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====== Int 21H, AH=40H ======

===== Version =====

2 and higher

===== Brief =====

"WRITE" - WRITE TO FILE OR DEVICE

===== Family API =====


===== Input =====

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

===== Return =====

 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)

===== Notes =====


  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
          
===== See also =====

 AH=28h,AH=3Fh"DOS",AH=93h,INT 2F/AX=1109h

===== Note =====

Text based on [[http://www.cs.cmu.edu/~ralf/files.html|Ralf Brown Interrupt List Release 61]]

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