Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
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Int 21h Function 23h
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  Interrupt:   21h     Function:   23h
 
  Title:       Get File Size
 
  See also:    Set File Pointer
 
  Description:
 
     Searches for a matching file in the current directory; if one is
     found, updates the FCB with the file's size in terms of number of
     records.
 
     Input                        Output
 
     AH = 23h                     If function successful (matching file
     DS:DX = segment:offset       found)
             of unopened file       AL = 00
             control block        And FCB relative-record field
                                  (offset 21h) set to the number of
                                  records in the file, rounded up if
                                  necessary to the next complete
                                  record
 
                                  If function unsuccessful (no
                                  matching file found)
                                    AL = FFh
                                    -♦-