[TUHS] history of virtual address space

[Ron Natalie via TUHS]

There was a start address in the a.out header.  This dates back to the 
V6 days (or perhaps earlier).

The header was on the PDP-11:
Magic number
Text size
Data Size
BSS size
Symbol Table Syze
ENTRY LOCATION
and the last two 16 bit values were unused.

When 4BSD came to the vax, the new header was pretty much the same only 
with longs instead of shorts.

------ Original Message ------
>From "Warner Losh via TUHS" <tuhs at tuhs.org>
To "Rich Salz" <rich.salz at gmail.com>
Cc "The Eunuchs Hysterical Society" <tuhs at tuhs.org>
Date 1/20/2026 4:51:00 PM
Subject [TUHS] Re: history of virtual address space

>Looking at 4.2BSD sources, I don't see any offset for address 0. a.out
>makes it kinda hard to unmap page 0. The compiler would have to put a start
>address into the a.out header, and the 4 binaries I checked had an a_entry
>of 0.. But it looks like only the stand alone programs use a_entry...  But
>that's hidden as this inside setregs() in kern_exec.c:
>         u.u_ar0[PC] = u.u_exdata.ux_entloc+2;
>and that type puns to a.out's a_entry, so that's going to jump to 0.
>
>4.3BSD spells it a little differently:
>         setregs(exdata.ex_exec.a_entry);
>but has the same a_entry == 0. setregs moved to vax/machdep.c where it adds
>2.
>
>So maybe my memory is bad, maybe we had a local hack. I acn't find out for
>sure, but stock 4.3BSD/vax necessarily maps page 0.
>
>Warner
>
>On Tue, Jan 20, 2026 at 12:38 PM Warner Losh <imp at bsdimp.com> wrote:
>
>>
>>
>>  On Tue, Jan 20, 2026, 9:49 AM Rich Salz <rich.salz at gmail.com> wrote:
>>
>>>
>>>
>>>  On Mon, Jan 19, 2026 at 7:05 PM Warner Losh via TUHS <tuhs at tuhs.org>
>>>  wrote:
>>>
>>>>  4.2BSD on the Vax dereferencing 0 would seg fault. I'm not sure when this
>>>>  started. I am sure that System V or 32V on the Vax dereferencing 0 gave
>>>>  0.
>>>
>>>
>>>  Are you sure?  My memory is the opposite.  For example,
>>>  https://www.tuhs.org/Usenet/comp.bugs.4bsd/1989-July/003016.html
>>>
>>
>>
>>  Maybe it was a local hack. I know for sure that the vax assembler i wrote
>>  definitely core dump when i did *NULL... it may have been 4.3 by then... I
>>  also recall a prof or TA saying that 4.1BSD burned a lot of time on
>>  infinite loops that students wrote, but now they are core dumps... But I've
>>  not looked at the 4.3 sources to see if that's unmapped.
>>
>>  Warner
>>
>>>
>>>