Happy Birthday VAX 11/780

Ethan Dicks ethan.dicks at gmail.com
Mon Oct 25 09:58:28 CDT 2010


On 10/25/10, Christian Liendo <christian_liendo at yahoo.com> wrote:
> First of all if you have a problem with the article, please just say so...

Besides the typos and the misplaced smart quotes, this stood out for me:

o The VAX instruction set well revered would later on influence
Motorola engineers when designing the 68000 CPU,

Despite the obvious (to those that know all three relevant
architectures and instruction sets) differences between the VAX and
68000 and the similarities between the *PDP-11* and the 68000, it's
unlikely there was time for Motorola engineers to be influenced by the
VAX.  I have held an XC68000 (still owned by my former boss) with a
manufacture date in late 1979.  I also have preliminary (so marked on
each page) data sheets for the 68000 from the same time frame that
both point to the design history of the 68000 to be well before the
release of the VAX.  A quick google of the history of the 68000
reveals that its design origins began around 1976, which is consistent
with the product development cycles of the era - three years from
inception to shipping silicon.

The 68000 is similar enough in instruction set and register
architecture (yes, I know there are plenty of differences) that at
that same company where my boss had the XC68000, they were able to
expedite the porting of the entire PDP-11 codebase to the 68000 with a
TECO macro (and the code shows it).  They munged the code (literally)
through a filter and only had to do some cleanup work around
interrupts and such.  This is something that is non-trivial for, say,
turning 8080 code into 6502 code - it _was_ trivial for PDP-11 ->
68000.

So there's my issue with point #2 of the "three important reasons" in
the article.

Disclaimer:  I wasn't "there" in 1977.  My first professional exposure
to the PDP-11, the VAX, and the 68000 all came in mid-1984, several
years after the invention of each, but I did spend quite a bit of time
in the late 1970s in my 6502 world, drooling over the 68000 datasheet.
 The mere idea of moving from three 8-bit registers and zero page
indirect references to eight 32-bit data registers and eight 32-bit
address registers made my head spin with the possibilities.  Six
months after my first hands-on experience with the 68000, I bought an
Amiga 1000 (the one I brought to VCFmw with the Rejuvenator).

-ethan



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