[TUHS] Another odd comment in V6

[Paul Ruizendaal]

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.