Size of microprocessor

Question

I have read that the microprocessor consists of several components, each having same/different "sizes". But what really confuses me is what determines the stated size of a microprocessor as 16-bit, 32-bit or 64-bit...

Is it:

1. the the ALU's capacity?
2. the size of the data bus?
3. the size of the address bus?
4. the "least common denominator" of the above?
5. or some other factor that I hitherto don't know about?

Answer 1

Generally the bit-size of a processor is the size of its general purpose registers. this often corresponds to the size of the memory bus and possibly the address bus, but doesn't necessarily.

For example, Intel sold a version of the 386 chip called the 386SX (http://en.wikipedia.org/wiki/Intel_80386#The_i386SX_variant) that internally was a 386 with 32-bit registers, but only has a 16 bit data bus. I think that most people would consider the chip to still be a 32-bit processor instead of a 16 bit processor.

Answer 2

Generally this refers to the amount of the memory (2^n) bytes that is addressable by the CPU. Usually it's the same as data bus, but the hardware may do multiple accesss to retrieve that amount so is not 100% guaranteed. Sometimes it also corresponds to CPU register size, however it too can be different.