# of disk drives 1/2

 BBS: Inland Empire Archive
Date: 04-24-92 (05:10)             Number: 99
From: STEVE HALKO                  Refer#: NONE
  To: LOGAN ASHBY                   Recvd: NO  
Subj: # of disk drives      1/2      Conf: (2) Quik_Bas

LA>    Ok, now that you've piqued our interest, let's see the code. :-)  I
LA>bet it's sitting right over there ( ---->> ) on your hard
LA>drive, crying out to be posted. <grin> ( <F/X, small voice
LA>heard from distance> post me ........post me .........post me
LA>.........post me ..........)  See, what did I tell you!!! <BIG grin>

  <G>  Well actually, I already posted it.  It was about a year ago.
  What??  You mean you missed it??  Okay, I guess I can post it again.

  Actually, I was hoping someone would have had enough interest to dig
  out Ralf Brown's Int List and figure it out - that's what I did, and
  it was very educational.  A nice learning experience.  I just LOVE
  rooting around in DOS's innards!

  Here it is:

  'LOGDRIVE.BAS by Steve Halko
  '|
-|
  '|  This program explores the use of undocumented DOS function 52h to|
  '|  examine DOS's Logical Drive Table and extract some parameters    |
  '|  from it.  Details are described in the LogicalDrives SUB         |
  '|
-|
  DEFINT A-Z
  DECLARE SUB LogicalDrives ()
  DECLARE FUNCTION Int2Hex$ (n&)
  '$INCLUDE: 'QB.BI'
  DIM SHARED inreg AS regtypex, outreg AS regtypex
  COLOR 14, 1
  CLS
  CALL LogicalDrives
  END

  FUNCTION Int2Hex$ (n&)

  'This function converts a word to a 4 character hex string

  Int2Hex = STRING$(4 - LEN(HEX$(n&)), "0") + HEX$(n&)

  END FUNCTION

  SUB LogicalDrives

  '
--'
  '| This subprogram determines the logical drives including those     |
  '| created by the DOS commands SUBST and JOIN.  The procedure uses   |
  '| the undocumented DOS service call function 52H which returns a    |
  '| pointer to the DOS List of Lists (also called the Configuration   |
  '| Variable Table or CVT).  For DOS versions 3, 4, and 5, offset     |
  '| 16H of the CVT is a double word pointer to the Logical Drive      |
  '| Table (LDT). The LDT contains one entry for each possible         |
  '| logical drive in the system starting at drive A and continuing    |
  '| to the drive letter specified by the LASTDRIVE= line in the       |
  '| CONFIG.SYS file.  With no LASTDRIVE line, the LDT contains the    |
  '| default of 5 drives, A through E.  The entire LDT is in one       |
  '| contiguous memory location, that is the entry for each drive      |
  '| follows the previous drive entry.  In DOS version 3, each         |
  '| drive entry is 81 bytes long. In DOS versions 4 and 5, each       |
  '| drive entry is 88 bytes long. To find out which of these logical  |
  '| drives listed in the LDT are valid, check offset 43H of the LDT   |
  '| which is a word containing the Current Drive Status, as follows:  |
  '|                                                                   |
  '|                     0000H  -  invalid drive                       |
  '|                     1000H  -  SUBSTed unit                        |
  '|                     2000H  -  JOINed unit                         |
  '|                     4000H  -  local drive                         |
  '|                     8000H  -  network drive                       |
  '|                                                                   |
  '| This information is from the book, DOS Programmer's Reference, 2nd|
  '| Edition, from Que, and PC Magazine, August 1991.                  |
  '|                                                                   |
  '| Steve Halko 6/1/91                                                |
  '|
-|



  'First get the major DOS version
  inreg.ax = &H3000
  CALL interruptx(&H21, inreg, outreg)
  MajorVersion = outreg.ax AND &HFF

  'Now get into the CVT
  inreg.ax = &H5200
  CALL interruptx(&H21, inreg, outreg)
  DEF SEG = outreg.es                                'Segement of CVT
  CVTOffset = outreg.bx                              'Offset of CVT

  'Print # of physical drives and # of logical drives
  LOCATE 1, 2
  PRINT "Number of physical drives = "; PEEK(CVTOffset + &H20)
  NrLogDrives = PEEK(CVTOffset + &H21)
  LOCATE 2, 2
  PRINT "Number of Logical Drives = "; NrLogDrives

  'Print heading for drive table
  LOCATE 4, 15
  PRINT "Drive Type"
  LOCATE 5, 8
  PRINT "------------------------"
  PRINT " Drive Network Local JOIN SUBST  DPB Ptr  Current Directory"
  PRINT "      ";
  PRINT " "
  VIEW PRINT 8 TO 25

  SELECT CASE MajorVersion

  'For DOS Version 2
  CASE 2                        'LDT not introduced until DOS 3.0
     PRINT
     PRINT "DOS Version 3.0 and above required."
     END

  'For DOS Versions 3, 4, 5
  CASE 3, 4, 5

     'Get Segment/Offset of LDT
     LDTSeg = PEEK(outreg.bx + &H18) + 256& * PEEK(outreg.bx + &H19)
     LDTOffset = PEEK(outreg.bx + &H16) + 256& * PEEK(outreg.bx + &H17)
     DEF SEG = LDTSeg
     IF MajorVersion = 3 THEN
          SizeOfLDT = 81
     ELSE
          SizeOfLDT = 88         'LDT is 7 bytes larger in DOS 4, 5
     END IF

     'Step through LDT
     FOR i = 0 TO NrLogDrives - 1

          TableOffset = LDTOffset + i * SizeOfLDT

          'Print drive letter
          LOCATE 8 + i, 4
          PRINT CHR$(i + 65);

          'Check flag at offset &H44 (Current Drive Status)
          flag = PEEK(TableOffset + &H44)

          IF flag AND &H80 THEN             'If network drive
               LOCATE 8 + i, 11
               PRINT "X";
          END IF
          IF flag AND &H40 THEN             'If local drive
               LOCATE 8 + i, 18
               PRINT "X";
          END IF
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