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From: brandis@inf.ethz.ch (Marc Brandis)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Comparing 486 to 386 Systems
Message-ID: <27898@neptune.inf.ethz.ch>
Date: 8 Apr 91 11:58:37 GMT
References: <1991Apr4.142742.20601@lonex.radc.af.mil> <1991Apr4.204923.29300@agate.berkeley.edu> <27865@neptune.inf.ethz.ch> <1991Apr6.045408.15395@agate.berkeley.edu>
Sender: news@neptune.inf.ethz.ch
Reply-To: brandis@inf.ethz.ch (Marc Brandis)
Organization: Departement Informatik, ETH, Zurich
Lines: 75

In article <1991Apr6.045408.15395@agate.berkeley.edu> c60b-1eq@e260-1d.berkeley.edu (Noam Mendelson) writes:
>>>Most people also subscribe to the notion that if the number is higher it's
>>>necessarily much more powerful, where in fact:
>>>		(486 - 386) < (286 - 86) << (386 - 286).
>>Well, I guess you subscribe to a similar notion which is also wrong, like most
>>performance comparisons that oversimplify. In fact, (386-286) is the smallest
>                                                     ^^^^^^^^^^^^^^^^^^^^^^^^^
>>of the differences, not by far the largest as your relation indicates. Give
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>me any instruction on the 386 except multiplies that are faster than on the 
>>286.  I know you will not find one.
>
>My comparison was of features, not speed.  The difference between the
>386 and the 286 is vast.  The 386 is 32-bit while the 286 is 16-bit.

Well, if you are comparing features you are right. But this discussion was not
about features. Probably we should define what we mean by 'powerful'. Anyway,
as long as you are running DOS software you will not get too many benefits
from the 32-bit architecture, if none. E.g. the advantages the 386 offers in
running Windows does not have anything to do with the 32-bit architecture but
with the added paging-unit and the added VM-86 mode.

>The 386 can split itself into four concurrent 8086's, each with their
>own 1M memory space, thereby allowing true multitasking.  

Wrong. The 386 can support as many "concurrent 8086's" as you like, even more
than you have memory (using virtual memory). I do not know where you have this
number four from, but it is wrong.

>And the 386
>can access a 4G address space, while the 286 is limited to a mere 16M.
>The 286's main advantage over the 86 is its ability to map 16M of
>extended memory.  Therefore, I reason that (386 - 286) >> (286 - 86).

The 286 offers protection, 1 GB virtual memory etc. If you do not count these
features because they are not used by DOS (software developers programming for
protected mode really appreciate these features), you should not count the 386
features that DOS does not use either. So, where is the difference then.

Anyway, as long as you stay in the DOS world, 16M of memory is plenty enough.

>The 486's main advantage over the 386 is the built-in math coprocessor.
>However, this is somewhat of a null feature because it just saves
>you some money and some space on the motherboard.  That is why I reason
>that it is the smallest difference.

I do not agree. The main advantage in my opinion is the reduced CPI (cycles
per instruction) count. The fact that you do not see this advantage well
enough when running DOS programs is not the 486's fault.
>
>>Of course, I am comparing apples to apples and oranges to oranges,
>>namely CPUs running in the same mode at the same clock frequency.
>
>What you are comparing is apples to oranges.  The 386 is a 32-bit processor
>while the 286 is a 16-bit processor.  It is in a different league altogether.

Since end-users are not in the role to recompile programs, they have to
compare the same program running on different processors. They will not notice
any of the benefits of the 32-bit architecture.
>
>>While it is true that the 486 is not that much faster than the 386 on non-fp
>>code running 16-bit software, you should see a larger difference running 
>>32-bit code.
>
>Perhaps, but since the majority of MSDOS software contains 16-bit code,
>the 486 does not offer much realistic speed improvement.

If you use this argument (compatibility to DOS programs), you should use it
uniformly over all members of the family, including the 386. 


Marc-Michael Brandis
Computer Systems Laboratory, ETH-Zentrum (Swiss Federal Institute of Technology)
CH-8092 Zurich, Switzerland
email: brandis@inf.ethz.ch