From mboxrd@z Thu Jan  1 00:00:00 1970
From: pnr@planet.nl (Paul Ruizendaal)
Date: Tue, 14 Feb 2017 15:35:46 +0100
Subject: [TUHS] Another odd comment in V6
In-Reply-To: <20170214141424.A5BC518C0A2@mercury.lcs.mit.edu>
References: <20170214141424.A5BC518C0A2@mercury.lcs.mit.edu>
Message-ID: <9D5768C7-995F-4C38-A316-E2B8F43E710F@planet.nl>


On 14 Feb 2017, at 15:14 , Noel Chiappa wrote:

>> From: Paul Ruizendaal
> 
>> Actually, research Unix does save the complete state of a process and
>> could back up the PC. The reason that it doesn't work is in the syscall
>> API design, using registers to pass values etc. If all values were
>> passed on the stack it would work.
> 
> Sorry, I don't follow this?
> 
> The problem with 'backing up the PC' is that you 'sort of' have to restore the
> arguments to the state they were in at the time the system call was first
> made. This is actually easier if the arguments are in registers.

Yeah, you're right. I was thinking of the 2.9BSD code which only does the backup
in certain cases and when stack parameter mode was used (the 0200 bit), but
stating it like I did is indeed incomplete to say the least.

Another difficulty in stock V6 would be code like this:
http://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/s4/chmod.s
where the data at label 9: could be overwritten by a signal handler
and re-executing the sys call would not work as intended.