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

I think I'll have a go. Traditionally I think that "size" meant the width (number of bits) in the register set. On "my" first computer the DEC PDP-8/E the single register available - the accumulator - was 12 bits wide and it was a 12-bit computer, on the PDP-11 the registers were 16 bits wide and it was a 16-bit computer. IBM 370 and VAX had 32-bit registers and were 32-bit computers.

Starting with the 80386 things became difficult. Depending on operating mode it could present itself to be a real mode 8086, a protected mode (PM) 80286 or a PM 80386. With the 64-bit processors using AMD64 or x86-64 you have all of the above and also 64-bit PM. So what are they? It should depend on the basic operating mode of the OS that's running on it. Windows NT 2000, Windows XP-32, Vista-32 7-32 make the processor 32-bit. OS with "64" in them make the processor 64 bit.

As to buses and stuff. There are two physical buses on x86-processors address+data and two logical buses memory+i/o. Special pins on the processor determine if the operation is memory or i/o, read or write and so on. On the 8086/8088 the data and address buses shared the same pins A0-A15 with D0-D15/A0-A7 with D0-D7 with bits A16-A19/A8-A19 being strictly address. On the 80286 they were separate, not sure about the 80186/80188. On the 80286 there were 24 address and 16 data lines. On the 80386 and 80486 there were 32 each for address and data. The 80386SX had the same external configuration as the 80286.

After this buses get complicated. The processors run so fast internally that they are more or less constantly waiting for their caches which in turn are more or less constantly waiting on external RAM. To satisfy the caches' insatiable hunger for data external memory began delivering it in 64 bit wide chunks starting with the Pentium and Pentium MMX that are both 32 bit processors with 32 address lines but with 64 data lines.

With later processors the number of address lines were increased to 36 allowing a total addressable external memory of 64 GB. The processors remained 32-bit internally.

On multi-core processors the hunger for data is even more pronounced so they may have several sets of address and data buses to faciltiate data being crammed into the processor. Desktop processors may have two or three and server processors three to four. I'm not sure but I believe some have switched to 128 bit wide data buses.

For modern 64-bit processors it is not feasible to also have 64 address lines since that would allow memory up to 16 billion Gigabytes which is not possible today. Some motherboards allow 128 GB which means that the processor needs at least 37 address lines.

As you can see address and data buses are no longer really usable to determine processor size. They actually haven't for the last 25 (80386 modes) years.

In C the int type is supposed to be equivalent to the register width. On AMD64 it isn't because there just isn't that great a need for 64 bit ints: 32 bit ints do just nicely in most cases. The width of a pointer in C on AMD64 is 64 bits though.

Answer 3

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.