Assembly Language Help (alang.hlp) (Table of Contents; Topic list)
Int 21h Function 42h
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     This function uses a method code and a double-precision (32-bit)
     value to set the file pointer. The next record read or written in
     the file will begin at the new file pointer location.
 
     Method 02h may be used to find the size of the file by calling Int
     21h Function 42h with an offset of 0 and examining the pointer
     location that is returned.
 
     Using methods 01h or 02h, it is possible to set the file pointer
     to a location before the start of file. If this is done, no error
     is returned by this function, but an error will be encountered
     upon a subsequent attempt to read or write the file.
 
     No matter what method is used in the call to this function, the
     file pointer returned in DX:AX is always the resulting absolute
     byte offset from the start of the file.
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