thanks for your replies
why letter a is 01000001? why a letter or generaly character is represented by 8 bits?
since I suppose the total number of possible characters is less than 99,999,999, so why not representing letters/characters as 4 bits, or 6 bits, etc
It's just an arbitrary design decision that got made based on the available resources of the architecture in question. I suspect that a=01000001 seemed logical to the person designing the system.
The more bits you have in a byte, the more items of information that can be represented by each. If you use 8-bits, there are 256 possible things each byte can represent. Using 4-bits drops it down to 16 possible things. These things can be letters, numbers, machine states, stack counters, machine instructions, information stored in memory, location in the memory address space, etc.
So the greater the number of bits you use for the basic architecture of your system, the more "bitspace" you have to represent whatever you want. And because most CPUs are designed to work by moving bits around on the byte level (hence 8-bit, 16-bit, 32-bit, 64-bit architectures) the general rule of thumb is: the more bits in a byte, the more powerful the architecture. If you think of a byte as a word and a bit as a letter, then a machine with more bits (or letters) has the potential for having a much bigger vocabulary. And since it has a bigger vocabulary, it also (theoretically) has the potential to be used for much more complex and elegant tasks.
At least on paper. (Note: There's a lot more involved in CPU design than just how big its bitspace
is. Instruction sets, cache design, multithreading, and a pile of other things are also crucial to the overall performance and power of the system.)
You might want to read Soul of a New Machine
by Tracy Kidder. This book tells the real life story of how an actual computer, code-named "Eagle", got designed and built. Although the technology described in the book is somewhat dated (the story took place in the 70s) the methodology, challenges, frustrations and politics of designing a computer have not. Well worth the time to read. It's a classic.
I make it a point to reread this book every couple of years. Each time I do, I never cease to be amazed at just how much - and just how little
- has actually changed between readings!