Function  37h  SWITCHAR / AVAILDEV 
 *   Get/set option marking character (is usually "/"), and device type 
entry   AH      37h 
	AL      00h     read switch character (returns current character in DL) 
		01h     set character in DL as new switch character 
      (DOS 2.x) 02h     read device availability (as set by function AL=3) into 
			DL. A 0 means devices that devices must be accessed in  
			file I/O calls by /dev/device. A non-zero value means  
			that devices are accessible at every level of the  
			directory tree (e.g., PRN is the printer and not a file  
			PRN). 
			AL=2 to return flag in DL, AL=3 to set from DL (0 = set, 
			1 = not set). 
      (DOS 2.x) 03h     get device availability, where: 
	DL      00h     means /dev/ must precede device names 
		01h     means /dev/ need not precede device names 
return  DL      switch character (if AL=0 or 1) 
		device availability flag (if AL=2 or 3) 
	AL      0FFh    the value in AL was not in the range 0-3. 
note 1) Functions 2 & 3 appear not to be implemented for DOS 3.x. 
     2) It is documented on page 4.324 of the MS-DOS (version 2) Programmer's 
	Utility Pack (Microsoft - published by Zenith).   
     3) Works on all versions of IBM PC-DOS from 2.0 through 3.3.1. 
     4) The SWITCHAR is the character used for "switches" in DOS command  
	arguments (defaults to '/', as in "DIR/P"). '-' is popular to make a  
	system look more like UNIX; if the SWITCHAR is anything other than '/', 
	then '/' may be used instead of '\ or pathnames  
     5) Ignored by XCOPY, PKARC, LIST 
     6) SWITCHAR may not be set to any character used in a filename 
     7) In DOS 3.x you can still read the "AVAILDEV" byte with subfunction 02h  
	but it always returns 0FFh even if you try to change it to 0 with 
	subfunction 03h. 
     8) AVAILDEV=0 means that devices must be referenced in an imaginary 
	subdirectory "\\dev" (similar to UNIX's /dev/*); a filename "PRN.DAT" 
	can be created on disk and manipulated like any other. If AVAILDEV != 0 
	then device names are recognized anywhere (this is the default): 
	"PRN.DAT" is synonymous with "PRN:".  
     9) These functions reportedly are not supported in the same fashion in  
	various implementations of DOS.  
    10) used by DOS 3.3 CHKDSK, BASIC, DEBUG