Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!uw-beaver!tektronix!tekcrl!tekgvs!toma
From: toma@tekgvs.UUCP
Newsgroups: comp.sys.m68k,comp.sys.intel
Subject: Re: Re: Re: Recent Motorola ad seen in Byte
Message-ID: <2238@tekgvs.TEK.COM>
Date: Thu, 23-Apr-87 10:28:59 EST
Article-I.D.: tekgvs.2238
Posted: Thu Apr 23 10:28:59 1987
Date-Received: Sat, 25-Apr-87 08:11:02 EST
References: <930@intsc.UUCP> <1517@ncr-sd.SanDiego.NCR.COM> <932@intsc.UUCP> <652@desint.UUCP>
Reply-To: toma@tekgvs.UUCP (Thomas Almy)
Distribution: comp
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 107
Xref: utgpu comp.sys.m68k:371 comp.sys.intel:171

In article <652@desint.UUCP< geoff@desint.UUCP (Geoff Kuenning) writes:
<In article <932@intsc.UUCP< tomk@intsc.UUCP (Tom Kohrs @fae) writes:
<
<< Not at all!  The 8086 architecture was a real nice way of extending the 
<< address range back in the days when the dominate machine was the 8085 and
<< Z-80.  Swapping segment registers to change address was so much nicer than
<< doing bank selections or overlays.  The 286 architecture is considerably
<< nicer than what was the dominate 16bit mini at the time, the PDP-11.  In
<< 1980 the idea that you could get the cpu power of an 11/70 on a chip with
<< a cleaner memory management model got a lot of people excited.  
<
<What a blatant misrepresentation:
<
<    (1) The PDP-11 is a 1960's architecture.  The 8086 was designed in the
<	late 70's, *after* the publication of Gordon Bell's book where he
<	states, "the most embarassing thing about the PDP-11 was that, only
<	two years after its introduction, we found it necessary to widen
<	the processor address."  But did Intel learn from DEC's mistake?

The PDP-11 came out in 1971 (which makes it a 60's architecture like the
PDP-10 and IBM 360), and was (and probably still is) the 
dominate minicomputer by 1980.  The initial PDP-11 had a 64k address space.
The initial 80x86 had a 1meg address space.  

<    (2) The *only* place where the PDP-11 loses to the 8086 is in the 8086's
<	clever use of instruction prefixes to provide bank switching (that's
<	all segments are).  The PDP-11 doesn't have any registers with
<	wierd private characteristics.  By contrast, even on the 80386, if
<	you want to shift by a variable shift count, it *has* to be in a
<	particular register.  How flexible.

Integer Divides and multiplies (on models that had them) had to use even/odd
register pairs.  MMU registers were memory mapped (which I would call "wierd".
Early PDP-11s could not shift more than one bit per instruction.  The 80x86
instruction set has more serious register limitiations than the variable
shift count (which is rarely used).  Even worse are its dedicated use of SI,
DI, and AX for the string instructions.

Segments can be used for memory protection/management and provide a convenient
way to pass data between processes on 80286 or 80386 processors.  The
unfortunate feature of segments is that the have no use in a UNIX environment.

The PDP-11 looses to the 8086 in price/performance.  It also suffers from
several non-compatible hardware floating point units and two types of hardware
integer multiply divide units.

<    (3) Although it is true that the 808x series was the dominant
<	MICROPROCESSOR CHIP (not the dominant machine, by any stretch of
<	the imagination), it is also true that there were *lots* of better
<	architecture examples around.  The 6800 literally leaps to mind.

The 6800 was better for numeric calculation, but not as good for moving data
around.  But the 6502 and the 6809 were better yet!  Unfortunately look at
the sales figures.

<    (4) Tom also conveniently ignores what a loser the 8085 was.  Essentially
<	no changes from the 8080.  Check out the 6809 by contrast, or compare
<	the PDP-11/70 with the original 11, the 11/20.

Sure the instruction set was no better (go to a Z-80 for that), but the 8085
did reduce the system chip count -- it was mainly better from a hardware
point of view.


<    (5) The basic reason for the 8086's kludgey architecture was [...]
<	because Intel saw binary compatibility with the 8080/8085 as a 
<       critical goal.  That's
<	forgivable;  nobody at the time could have seen that MS/DOS was
<	going to kill CP/M-86.  But contrast DEC's approach with the VAX.
<	They got compatibility with no kludges, and their customers don't
<	seem too unhappy.

What binary compatibility??  It was sort of compatible from source, if you
ran it through a conversion program.  The result was sluggishly running
8086 programs (such as WordStar which on a good Z80 machine would run rings
around the 8086 version on an IBM-PC).  MS-DOS was a closer CP/M-80 clone
than CP/M-86 since only the former supported CP/M 80 style system calls
(that is what CALL 5 is for).  DEC's approach with the VAX matches that of
NEC's V20/30 chip which does maintain compatibility, and I only wish came
out five years sooner (and emulated Z80 as well).

<
<< What caused most of the frustration toward the 286 was DEC and Motorola 
<< both went to a 32bit programming model at that time. Programmers quickly 
<< jumped to arms to adhere to the old maxim of using all of the available 
<< memory plus one byte. When these neat new programs (ie BSD 4.x) were 
<< forced back down to the 16 bit architecture things got tricky.  Many 
<< programmers decided that programming in a 32 bit environment required
<< less effort and less need for structure than the 16 bit environment and
<< so to justify their not liking to work on 16 bit machines they were labled
<< as being kludges or obsolete.
<
<[...]I guess it's just "lazy programming" that kept me from doing arbitrary
<complexity within 64K...

OK, two examples of lazy C programming are 1). the assumption that 
sizeof(int)==sizeof(char *), 2)Using int instead of long for numbers that
need to be longer than 16 bits.


Tom Almy
Tektronix, Inc.

(If I had a choice, I would still be doing assembly languange programming
on a PDP-11, with the best intruction set of all time, rather than a 
680x0 or 80x86, both of which have more kludges than I would have time
to talk about.)