Function  51h    "Used Internally by DOS" - Get Program Segment Prefix 
 *               Returns the PSP address of currently executing program 
entry   AH      51h 
return  BX      address of currently executing program 
		offset 
		00h     program exit point 
		02h     memory size in paragraphs 
		04h     unused (0) 
		05h     CP/M style entry point (far call to DOS) 
		0Ah     terminate address (old int 22h) 
		0Ch     terminate segment 
		0Eh     break address (old int 23h) 
		10h     break segment 
		12h     error address (old int 24h) 
		14h     error segment 
		16h     parent PSP segment 
		18h     DOS 2.0+ open files, 0FFh = unused 
		2Ch     DOS 2.0+ environment segment 
		2Eh     far ptr to process's SS:SP 
		32h     DOS 3.x max open files 
		34h     DOS 3.x openfile table address 
		36h     DOS 3.x openfile table segment 
		38h     unused by DOS versions struct#fcb">= 3.3 
		50h     DOS function dispatcher (FAR routine) 
		53h     unused 
		55h	Struct -FCB #1 extension
		5Ch	Struct -FCB #1
		6Ch	Struct -FCB #2
		80h     command tail / default DTA buffer 
note 1) Used in DOS 2.x, 3.x uses 62h 
     2) Function 51h 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)                                            
     3) 50h and 51h might be used if you have more than one process in a PC. 
	For instance if you have a resident program that needs to open a file 
	you could first call 51h to save the current id and then call 50h to set 
	the ID to your PSP. 
     4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler 
	without setting the Critical Error flag 
     5) Used by DOS 3.3 PRINT, DEBUG