Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!sun-barr!cs.utexas.edu!ico!nbires!maa
From: maa@nbires.nbi.com (Mark Armbrust)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Help from TSR Gurus requested
Summary: PDB definition
Keywords: TSR, PSP, Process, Allocation
Message-ID: <426@nbires.nbi.com>
Date: 14 Jul 89 20:40:28 GMT
References: <536@cybaswan.UUCP>
Reply-To: maa@nbires.UUCP (Mark Armbrust)
Organization: NBI Inc, Boulder CO
Lines: 45

In article <536@cybaswan.UUCP> iiit-sh@cybaswan.UUCP (Steve Hosgood) writes:
>
>Now the Question: Can the resident part of a TSR get away with using the area
>below 5Ch? I would *think* that DOS has no further use for the PSP after the
>program becomes resident, but of course the area may still figure
>in some sort of linked-list internally in DOS.

The first 5Ch bytes of the PSP are called the Process Data Block (PDB).  The
information in the PDB is used during program execution as well as program
spawn and termination.  The format of the DOS 3.3 PDB is:

OFFSET	SIZE	CONTENTS
------	----	--------
00h	2	INT 20h instruction.
02h	2	Size of the process in paragraphs when it was started
04h	1	unused
05h	5	Far call to DOS -- Ancient CPM compatible OS call
0Ah	4	Far pointer to exit routine, normally points to instruction
		after spawn call in parent process.
0Eh	4	Far pointer to ^C interrupt routine.
12h	4	Far pointer to fatal error routine, normally points to code
		that gives you the "ABORT, RETRY, IGNORE?" message.
16h	2	Process ID (PID) of parent process
18h	20t	Job File Number (JFN) table -- used to process's file handle
		into system file handles
2Ch	2	Segment address of process's environment variables
2Eh	4	Process stack pointer save area during system calls
32h	2	JFN table size
34h	4	Far pointer to JFN -- normally points to JFN table in PDB, but
		can be changed if the JFN table needs to be bigger.
38h	4	Pointer to next PDB in list -- I've never seen this to be any-
		thing other than 0FFFF:0FFFFh
3Ch	14h	unused
50h	5	INT 21h; RETF; 0; 0 -- "Portable DOS call" interface
55h	7	unused

As you can see, when a TSR is running on it's own PID (as it has to to do file
operations), there are several things in the PDB that are still required.

Hope this helps.
-- 

Mark Armbrust
maa@nbires.nbi.com
maa@nbires.UUCP