the FlashTek X-32 DOS extender, the pointer is in DS:EDX

AH=28h,AH=3Fh,INT 2F/AX=1109h

Text based on Ralf Brown Interrupt List Release 61