INT 2fH 1500H: Is MSCDEX Installed?/Count Drive Letters

                                                         Compatibility: 6.0+ 
 Expects: AX    1500H
          BX    0000H
          ──────────────────────────────────────────────────────────────────
 Returns: BX    number of CD-ROM drive letters (0=not installed)
          ──────────────────────────────────────────────────────────────────
    Info: Checks to see if MSCDEX.EXE (the Microsoft CD-ROM support driver)
          has been installed and returns the number of drive letters in use
          by CD-ROMs.

          Use INT 2fH 150dH (query CD-ROM drive letters) to learn which
          drive letters that are in use.

See Also: INT 2fH: Multiplex Interrupt
          DOS Functions
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