From mboxrd@z Thu Jan  1 00:00:00 1970
From: michael_davidson@pacbell.net (Michael Davidson)
Date: Sun, 23 Feb 2003 13:40:40 -0800
Subject: [pups] Re: PDP11 - RQDX1 standalone format utility? --> XXDP
References: <001201c2d98f$b4ff34d0$77b95b8b@ics.forth.gr> <020901c2dad6$e0afcf70$8a00a8c0@arctura>
Message-ID: <3E593FD8.2090207@pacbell.net>

Jonathan Engdahl wrote:

>I documented the procedure to extract a program from XXDP and put a header
>on it so that you can boot it from VTserver or a UNIX disk. It has been a
>long time since I did this to ZRQCH0, so I practiced on the program that I
>think Christos needs (ZRQBC1) and sent it to him.
>
>There's a lot I don't know about the a.out header, so if someone can improve
>my method, let me know.
>
>http://users.safeaccess.com/engdahl/xxdp.htm#hackXXDP
>
There isn't very much to know about the a.out header,
and what you are doing looks OK although I would probably
have set a_text to 160000 and left a_data and a_bss as 0.

While I am not really familiar with the BSD boot code that
vtserver uses when loading a program I am almost certain that
it will use the entrypoint address in the a.out header and
not just jump to address 0. (remember it's an actual program
loader that understands a.out format, not just something that
only knows how to load a boot block and jump to it)

This is confirmed by the fact that the code you are patching
into the image at address 0 is wrong if you want it to get
you to the restart address.

In your example you have:

000020/ 000167
000022/ 145702

If this was loaded at address 0 in memory and the boot code
jumped to address 0, you would end up at 145706 *not* 145702
(of course, you might very well get lucky and still land on
an instruction boundary, but you would have skipped the first
2 words of the restart code).

If this code were really necessary (and I'm 99.99% sure that
it isn't) you would want either:

000137
145702

or:

000167
145676