From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Subject: Re: octal to decimal converter?
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University
Date: Wed, 23 Dec 1992 16:00:40 GMT

In article <16098357@zl2tnm.gen.nz> don@zl2tnm.gen.nz (Don Stokes) writes:
>lasner@watsun.cc.columbia.edu (Charles Lasner) writes:
>
>> In any case, the point is made:  with all of its complications, it doesn't
>> seem that a VAX can create significantly tighter code thana PDP-8 RISC
>> machine.  In some cases, specialized instructions will beat it out, calling
>> up a lot of hardware to do it, but even then not by much.  And in some cases
>> it's still too clumsy to do as well.
>
>You made the assertion that PDP-8 code was significantly smaller; I believe
>I've demonstrated that this isn't necessarily the case.  It _is_ fair to 
>say that code on a VAX is not bummed in like PDP-8 code, but on a VAX, *you*
>*don't* *need* *to*.  There isn't the kind of address space cram on a VAX
>or even PDP-11 that there is on an -8.  Again, the tradeoffs of the '1960s
>that the PDP-8 represents were not the same as the tradeoffs of the 1970s 
>that produced the VAX.

But the VAX has to do all sorts of odd-sized instruction fetches that evade
many forms of optimization while the -8-type form is easily improved on.
As to whether or not you need to squeeze or not depends on the application.
Just because there is the potential of a large address space doesn't mean
that squandering it is useful.  There are no VM -8 systems, as all of the
address space is quite real.  How much performance lossage is due to the
excessively large program swapping itself?

I wouldn't put the -11 and the VAX into that statement above.  I think you'll
find that the -11 programs are longer; your own examples were -11-ish and you
had to resort to VAX-specific tricks to keep up with the -8.  MOV instructions
can be useful in certain places, but if it's the only mechanism available, it
will often get in the way.

>
>> Considering how much hardware the VAX is, don't you think the problems
>> that can be so solved are better served by multiple PDP-8's that amass to
>> the same amount of electronics! :-).
>
>Hmmm -- there's a VAX humming away quietly beside my left knee here, and
>a PDP-8 (well, a DECmate III+) right behind me, acting as a terminal to the
>VAX....

All that proves is that there are small-scale almost-no peripherals PDP-8's.
>
>Physically, the DECmate has two serial ports, character mode video, a 20MB
>hard disk and 64kwords (96kbytes) of memory.  There are three circuit boards,
>a main board, hard disk controller daughtercard and APU (z80 copro) card.
And it could have 800x300 color graphics if you had the graphics board
which btw doesn't have a CPU on it, just the NEC 7220 graphics chip and local
RAM, etc.  Also, the 20 MB disk is a marketing consideration, as there are
disks up to about 90 megs that could fit into the little box and still be
MFM.  Also, the decision to be MFM was so it was compatible with the older
DM II design.  Since the disk controller has its own 8751, it need only be
compatible with the DM II controller's calls, and could have been ESDI, SCSI
or even IDE for that matter, since the disk is access logically.

PDP-8's have been created with 40 or more serial ports.  The largest one I
worked on had 4 groups of 8 plus 3 console terminals and a few modems, and
3 paper-tape readers and punches.  DEC's decision to limit the DECmates to
only a comm port and a limited serial printer port is an economy move (no
buss).  Also, they didn't implement a memory manager so the DECmates can't
have the extended memory of an -8, which is available up to 512K words
(768K bytes).  Moreover, they didn't implement a terminal to it either; the
CP memory program's primary purpose is to emulate a VT-220 terminal via
software traps of the standard interface.  (And they didn't get that quite
right either.)
My PDP-8/e-a system has two 80 Meg MFM disks and a SyQuest 45 Meg removable
cartridge disk on it.  I could have more if I wanted to as it is an SCSI
buss disk subsystem.  Also, CESI sold a board-level machine based on the
6120 that had 8 serial ports and an SCSI buss that would be a much better
configuration than a DM.

>
>The VAX has 100MB of disk, a TK50 tape drive, 9 serial ports, 16MB of
>memory, 768x1024 bitmapped video, keyboard, mouse and ethernet in eight
>boards.  Somehow methinks you're not going to get quite the same grunt
>out of the equivalent load of PDP-8s, and they're both of similar vintage
>(the VAX is a little older).  OK, it's not quite the same as the 1977 
>versions....

All that proves is that DEC didn't put any effort whatsoever in adding the
stuff onto the -8, not any inherent limitations of the architecture.

>
>Of course DEC did build a VAX out of microprocessors -- the 11/730 (and
>11/725, which is the same CPU in a smaller box with a built-in (RC25)
>disk drive).  Pull the main CPU board out, and you find eight AM2901 
>chips in a bunch.....

Also the 2020 was done this way.  So, what if there was a bit-slice PDP-8?

cjl

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