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.

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