Function  50h   "Used Internally by DOS" - Set PSP 
 *              Set new Program Segment Prefix (current Process ID) 
entry   AH      50h 
	BX      segment address of new PSP 
return  none - swaps PSP's regarded as current by DOS 
note 1) By putting the PSP segment value into BX and issuing call 50h DOS stores 
	that value into a variable and uses that value whenever a file call is  
	made.  
     2) Note that in the PSP (or PDB) is a table of 20 (decimal) open file  
	handles. The table starts at offset 18h into the PSP. If there is an  
	0FFh in a byte then that handle is not in use. A number in one of the  
	bytes is an index into an internal FB table for that handle. For  
	instance the byte at offset 18h is for handle 0, at offset 19h handle  
	1, etc. up to 13h. If the high bit is set then the file associated by  
	the handle is not shared by child processes EXEC'd with call 4Bh. 
     ) Function 50h is dangerous in background operations prior to DOS 3.x as  
	it uses the wrong stack for saving registers.  (same as functions  
	0..0Ch in DOS 2.x) 
     4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler 
	without setting the Critical Error flag 
     5) Open File information, etc. is stored in the PSP DOS views as current.  
	If a program (eg. a resident program) creates a need for a second PSP, 
	then the second PSP should be set as current to make sure DOS closes  
	that as opposed to the first when the second application finishes. 
     6) See PC Mag Vol.5, No 9, p.314 for discussion.  
     7) Used by DOS 3.3 PRINT & DEBUG, DesQview 2.01, Windows 1.03, SYMDEB 
	from MASM 4.0