Yeah, ISVTX only worked on 410 and 411 files.   It locked the text segment in swap.   It kind of got obsoleted by later versions that could load stuff direct to memory from the disk image.

From: Clem Cole <clemc@ccc.com>
Sent: Monday, December 31, 2018 10:54 AM
To: Ronald Natalie <ron@ronnatalie.com>
Cc: Will Senn <will.senn@gmail.com>; The Eunuchs Hysterical Society <tuhs@tuhs.org>
Subject: Re: [TUHS] Upgrading from 11/40 to 11/45 in Unix v6

On Mon, Dec 31, 2018 at 10:05 AM <ron@ronnatalie.com> wrote:

Yep, and notwithstanding split I/D, we switched to -n (410 magic) read-only code space.

Agreed, that was pretty standard and used the sticky bit as needed.  IIRC this was to help sharing, but I admit those bits in my brain had not been refreshed in years.

Of course, the kludge nargs() function didn’t work at all in split-I/D mode unless you made a hack to the processor to change the behavior of MFPI.

Ah yes, I've forgotten that hack.   Does simh or any of the other simulators support it?   It would need to be an option of course.

ᐧ