Int 21H, AH=45H

2 and higher

“DUP” - DUPLICATE FILE HANDLE

• AH = 45h
• BX = file handle

• CF clear if successful
  • AX = new handle
• CF set on error
  • AX = error code (04h,06h) (too many open files (no handles available), invalid handle)

moving file pointer for either handle will also move it for the other, because both will refer to the same system file table

for DOS versions prior to 3.3, file writes may be forced to disk by duplicating the file handle and closing the duplicate

AH=3Dh,AH=46h

Text based on Ralf Brown Interrupt List Release 61