17 Mar
2018
17 Mar
'18
11:49 p.m.
On 3/17/2018 8:54 AM, Dave Horsfall <dave(a)horsfall.org> wrote:
... Was it the 704, or the 709? I recall that the
array indexing order mapped directly into its index register or something
...
It first ran on the IBM 704, whose index registers subtracted (as did
the follow-on 709, 7090, etc), so array indexing went from higher memory
addresses to lower.
The bookshelf: I had most of those books once;
what's the one on the
bottom right? It has a "paperback" look about it, but I can't quite make
it out because of the reflection on the spine.
I'm not sure, and things have shifted since then on the shelves, but I
sent the original photo to your email address.
18 Mar
18 Mar
12:52 a.m.
New subject: RIP John Backus
On 3/17/2018 1:49 PM, Paul McJones wrote:
It first ran on the IBM 704, whose index registers
subtracted (as did
the follow-on 709, 7090, etc), so array indexing went from higher
memory addresses to lower.
Leave it to IBM to do something backwards.
Of course, that was in 1954, so I can't complain, it was 11 years before
I was born. But that's ... odd.
Was subtraction easier than addition with digital electronics back then?
I would think that they were both the same level of effort (clock
cycles) so why do something obviously backwards logically?
ak
9:39 a.m.
New subject: RIP John Backus
On Saturday, 17 March 2018 at 14:52:29 -0400, Arthur Krewat wrote:
On 3/17/2018 1:49 PM, Paul McJones wrote:
Yes. The basic arithmetic operation on ones-complement machines
(which meant just about every big computer of the day) was
subtraction.
It first ran on the IBM 704, whose index
registers subtracted (as did
the follow-on 709, 7090, etc), so array indexing went from higher
memory addresses to lower.
Leave it to IBM to do something backwards.
Of course, that was in 1954, so I can't complain, it was 11 years before
I was born. But that's ... odd.
Was subtraction easier than addition with digital electronics back
then? I would think that they were both the same level of
effort (clock
cycles) so why do something obviously backwards logically?
If I recall this correctly, the big issue was -0. It was easier to
avoid this value with a subtractive adder than with an additive adder.
Possibly the adder itself was also marginally simpler as a result.
The other interesting thing about the 704 and 709 was that there was a
3 bit field for the decrement registers (as the index were called),
and each bit selected one register, so you could use any or all of the
registers in an operation. I'd like to claim that this was the reason
for 3 array subscripts in early FORTRAN, but last time I made a claim
like that I was soundly trounced.
Later 70* machines expanded to 7 decrement registers, and this feature
was lost.
Greg
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1:35 p.m.
New subject: RIP John Backus
On Sat, Mar 17, 2018 at 02:52:29PM -0400, Arthur Krewat wrote:
On 3/17/2018 1:49 PM, Paul McJones wrote:
Speculation:
If you only have a conditional jump on zero, a loop that ends at an
index becoming zero is more easy, it saves an extra subtraction to
test for the end condition. You load the size of the array into the
index register, and then loop until it becomes zero. If you then use
the end the array as the base, you still have a forward loops since the
effective address will be computed as end - index with index begin
decremented in the loop.
-Otto
It first ran on the IBM 704, whose index
registers subtracted (as did
the follow-on 709, 7090, etc), so array indexing went from higher memory
addresses to lower.
Leave it to IBM to do something backwards.
Of course, that was in 1954, so I can't complain, it was 11 years before I
was born. But that's ... odd.
Was subtraction easier than addition with digital electronics back then? I
would think that they were both the same level of effort (clock cycles) so
why do something obviously backwards logically?
ak
2254
days inactive
2255
days old
3 comments
4 participants
participants (4)
-
Arthur Krewat -
Greg 'groggy' Lehey -
Otto Moerbeek -
Paul McJones