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I am having windows 7 32 bit operating system and 8 gb of RAM. I know that maximum usage of RAM for windows 32 bit is 4GB. I have installed virtual box which contains two Ubuntu(both 32bit and 64bit)operating systems. Is it possible to allocate the remaining 4GB RAM to operating systems in virtual box so that I can use complete 4 GB RAM dedicatedly to Windows 32 bit.

Right now i have allocated 2.5 GB to windows and 1.5GB RAM to Ubuntu 32 and 64 bit each. I can't run both the operating systems simultaneously.

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There is no 'remaining' ram. Your 32 bit operating system doesn't know it exists, so no VM inside that operating system would either. –  Christian Varga Jun 20 '12 at 5:22
    
Also I don't know about VirtualBox but I know some of the other products require you to have a 64bit host in order to support 64bit guests. –  Scott Chamberlain Jun 20 '12 at 5:35
    
VMware products support install 64bit guest OS on 32bit host OS, if the underlying hardware is 64bit architecture. –  LiuYan 刘研 Jun 20 '12 at 7:53
    
Couldn't you just run Windows as the virtual machine inside an Ubuntu 64-bit host? (A hypervisor architecture could theoretically do what you're asking for, but I don't think there are any existing products that would support it.) –  Harry Johnston Jun 21 '12 at 2:54
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2 Answers

Replace your Windows 7 host from 32-bit to 64-bit and this is the only solution.

In a Windows (exclude servers) 32-bit operating system there is no way to assign address more than 32-bit (and also some reserved by other hardware), so your 4.3GB RAM will be totally invisible to VM.

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How about PAE ? –  Mehrdad Jun 20 '12 at 5:28
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Windows "user" editions seems to limit the maximum amount of RAM you can use even with PAE, but those "server" editions don't. Still need to install another OS –  Alvin Wong Jun 20 '12 at 5:30
    
Yup. Just mentioned it since your answer said "in a 32-bit OS". –  Mehrdad Jun 20 '12 at 5:31
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@AlvinWong: Your statement is incorrect. A 32-bit program can not address more than 4GB of virtual memory at once. But this only limits its use of virtual memory, not its use of RAM. A 32-bit application can use as much memory as it wants. There is no requirement that it fit in the virtual address space. (For example, through the read and write calls, you can use huge amounts of RAM without mapping a single byte.) –  David Schwartz Jun 20 '12 at 6:01
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@AlvinWong: Your distinction between the OS and the program itself is a meaningless one. Programs always use RAM through the OS in this way. When a program handles a string, it has no idea if it's physically resident in RAM (unless it locks it or something very unusual like that). It's up to the OS to decide how best to use physical RAM -- invisible to the process except in performance implications. –  David Schwartz Jun 20 '12 at 6:33
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According to the official Microsoft documentation, Windows 7 ("all supported releases", whether 32-bit or 64-bit), support from 8GB (Home Basic version) to 192GB (Professional, Enterprise and Ultimate). This depends not on the release, but on the CPU architecture (32-bit, or 64-bit, which is what most (all?) desktop and laptop CPUs manufactured within the past 5 years have, being them Intel of AMD, despite the confusing "amd64" name).

Limits on physical memory for 32-bit platforms also depend on the Physical Address Extension (PAE), which allows 32-bit Windows systems to use more than 4 GB of physical memory.

Memory and Address Space Limits

The following table specifies the limits on memory and address space for supported releases of Windows. Unless otherwise noted, the limits in this table apply to all supported releases.

╔════════════════════════╦══════════════╦══════════════╗
║        Version         ║ Limit on X86 ║ Limit on X64 ║
╠════════════════════════╬══════════════╬══════════════╣
║ Windows 7 Ultimate     ║ 4 GB         ║ 192 GB       ║
║ Windows 7 Enterprise   ║ 4 GB         ║ 192 GB       ║
║ Windows 7 Professional ║ 4 GB         ║ 192 GB       ║
║ Windows 7 Home Premium ║ 4 GB         ║ 16 GB        ║
║ Windows 7 Home Basic   ║ 4 GB         ║ 8 GB         ║
║ Windows 7 Starter      ║ 2 GB         ║ N/A          ║
╚════════════════════════╩══════════════╩══════════════╝
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