From: pnr@planet.nl (Paul Ruizendaal)
Subject: [TUHS] Another odd comment in V6
Date: Tue, 14 Feb 2017 15:35:46 +0100 [thread overview]
Message-ID: <9D5768C7-995F-4C38-A316-E2B8F43E710F@planet.nl> (raw)
In-Reply-To: <20170214141424.A5BC518C0A2@mercury.lcs.mit.edu>
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.
next prev parent reply other threads:[~2017-02-14 14:35 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-14 14:14 Noel Chiappa
2017-02-14 14:35 ` Paul Ruizendaal [this message]
2017-02-14 15:48 ` Random832
2017-02-14 16:06 ` Dan Cross
-- strict thread matches above, loose matches on Subject: below --
2017-02-21 19:23 Norman Wilson
2017-02-22 9:59 ` Tony Finch
2017-02-14 16:17 Noel Chiappa
2017-02-14 15:40 Noel Chiappa
2017-02-14 8:46 Paul Ruizendaal
2017-02-14 11:27 ` Nick Downing
2017-02-14 12:27 ` Paul Ruizendaal
2017-02-14 12:46 ` Nick Downing
2017-02-14 14:03 ` Lars Brinkhoff
2017-02-14 14:18 ` Nick Downing
2017-02-14 15:50 ` Random832
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