Runlevels and opcodes
I am a bit familiar with the design of operating systems
but i definitely lack practical experience so please
apologize if i am confusing things ... Anyway i think
the subject is likely to interest our readers !
On most modern operating systems, system calls provide
control and isolation for resource access (memory, drivers...)
and thereby security.
*** But what does prevent a user-level process from executing
wild instructions (RESET, traps, other dangerous instructions
and undocumented features) ?
More generally, does any of you know if some architectures
provide the possibility of designing custom transitions
between processor "runlevels" (usually there are only 2
available, superuser and user modes) ?
E.g.
processor starts in super-user mode ----> thread 1: switch to user-level
with some opcodes masked
----> thread 2: switch to another
user-level
with other opcodes masked
How do context switches occur on existing architectures ?
Is it some kind of "forking" from super-user mode to user mode and
multiple simultaneous user mode contexts (no switching back to
super-user mode) ?
Basically, what are the main differences between (intel, sparc, alpha, ...)
from this point of view ?
Best regards
Phil
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