[Yossarian]

If you don't need 64-bit addressing (i.e. if you don't need to access more than, say, 2 GB of RAM at any given time), I'd stay away from a 64 bit chip.

Why? Because all those 64-bit pathways make the chip bigger, and bigger chips are slower chips.

Of course, some media instructions are good to have as wide as reasonable, and operating systems will use more and more RAM as disk caches. But in general, it's like renting a limo when a Ferrari would do.

[krb]

If you don't need 64-bit addressing (i.e. if you don't need to access more than, say, 2 GB of RAM at any given time), I'd stay away from a 64 bit chip.

It's not the address space that interests me, it's the 64-bit registers I would like to be able to use.

[antiRepublicrat]

Why? Because all those 64-bit pathways make the chip bigger, and bigger chips are slower chips.

I think AMD did very well with its 64-bit extensions to the x86 ISA. IBM also did well with its PPC 970 chip that's designed from scratch to do 32- and 64-bit natively with no performance hit in either mode. The Itanium is a different monster, a 64-bit chip that has to run 32-bit code in emulation -- a disaster when general computing is still mostly 32-bit (although they do well in supercomputers).