Compatibility: 6.0+
Expects: AX    4a11H (mux number)
BX    0000H (DoubleSpace fn code)
Returns: AX    DoubleSpace API Error Code (0000H=success)
BX    444dH signature; 'M','D')
CL    first drive letter used by DoubleSpace (41H='A', etc.)
CH    count of drives letters used by DoubleSpace
DX    Version number (an internal code) high bit indicates current
code location (high or low memory)
Info: You may use this fn to see if DoubleSpace is installed and to
obtain the range of drive letters that it uses.

CL is the ASCII code for a drive letter (for instance, 45H=E).
CH is a count of how many drive IDs are controlled by DoubleSpace;
for instance, CX=0544H means drives D,E,F,G, and H are available
to DoubleSpace for use as drive IDs.

DX The low 15 bits identify the version number (this shows up as
0006H on my PC).

Bit 15 is a location flag:  When set, it indicates that
DBLSPACE.BIN is not at its final location (for instance, the
DEVICEHIGH=DBLSPACE.SYS command is not in CONFIG.SYS or has not
been executed yet.

- -

INT 2fH 4a11H BX=0000H: Get Dblspace Version & Drives