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From: davidsen@steinmetz.steinmetz.UUCP (William E. Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: 80286 vs. 432
Message-ID: <5987@steinmetz.steinmetz.UUCP>
Date: Thu, 14-May-87 11:57:21 EDT
Article-I.D.: steinmet.5987
Posted: Thu May 14 11:57:21 1987
Date-Received: Sat, 16-May-87 14:16:48 EDT
References: <5981@shemp.UCLA.EDU>
Reply-To: davidsen@kbsvax.steinmetz.UUCP (William E. Davidsen Jr)
Distribution: world
Organization: General Electric CRD, Schenectady, NY
Lines: 64
Keywords: 432, 80286

I am just cross posting this to arch, because many people don't (won't)
read comp.intel.
================================================================
In article <5981@shemp.UCLA.EDU| kong@CS.UCLA.EDU writes:
|Here is a interesting issue,
|What are the pros and cons of object based (or capability based) systems, say
|iAPX 432, in terms of protection mechanisms compared to the contentional system,say 80286, from the architecture's point of view ?
|
|Here are some comparisons between them,
|1. protection granularity
|	80286 uses segments as the basic protection unit.
|	432 uses objects.
|
|2. protection implementation
|	80286 uses global and local descriptor tables(GDT and LDT) to define
|	the rights to access the segments. Each memory reference has to check
|	those access rights.
|	432 uses Access Descriptors (actually, capabilities) to define the
|	access right the user's process has.
|
|3. protection hierarchy
|	80286 uses ring structured protection hierarchy. The inner most ring is
|	the most trusted OS kernel, the outer ring is the user's process. Each
|	ring has a privilege level which defines the current process privilege,
|	the privilege level will change as the process call a function which 
|	resides at the inner ring.
|	432 uses plain object spaces. There is no hierarchical structure. User
|	can access every objects as long as he has the right capability.
|
|4. instruction set
|	80286 has certain privileged instructions which can only be executed
|	in the kernel mode. This is the direct consequence of the ring structure	and use the segment as the basic protection unit.
|	in 432, you can execute every instructions as long as you have the right	capability to access the instruction object.
|
|5. sharing
|	80286 uses some sharable LDT or aliases to share some data or 
|        instructions between some processes. Or you can put them into GDT to be 
|	shared by every processes in the system.
|	432 sharing is achieved by passing capabilities to the sharable object.
|
|***	It seems the conventional architecture such as 80286 has different
|	approaches for different problems (i.e. privilege level, ring,...).
|	Sometimes it seems that the conventional architecture can't guarantee
|	a true protection. On the other hand, the architecture of 432 is very 
|	uniform; it solves each problems by using the same approach (I.e. 
|	the object model), but it has severe ovehead for object access. As we 
|	know, the 432 suffers from the slow performance problems which is a 
|	direct consequence of this "safest" object model. As a matter of fact, 
|	every object based systems has this kind of problems. So, what should 
|	be our choice ? a conventional architecture whose performance is
|	acceptable but sometimes has protection problems, or a object based 
|	system which has less protection problems but an (unacceptable ?) poor 
|	performance ?
|
|'Kong
|UUCP : ..!{sdcrdcf,ihnp4,cepu,trwspp,ucbvax}!ucla-cs!kong
|ARPA : kong@cs.ucla.edu 


-- 
bill davidsen			sixhub \	ARPA: wedu@ge-crd.arpa
      ihnp4!seismo!rochester!steinmetz ->  crdos1!davidsen
				chinet /
"Stupidity, like virtue, is its own reward"