Compatibility: 6.0+
Expects: AX    4404H (INT 21H IOCTL Read service number)
BL    any compressed drive number (1=A, 2=B, etc)
CX    10 (size of an DsIoctlRec; see below)
DS:DX address of a DsIoctlRec with an 'F' (ASCII 46H) in
the bCmdCode field)
                                                                  
Returns: DS:DX bResult contains a DoubleSpace API Error Code (00H=success)
                                                                  
Info: DoubleSpace maintains internal caches for its cluster data,
BitFAT and MDFAT data (see CVF Layout).

This IOCTL fn forces DoubleSpace to update the CVF  on disk to
reflect the currently-cached data.

The cache is still considered valid.  Use DS IOCTL 'I' to force
DoubleSpace to invalidate the cache and re-read the disk.

DS:DX points to a DsIoctlRec as follows:

DsIoctlRec
Offset Size Contents
                                                               
+0      2  abSig       must be 444dH ('M','D')
+2      1  bCmdCode    must be 46H ('F') Flush command
+3      2  bResult     returns a DoubleSpace API Error Code
+5      5  res         (reserved; 0)
10              size of a DsIoctlRec structure

To use this fn, set up the packet and the registers, and call
INT 21H.  See DOS fn 44H (device I/O Control) for related DOS
functions.

On return, check bResult for an error code.  The value 4f4bH
('OK') indicates success.

Note: You can force DoubleSpace and SmartDrive to flush their disk-
sector caches via DOS fn 0dH (reset disk).

- -

DoubleSpace IOCTL fn 'F': Flush DoubleSpace Cache