The QBNews                                                     Page  3
     Volume  1, Number  1                                 November  1, 1989



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        W h o   y a   g o n n a   c a l l ?   C A L L   I N T E R R U P T
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     QuickBASIC Directories
     by Hector Plasmic
     
        While QuickBASIC has a lot of features, there are a few it lacks.
     Ever wanted to do a directory search?  Chances are you had to Shell to
     DOS, create a temporary file by redirecting the DIR command, then
     return to QB and read the file line-by-line, parsing out the filenames.
     Either that or you resorted to a third-party library function.  It's
     not necessary.  QB can be cajoled into doing directory searches by
     utilizing the DOS directly via Interrupt.
     
        Interrupt 21h functions 4Eh and 4Fh do find-first-match and
     find-next-match respectively.  If the call to these functions is
     successful, information on the matched file is placed by DOS into the
     current DTA (disk transfer area) as follows:
     
       Bytes 0-20   =   (reserved, used during findnext calls by DOS)
       Byte 21      =   attribute of matched file
       Bytes 22-23  =   file time
       Bytes 24-25  =   file date
       Bytes 26-27  =   least significant word of file size
       Bytes 28-29  =   most significant word of file size
       Bytes 30-42  =   filename and extension in form of ASCIIZ string
     
        All this brings up another question:  where is the current DTA?  The
     answer is given by a call to interrupt 21h function 2Fh, which returns
     the DTA's offset in BX and the segment in ES.
     
     So, we make a call to interrupt 21h with register .ah set to 2Fh like
     this:
     
       InRegs.AX = &H2F * 256 'Remember, .ah is the high-byte of .ax
       INTERRUPTX &H21, InRegs, OutRegs
     
     And retrieve the returned information:
     
       DTASeg = OutRegs.ES            'Segment of current DTA
       DTAOff = OutRegs.BX            'Offs of current DTA
     
     If we now say DEF SEG = DTASeg we can directly PEEK into the DTA,
     wherever it may be.
     
     Okay, we know where the data will be, now how do we get it there?  Like
     this:
     
       FLName$ = "*.*" + CHR$(0)     'ASCIIZ filespec (null terminated)
       InRegs.AX = &H4E * 256        'Which function (4Eh)
       InRegs.CX = 0                 'Attribute (normal)
       InRegs.DS = VARSEG(FLName$)   'Segment of ASCIIZ filename
       InRegs.DX = SADD(FLName$)     'Offset of ASCIIZ filename
       InterruptX &H21, InRegs, OutRegs
     
     The QBNews                                                     Page  4
     Volume  1, Number  1                                 November  1, 1989

     
        This calls interrupt 21h function 4Eh and places the information on
     the first matched file into the DTA.  There's a catch, though.  There
     may not be a matching file.  If there isn't, this function returns an
     error by setting the carry flag and returning a code in AX.  We can
     check for this with:
     
       IF OutRegs.Flags AND 1 THEN WhichError = OutRegs.AX
     
     The two possible errors are "invalid path" (2h) and "no (more) matching
     file(s)" (12h).
     
     Now all that remains is to PEEK the information out of the DTA and do
     something useful with it.
     
     FINDEM.BAS is a sample program together with a Function FindFirst that
     will do directory searches for you.  Modify the main module code to
     load the filenames into an array or whatever you need it to do.