Why are 8 and 256 such important numbers in computer sciences?

Question

I don\'t know very well about RAM and HDD architecture, or how electronics deals with chunks of memory, but this always triggered my curiosity: Why did we choose to stop at 8 bi

Answer 1

Not all bytes are 8 bits. Some are 7, some 9, some other values entirely. The reason 8 is important is that, in most modern computers, it is the standard number of bits in a byte. As Nikola mentioned, a bit is the actual smallest unit (a single binary value, true or false).

As Will mentioned, this article http://en.wikipedia.org/wiki/Byte describes the byte and its variable-sized history in some more detail.

The general reasoning behind why 8, 256, and other numbers are important is that they are powers of 2, and computers run using a base-2 (binary) system of switches.

Answer 2

ASCII encoding required 7 bits, and EBCDIC required 8 bits. Extended ASCII codes (such as ANSI character sets) used the 8th bit to expand the character set with graphics, accented characters and other symbols.Some architectures made use of proprietary encodings; a good example of this is the DEC PDP-10, which had a 36 bit machine word. Some operating sytems on this architecture used packed encodings that stored 6 characters in a machine word for various purposes such as file names.

By the 1970s, the success of the D.G. Nova and DEC PDP-11, which were 16 bit architectures and IBM mainframes with 32 bit machine words was pushing the industry towards an 8 bit character by default. The 8 bit microprocessors of the late 1970s were developed in this environment and this became a de facto standard, particularly as off-the shelf peripheral ships such as UARTs, ROM chips and FDC chips were being built as 8 bit devices.

By the latter part of the 1970s the industry settled on 8 bits as a de facto standard and architectures such as the PDP-8 with its 12 bit machine word became somewhat marginalised (although the PDP-8 ISA and derivatives still appear in embedded sytem products). 16 and 32 bit microprocessor designs such as the Intel 80x86 and MC68K families followed.

Answer 3

Historical reasons, I suppose. 8 is a power of 2, 2^2 is 4 and 2^4 = 16 is far too little for most purposes, and 16 (the next power of two) bit hardware came much later.

But the main reason, I suspect, is the fact that they had 8 bit microprocessors, then 16 bit microprocessors, whose words could very well be represented as 2 octets, and so on. You know, historical cruft and backward compability etc.

Another, similarily pragmatic reason against "scaling down": If we'd, say, use 4 bits as one word, we would basically get only half the troughtput compared with 8 bit. Aside from overflowing much faster.

You can always squeeze e.g. 2 numbers in the range 0..15 in one octet... you just have to extract them by hand. But unless you have, like, gazillions of data sets to keep in memory side-by-side, this isn't worth the effort.

Answer 4

Computers are build upon digital electronics, and digital electronics works with states. One fragment can have 2 states, 1 or 0 (if the voltage is above some level then it is 1, if not then it is zero). To represent that behavior binary system was introduced (well not introduced but widely accepted).

So we come to the bit. Bit is the smallest fragment in binary system. It can take only 2 states, 1 or 0, and it represents the atomic fragment of the whole system.

To make our lives easy the byte (8 bits) was introduced. To give u some analogy we don't express weight in grams, but that is the base measure of weight, but we use kilograms, because it is easier to use and to understand the use. One kilogram is the 1000 grams, and that can be expressed as 10 on the power of 3. So when we go back to the binary system and we use the same power we get 8 ( 2 on the power of 3 is 8). That was done because the use of only bits was overly complicated in every day computing.

That held on, so further in the future when we realized that 8 bytes was again too small and becoming complicated to use we added +1 on the power ( 2 on the power of 4 is 16), and then again 2^5 is 32, and so on and the 256 is just 2 on the power of 8.

So your answer is we follow the binary system because of architecture of computers, and we go up in the value of the power to represent get some values that we can simply handle every day, and that is how you got from a bit to an byte (8 bits) and so on!

(2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, and so on) (2^x, x=1,2,3,4,5,6,7,8,9,10 and so on)