Compatibility: 6.0+
Expects: AX    150dH
ES:BX address of buffer to receive drive letters
Returns: ES:BX buffer contains data
Info: Obtains a list of DOS logical drive letters that are mapped to

ES:BX on entry, this buffer must be at least as long, in bytes, as the
value obtained by a previous call to INT 2fH 1500H (count

On return, the buffer contains a series of one-byte values
identifying the CD-ROM drives, where 0=A, 1=B, 2=C, etc.

For instance, if there are two CD-ROM drives, and ES:[BX+0]=0cH
and ES:[BX+1]=12H, then drives M and S are CD-ROMs (i.e.,
'A'+0cH='M' and 'A'+12H='S').

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INT 2fH 150dH: Query CD-ROM Drive Letters