Dying Architectures

[Dick Dunn]

> > ...It all started with the PDP-11,
> > which defined the basic instruxion set architexure for practically all
> > new machines. Take a look at the 68000 & say, `this is a pdp-11.'

If I say that, I quickly argue with myself.  "No it's not--the PDP-11 had
one kind of register, not two.  It didn't have eleventeen different
combinations of addressing modes.  The addressing modes of two-address
instructions were distinct, so that it could do memory-memory operations.
Position-independent code was easy.  It had a comparatively small address
space."  What are the similarities?

> PDP-11's?  The IBM 360 series was commercially introduced in 1965,...
> ...  I'm not a big fan of IBM's,
> nor the architecture of the 360/370/303x/308x/309x machines, but in terms
> of "defining architectures" for new machines, they have NO competition
> by just about any measure (number of machines installed [in the appropriate
> class], quantity of installed-base code [$375 Billion+, is the current
> estimate], etc.)

I know of no basis for claiming that commercial success has any relation to
the definition of computer architecture.  The single notable architectural
concept of the 360... series is having essentially the same instruction set
spanning a wide price and performance range of machines.  In terms of
useful architectural concepts (and particularly in terms of characteristics
of interest in implementing programming languages) the whole IBM mainframe
line is architecturally right where it started--in the mid-'60's.  That
they have such a huge installed base in spite of a crippled architecture
(like a 4K forward-only offset in instructions) is a tribute to IBM's
marketing, sales, and support organizations--certainly not to the
architecture of their machines.
-- 
Dick Dunn	{hao,ucbvax,allegra}!nbires!rcd		(303)444-5710 x3086
   ...Simpler is better.