PSP - DOS Program Segment Prefix Layout

     Offset Size	      Description

	00   word	machine code INT 20 instruction (CDh 20h)
	02   word	top of memory in segment (paragraph) form
	04   byte	reserved for DOS, usually 0
	05  5bytes	machine code instruction long call to the DOS
			function dispatcher (obsolete CP/M)
	06   word	.COM programs bytes available in segment (CP/M)
	0A   dword	INT 22 terminate address;  DOS loader jumps to this
			address upon exit;  the EXEC function forces a child
			process to return to the parent by setting this
			vector to code within the parent (IP,CS)
	0E   dword	INT 23 Ctrl-Break exit address; the original INT 23
			vector is NOT restored from this pointer (IP,CS)
	12   dword	INT 24 critical error exit address; the original
			INT 24 vector is NOT restored from this field (IP,CS)
	16   word	parent process segment addr (Undoc. DOS 2.x+)
			COMMAND.COM has a parent id of zero, or its own PSP
	18  20bytes	file handle array (Undocumented DOS 2.x+); if handle
			array element is FF then handle is available.  Network
			redirectors often indicate remotes files by setting
			these to values between 80-FE.
	2C   word	segment address of the environment, or zero (DOS 2.x+)
	2E   dword	SS:SP on entry to last INT 21 function (Undoc. 2.x+) 
	32   word	handle array size (Undocumented DOS 3.x+)
	34   dword	handle array pointer (Undocumented DOS 3.x+)
	38   dword	pointer to previous PSP (deflt FFFF:FFFF, Undoc 3.x+) 
	3C  20bytes	unused in DOS before 4.01  
	50   3bytes	DOS function dispatcher CDh 21h CBh (Undoc. 3.x+) 
	53   9bytes	unused
	5C  36bytes	default unopened FCB #1 (parts overlayed by FCB #2)
	6C  20bytes	default unopened FCB #2 (overlays part of FCB #1)
	80   byte	count of characters in command tail;  all bytes
			following command name;  also default DTA (128 bytes)
	81 127bytes	all characters entered after the program name followed
			by a CR byte

	- offset 5 contains a jump address which is 2 bytes too low for
	  PSP's created by the DOS EXEC function in DOS 2.x+  
	- program name and complete path can be found after the environment
	  in DOS versions after 3.0.  See offset 2Ch.

	 see Bibliography for reference to "Undocumented DOS"