[plt-scheme] still confused about 32/64 bit

Hi all,

please excuse the off-topic question, but having started trying to  
understand this I would like to get it right now :-;

After reading some articles on Wikipedia, my main question is now:   
Does  "architecture" refer to the hardware or to the software (the OS)?
Because first I thought it referred to the hardware, like in "Intel  
x86 architecture". In that case, as my processor is an Intel Core 2  
Duo, I guess I should have an x86_64 architecture.
On the other hand, when I type

$ arch

I get "i386" which stands for 32 bit intel.
So "architecture" perhaps refers to how the OS uses the processor (I'm  
still running Leopard, where the kernel runs in 32bit  mode)?

Also, I wonder is "platform" just a synonym for "architecture", or  
does it mean something different?

Thanks a lot for any explanation!

Ciao
Sigrid