Determine word size of my processor

Question

How do I determine the word size of my CPU? If I understand correct an int should be one word right? I'm not sure if I am correct.

So should just printing sizeof(int) would be enough to determine the word size of my processor?

Answer 1

Many thinks of memory as an array of bytes. But CPU has another view of it. Which is about memory granularity. Depending on architecture, there would be 2, 4, 8, 16 or even 32 bytes memory granularity. Memory granularity and address alignment have great impact on performance, stability and correctness of software. Consider a granularity of 4 bytes and an unaligned memory access to read in 4 bytes. In this case every read, 75% if address is increasing by one byte, takes two more read instructions plus two shift operations and finally a bitwise instruction for final result which is performance killer. Further atomic operations could be affected as they must be indivisible. Other side effects would be caches, synchronization protocols, cpu internal bus traffic, cpu write buffer and you guess what else. A practical test could be run on a circular buffer to see how the results could be different. CPUs from different manufacturers, based on model, have different registers which will be used in general and specific operations. For example modern CPUs have extensions with 128 bits registers. So, the word size is not only about type of operation but memory granularity. Word size and address alignment are beasts which must be taken care about. There are some CPUs in market which does not take care of address alignment and simply ignore it if provided. And guess what happens?

Answer 2

In short: There's no good way. The original idea behind the C data types was that int would be the fastest (native) integer type, long the biggest etc.

Then came operating systems that originated on one CPU and were then ported to different CPUs whose native word size was different. To maintain source code compatibility, some of the OSes broke with that definition and kept the data types at their old sizes, and added new, non-standard ones.

That said, depending on what you actually need, you might find some useful data types in stdint.h, or compiler-specific or platform-specific macros for various purposes.

Answer 3

To use at compile time: sizeof(void*)

Answer 4

Your assumption about sizeof(int) is untrue; see this.

Since you must know the processor, OS and compiler at compilation time, the word size can be inferred using predefined architecture/OS/compiler macros provided by the compiler.

However while on simpler and most RISC processors, word size, bus width, register size and memory organisation are often consistently one value, this may not be true to more complex CISC and DSP architectures with various sizes for floating point registers, accumulators, bus width, cache width, general purpose registers etc.

Of course it begs the question why you might need to know this? Generally you would use the type appropriate to the application, and trust the compiler to provide any optimisation. If optimisation is what you think you need this information for, then you would probably be better off using the C99 'fast' types. If you need to optimise a specific algorithm, implement it for a number of types and profile it.

Answer 5

sizeof(int) is not always the "word" size of your CPU. The most important question here is why you want to know the word size.... are you trying to do some kind of run-time and CPU specific optimization?

That being said, on Windows with Intel processors, the nominal word size will be either 32 or 64 bits and you can easily figure this out:

• if your program is compiled for 32-bits, then the nominal word size is 32-bits
• if you have compiled a 64-bit program then then the nominal word size is 64-bits.

This answer sounds trite, but its true to the first order. But there are some important subtleties. Even though the x86 registers on a modern Intel or AMD processor are 64-bits wide; you can only (easily) use their 32-bit widths in 32-bit programs - even though you may be running a 64-bit operating system. This will be true on Linux and OSX as well.

Moreover, on most modern CPU's the data bus width is wider than the standard ALU registers (EAX, EBX, ECX, etc). This bus width can vary, some systems have 128 bit, or even 192 bit wide busses.

If you are concerned about performance, then you also need to understand how the L1 and L2 data caches work. Note that some modern CPU's have an L3 cache. Caches including a unit called the Write Buffer

Answer 6

What every may be the reason for knowing the size of the processor it don't matter.

The size of the processor is the amount of date that Arthematic Logic Unit(ALU) of One CPU Core can work on at a single point of time. A CPU Cores's ALU will on Accumulator Register at any time. So, The size of a CPU in bits is the the size of Accumulator Register in bits.

You can find the size of the accumulator from the data sheet of the processor or by writing a small assembly language program.

Note that the effective usable size of Accumulator Register can change in some processors (like ARM) based on mode of operations (Thumb and ARM modes). That means the size of the processor will also change based on the mode for that processors.

It common in many architectures to have virtual address pointer size and integer size same as accumulator size. It is only to take advantage of Accumulator Register in different processor operations but it is not a hard rule.