16 bit Int vs 32 bit Int vs 64 bit Int

Question

I\'ve been wondering this for a long time since I\'ve never had \"formal\" education on computer science (I\'m in highschool), so please excuse my ignorance on the subject.

Answer 1

It depends on whether speed or storage should be optimized. If you are interested in speed and you are running SQL Server in 64 bit mode then 64 bit keys are what you need. A 64 bit processor running in 64 bit mode, is optimized to use 64 bit numbers and addresses. Likewise, a 64 bit processor running in 32 bit mode is optimized to use 32 bit numbers and addresses. For example, in 64 bit mode, all pushes and pops onto the stack are 8 bytes etc. Also fetch from cache and memory are again optimized for 64 bit numbers and addresses. The processor, running in 64 bit mode, may need more machine cycles to handle a 32 bit number just like a processor, running in 32 bit mode needs more machine cycles to handle a 16 bit number. The increases in processing time come for many reasons, but just think about the example of memory alignment: The 32 bit number may not be aligned on a 64 bit integral boundary which means loading the number requires shifting and masking the number after loading it into a register. At the very least, every 32 bit number must be masked before each operation. We are talking at least halving the processor's effective speed while handling 32 or 16 bit integers in 64 bit mode.