18 Mar
2018
18 Mar
'18
8:35 a.m.
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