Why is number of bits always(?) a power of two?

Question

We have 8-bit, 16-bit, 32-bit and 64-bit hardware architectures and operating systems. But not, say, 42-bit or 69-bit ones.

Why? Is it something fundamental that makes 2

Answer 1

As others have pointed out, in the early days, things weren't so clear cut: words came in all sorts of oddball sizes.

But the push to standardize on 8bit bytes was also driven by memory chip technology. In the early days, many memory chips were organized as 1bit per address. Memory for n-bit words was constructed by using memory chips in groups of n (with corresponding address lines tied together, and each chips single data bit contributing to one bit of the n-bit word).

As memory chip densities got higher, manufacturers packed multiple chips in a single package. Because the most popular word sizes in use were multiples of 8 bits, 8-bit memory was particularly popular: this meant it was also the cheapest. As more and more architectures jumped onto the 8 bit byte bandwagon, the price premium for memory chips that didn't use 8 bit bytes got bigger and bigger. Similar arguments account for moves from 8->16, 16->32, 32->64.

You can still design a system with 24 bit memory, but that memory will probably be much more expensive than a similar design using 32 bit memory. Unless there is a really good reason to stick at 24 bits, most designers would opt for 32 bits when its both cheaper and more capable.

Answer 2

Because the space reserved for the address is always a fixed number of bits. Once you have defined the fixed address (or pointer) size then you want to make the best of it, so you have to use all its values until the highest number it can store. The highest number you can get from a multiple of a bit (0 either 1) is always a power of two

Answer 3

http://en.wikipedia.org/wiki/Word_%28computer_architecture%29#Word_size_choice

Different amounts of memory are used to store data values with different degrees of precision. The commonly used sizes are usually a power of 2 multiple of the unit of address resolution (byte or word). Converting the index of an item in an array into the address of the item then requires only a shift operation rather than a multiplication. In some cases this relationship can also avoid the use of division operations. As a result, most modern computer designs have word sizes (and other operand sizes) that are a power of 2 times the size of a byte.

Answer 4

We have, just look at PIC microcontrollers.

Answer 5

May be you can find something out here: Binary_numeral_system

Answer 6

Partially, it's a matter of addressing. Having N bits of address allows you to address 2^N bits of memory at most, and the designers of hardware prefer to utilize the most of this capability. So, you can use 3 bits to address 8-bit bus etc...