Function  36h  Get Disk Free Space 
     get information on specified drive 
entry   AH      36h 
	DL      drive number (0=default, 1=A:, 2=B:, etc) 
return  AX      number of sectors per cluster  
		0FFFFh means drive specified in DL is invalid 
	BX      number of availible clusters 
	CX      bytes per sector 
	DX      clusters per drive 
note 1) Mult AX * CX * BX for free space on disk 
     2) Mult AX * CX * DX for total disk space 
     3) Function 36h returns an incorrect value after an ASSIGN command. Prior  
	to ASSIGN, the DX register contains 0943h on return, which is the free 
	space in clusters on the HC diskette. After ASSIGN, even with no  
	parameters, 0901h is returned in the DX register; this is an incorrect  
	value. Similar results occur with DD diskettes on a PC-XT or a PC-AT. 
	This occurs only when the disk is not the default drive. Results are as 
	expected when the drive is the default drive. Therefore, the  
	circumvention is to make the desired drive the default drive prior to  
	issuing this function call. 
     4) Int 21h, function call 36h returns an incorrect value after an ASSIGN  
	command. Prior to ASSIGN, the DX register contains 0943h on return,  
	which is the free space in clusters on the HC diskette. After ASSIGN,  
	even with no parameters, 0901h is returned in the DX register; this is  
	an incorrect value. Similar results occur with DD diskettes on a PC-XT  
	or a PC-AT. This occurs only when the disk is not the default drive. 
	Results are as expected when the drive is the default drive. Therefore, 
	the circumvention is to make the desired drive the default drive prior  
	to issuing this function call. 
     5) This function supercedes functions 1Bh and 1Ch.