Compatibility: 6.0+

See: BX=0000H (query DoubleSpace version/first & last drive)
BX=0001H (get DoubleSpace drive mapping)
BX=0002H (swap drive letters)
BX=0003H (reserved; used by SmartDrive disk cache)
BX=0004H (reserved; used by SmartDrive disk cache)
BX=0005H (mount a DoubleSpace drive)
BX=0006H (unmount a DoubleSpace drive)

BX=0007H (get drive size & free space)
BX=0008H (get CVF file fragment info)
BX=0009H (get count of DISK_UNIT structures)

BX=000aH (set automount drives; DOS 6.2+)
BX=000bH (query automount drives; DOS 6.2+)

DS IOCTL 'F' (flush DoubleSpace cache)
DS IOCTL 'I' (flush and invalidate DoubleSpace cache)
DS IOCTL 'S' (get actual free space; DOS 6.2+)

Compressed Volume File Layout (CVF internal structures)
DoubleSpace API Error/Return Codes (codes returned by these fns)
MRCI API (Microsoft Real-time Compression Interface fns)

To use the API, set AX to 4a11H and set other registers as described
in each fn, then execute an INT 2FH.  The two IOCTL calls are just
DOS fn 4404H (IOCTL read) services.

Notes:   These services are available only if DoubleSpace (or compatible
compression service) was activated upon system startup.

  For most applications, the existence of DoubleSpace is entirely
transparent.  In general, only specialized disk utility
programs need to access these fns.

DOS fn 4404H (get control data from block device)
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INT 2fH 4a11H: DoubleSpace API Services