14 Feb
2017
14 Feb
'17
8:35 p.m.
On 14 Feb 2017, at 15:14 , Noel Chiappa wrote:
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.
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.