Path: utzoo!mnetor!uunet!oddjob!hao!ames!amdcad!cae780!tektronix!dadla!amadeus!jamesa
From: jamesa@amadeus.TEK.COM (James Akiyama)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Segmented vs. linear architectures
Message-ID: <1032@amadeus.TEK.COM>
Date: 25 Feb 88 01:46:34 GMT
References: <31@vsi.UUCP>
Distribution: comp
Organization: Tektronix Inc., Beaverton, Or.
Lines: 35
Keywords: 80x86, segmentation, linear, architecture


First, I should probably mention that I'm not sold on segmentation.  I have
examined both architectures and believe that each have advantages in certain
applications.

Intel's segmentation turns out to be an effective method of providing a
relatively low cost memory management scheme.  Privileges can be assigned thru
the segments resulting in certain memory locations being protected.  Using
this scheme, potential privilege violations need only be checked when crossing
segment boundaries--resulting in faster execution speeds and/or lower cost
(over schemes requiring privilege testing on every access).  This type of
protection scheme does have a drawback of not being entirely transparent to the
software (since memory areas with different privileges must live in different
segments; requiring software to load the appropriate segment register before
accessing).  Other protection schemes exists which are more transparent but
also, generally, more expensive (in either dollars and/or speed penalties).
Also, these software idiosyncrasies are often handled in the operating system,
making them relatively "transparent" to applications.

Execution speed between the Intel segmented architecture and a linear
architecture (such as Motorola's) is also very dependent on application.  I
think you'll find that both chips (80X86 and 680X0) both offer comparable
speeds--oftentimes speed being more related to the support hardware, compiler,
or operating system.

I think the biggest drawback to the 8086, 80286 segments are their 64K boundary
limits.  This is what causes all the memory model headaches.  This was, for the
most part, fixed in the 80386 if you are willing to lose downward compatibility
(to the older processors).

In summary, which architecture is appropriate is very dependent on your
particular application.

					James E. Akiyama
					Tektronix, Inc.