ITS approach to system calls

[ET]

I have posted Alan Bawden's excellent paper on how ITS
handled system calls.  I think it contains some interesting
food for thought for how a LispOS might want to handle
system calls.

It is at
http://www.eval-apply.com/LispOS/pclsr.htm

As a side note, the Lisp Machine handled interrupted system
calls by saving the state of the microcode in the stack group
that was interrupted.  This has a few negative consequences:

1.  It is insecure (security was never an issue on a Lisp Machine).

2.  The microcode needed to be careful about dangling pointers
in machine regs if it did anything that might trigger an interrupt.
I don't think the Symbolics machines had this problem, but it
would cause an occaisonal crash on an LMI machine.

			~jrm