In article ,
Puckdropper puckdropper(at)yahoo(dot)com wrote:
" wrote in
:

On Tue, 13 Apr 2010 15:58:38 -0500,
(Robert Bonomi) wrote:
*trim*
I agree that subtraction, GENERALLY, "*is* adding the negative".

Not generally. That is one way to do it, but certainly not the only
way. Hardware subtraction is no more difficult than addition. They're
really the same logic (twisted, but really the same).

Now go back and read what _this_ machine actually did. grin

There have been many such things talked about here. Specifically, the
IBM 1620, knows as the CADET, had a table look-up for addition. It
couldn't subtract, either.

I repeat, this multi-million-dollar super-computer "couldn't ADD".

The 1620 wasn't a "multi-million-dollar super-computer". It was, in
fact, rather mundane. That's the reason it couldn't add - they didn't
want to spend money on an adder. Note that it had no subtractor,
either.


Well that's completely different! Using a look up table for addition...
And apparently an extensive one if my 10 second internet search is worth
anything.

Depends on what you mean by 'extensive' it was a 'decimal' (although the
actual 'base' it used for arithmetic operations was programmable!) machine, so
it only needed 100 (2-digit) entries for a 1-digit x 1-digit table (base 10).
1x2 would have required 1000 (3-digit) entires, and 2x2 would have needed
1000 4-digit entires. As the smallest machines had only 20,000 'digits'
of storage, it is clear that the look-up table was a 1x1 matrix (200 digits
total). subtract, and multiply, were also performed using separate look-up
tables.