Int 67 Fn 2763  - Videms.sys V1.31+ - Return Hidden Block Size             [m]

   AX = 2763h
   CL = 04h
   BX = signature AAFFh

Return: AH = 00h if successful
       DX = block size in kilobytes
Desc:  Returns the amount of EMS which can be safely used at any time, and
     can't be destroyed by writing anything to the B800:0000 buffer.

Note:  this function normally returns 184K, while the driver provides up to
     240K of EMS.

See Also: AX=2763h/CL=01h,AX=2763h/CL=02h