Why use float and double instead of real?

[Don]

BCS wrote:
> Hello Don,
> 
>> Size. Since modern CPUs are memory-bandwidth limited, it's always
>> going to be MUCH faster to use float[] instead of real[] once the
>> array size gets too big to fit in the cache. Maybe around 2000
>> elements or so.
> 
> I was under the impression that the memory buss could feed the CPU at 
> least as fast as the CPU could process data but just with huge latency. 
> Based on that, it's not how much data is loaded (bandwidth) but how many 
> places it's loaded from. Is my initial assumption wrong or am I just nit 
> picking?
> 
Intel Core2 can only perform one load per cycle, but can do one floating 
point add per cycle.
So in something like a[] += b[], you're limited by memory bandwidth even 
when everything is in the L1 cache.
But in practice, performance is usually dominated by cache misses.