How much ram is supported by 32 bit os

i have got a laptop which has 4 gb installled ram but my laptop show that i can use only upto 2.9 gb ram.why is it so i have tried on vista windows 7 and other os but still its same
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  1. In a computer all bytes in the memory system need a unique name. This is called an address. For example, if you have 2 GB of main memory, then there are 2147483648 bytes of RAM in your machine, each of which require an address for the operating system to communicate to it. To give these all an address you need 31 bits to do it. Now, if/when you have 32 bits, you can name 4 GB (2 bytes to the 32nd power = 4GB).

    This is why the total addressable space available in a 32 bit OS is 4GB – the OS runs out of addresses and cannot communicate/locate any more bytes of memory because of that.

    You may think ”Hey, 4GB of address space… 4GB of RAM… What’s the problem” The problem is that memory isn’t the only thing needing an address. If you install a total of 4GB worth of RAM, the system will detect/use/display less than 4GB of total memory because of address space allocation for other critical functions, such as:

    - System BIOS (including motherboard, add-on cards, etc..)
    - Motherboards resources
    - Memory mapped I/O
    - Configuration for AGP/PCI-Ex/PCI
    - Other memory allocations for PCI devices

    Different onboard devices and different add-on cards (devices) will result of different total memory size. e.g. more PCI cards installed will require more memory resources, resulting of less memory free for other uses.

    This limitation applies to most chipsets & Windows XP/Vista 32-bit version operating systems. Again, this is a limitation of the Operating System not having enough address space to allocate to the system *and* the RAM. Not allocating address space to devices renders them inoperable. Not allocating addresses to RAM simply results in the unaddressed section not being used in an otherwise fully functional computer. Therefore the OS designers assign RAM last.

    If you install a Windows operating system, and if more than 3GB memory is required for your system, then the below conditions must be met:

    1. A memory controller which supports memory swap functionality is used. The latest chipsets like Intel 975X, 955X, Nvidia NF4 SLI Intel Edition, Nvidia NF4 SLI X16, AMD K8 and newer architectures can support the memory swap function.

    2. Installation of Windows XP Pro X64 Ed. (64-bit), Windows Vista 64, or other OS which can provide more than 4GB worth of address space.

    Note: Windows Vista 32bit SP1 will display the installed amount of RAM. This is a display change only.
  2. In general a 32 bit OS will only provide you access to up to 3.2GB of ram, this of course varies a bit by system. 2.9 seems reasonable. Try using a 64 bit copy of windows 7, it will grant you access to all of your memory as a 64 bit OS can address way more memory than you will need for a long time.
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