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I know there are 32/64 bit OS in the market.
Q1-> But what does this bit mean actually ?
Q2-> What is the lowest bit OS and highest bit OS ?

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I think it has to do with the size of an instruction in the cpu, but off top of my head I don't remember too well, look up at wikipedia. –  Cupcake Jul 2 '11 at 10:09
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2 Answers

up vote 6 down vote accepted

This means the number of bits used for addressing in memory.

On a 32bit system an address has 32bits. A bit is a 0 or 1, so if you have 32 bit you can have 2^32 possible combinations which is about 4 Billion. It also means that you cannot have more than 4 Billion addresses in memory. --> since every address usually links to one byte, a maximum of 4 Billion bytes can be addressed with a 32 bit system. Hence the 4 GB memory limit for 32 bit operating systems (Although Physical Address Extensions works around this by using a number of segments. With PAE you can address up to 64GB).

Increasing the number of bits for addressing makes address calculations slower, because one has to calculate with larger numbers, but it also increases the number of addresses available.

There is no limit for the bits you can use. A "one bit OS" does not really make sense, because that would mean that your computer cannot have more than 2 bytes of memory. I think descent operating systems start at 8 bit. 32 bit is the most common although 64bit is growing a lot and will eventually replace 32bit. I have heard of some mainframes that have 128bit, but I don't think anything higher really exists, because 128bit gives you already an unimaginable large number of addresses ;-).

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"because 128bit gives you already an unimaginable large number of addresses ;-)." That reminds me of Bill Gates' "640K ought to be enough for anybody.". (FYI : Actually, Bill Gates didn't say that... :() –  JiminP Sep 4 '11 at 8:34
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A processor's "bits" can actually mean a number of things; there's quite a few things inside and outside of a CPU that is limited by a number of bits. However, in the case of x86 CPUs, here are the following differences:

  • 64-bit CPUs can work with far more RAM without workarounds like PAE than 32-bit CPUs.
  • AMD's 64-bit architecture, which has become the dominant one, added more general purpose registers to the CPU. Registers are temporary holding areas for calculations that are much faster than RAM.
  • All registers in the CPU are 64-bit. This means the CPU can work with larger values with fewer instructions.

As far as PC-compatibile Intel CPUs, the lowest bit OS are the 16-bit OSes such as DOS, CP/M-86, Xenix, and Windows 3.x. Intel did make several 8-bit chips (8008, 8080) that ran a variant of CP/M if I'm not mistaken, and of course there's the old 4004 which is a 4-bit chip but it never ran any OS. No one is producing an x86-compatible 128-bit chip or higher at this time.

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