Re: Privilege Levels Below User

In article <v407ah$29fla$7@dont-email.me>, ldo@nz.invalid (Lawrence
D'Oliveiro) wrote:

On Fri, 07 Jun 2024 12:03:03 -0600, John Savard wrote:

So I am thinking it might be useful to have, for example, two
states less privileged than the user state, and some mechanism
for user programs to call subroutines which are in that state
until they return - the return instruction being limited, sort
of like a supervisor call, so it can only return in a proper
manner.

As a practical matter, ISA features requiring assembly coding are not
accessible to application programmers these days, because they mostly
don't know assembler and are scared by the idea. They also don't want to
know about privilege levels. Such features are accessible to compiler
writers, JIT creators and OS creators, but introducing additional
complexity into their work is not welcome.

If this feature existed and worked, it would hardly be used at all.

User code normally ran at ring 4. This left 5, 6 and 7 available
for ordinary users to impose their own additional isolation on code
they didn't quite trust.

Was this used? People were much more willing to do low-level programming
in those days, but I bet this went unused.

John