Organization: University of Maine System
Date: Thursday, 13 Aug 1992 09:35:26 EDT
From: <REE700A@MAINE.MAINE.EDU>
Newsgroups: misc.wanted,comp.sys.ibm.pc.hardware,misc.forsale.computers
Subject: Re: i486 info wanted

  Mike asks about the differences between '386, '486SX, '486DX and '486DX2:
This should be a permanently-posted FAQ... Oh well.

   The '386 is a protected mode / real mode processor which defies description
in the space alloted.  Let's just say it is a 32-bit CISC processor with no
inherent floating-point capabilities which employes an internal microcode
to implement a set of instructions which are upwardly compatible with the
8088, 8086, 80286.  The floating point processing can be (a) emulated or
(b) provided by an 80387 or equivalent (Cyrix, IIT...) or a Weitek chip.
   The 80386 has full memory management capabilities (Virtual Memory,
Paging and Segmant Relocation) but no internal cache.

   The 80486DX combines (in function - not true structure) an 80386, 80387
and an internal, 8K Byte, 4-way set associative cache (roughly equivalent
in performance to a 32kB direct-mapped cache).  In addition, the microcode
has been improved (reflecting improvements in internal structure, which is
transparent to you & me, the end user) to allow most instructions faster
execution time.  Because of this, the 80486, at the same clock speed, will
outperform the 80386 (both DX).

   The 80486SX is an attempt to reclaim some serious losses in chip yield,
and sell parts to customers who don't really need (or don't realize they
need) the floating point unit (FPU).  THE 80486SX IS STILL A 32 BIT INTERNAL
AND EXTERNAL PROCESSOR!  A 486SX system, at the same clock speed, will execute
many programs the same as a 486DX.

   Of course, many of these customers decided that an FPU was worth it, after
all.  Thus was born the 487SX - a 486DX under an assumed name, with a slightly
different pinout, that disabled the lobotomized 486SX and took over as a 486DX.

   Now, the 486DX family has become much too fast for the average circuit
board and SIMMs.  The only thing preventing a 486DX-100MHz is a fair amount
of cooling and the fact that no cost-effective memory or fiberglass-based
printed circuit board can keep up with it.  (50's were edge of the performance
envelope for a while...).   Thus were born the 486DX2.  These chips have a 50MH
z, and now a 66 MHz (and soon a 100MHz?) 486DX at their core, but the bus
interface logic runs at half the speed.  Since they are meant to run using the
clock speed of the circuit board (half the internal speed), they have a clock
doubler inside them, and hence are called "clock doubled" or DX2 chips.

   Intel, believing (unfortunately correctly) that the average user couldn't
change a chip in their own system, is about to release the Overdrive chip.
This is essentialy a 487SX2.....

   That should answer these questions...

   Jeff Andle    U. Maine

"Everyone tells me that COMMON blocks in Fortran are evil (and they are).  But
these assholes never learned Fortran and have probably crashed the system more
times by abusing null pointers in C than all Fortran programmers combined."

(I like C - even better than Fortran, but they've both got their places.  I
just hate these arrogant undergrad's that parrot what their Com Sci professors
tell them without thinking for themselves!)
Newsgroups: misc.wanted