How much memory can a 32 bit process access on a 64 bit operating system

Question

On Windows  under normal circumstances a 32 bit process can only access 2GB of RAM  or 3GB with a special switch in the boot ini file   When running a 32 bit process on a 64 bit operating system  how much memory is available  Are there any special switches or settings that can change this

User · Answer

The limit is not 2g or 3gb its 4gb for 32bit.

The reason people think its 3gb is that the OS shows 3gb free when they really have 4gb of system ram.

Its total RAM of 4gb. So if you have a 1 gb video card that counts as part of the total ram viewed by the 32bit OS.

4Gig not 3 not 2 got it?

User · Answer

4 GB minus what is in use by the system if you link with  LARGEADDRESSAWARE   Of course  you should be even more careful with pointer arithmetic if you set that flag

User · Answer

An single 32-bit process under a 64-bit OS is limited to 2Gb  But if it is compiled to an EXE file with IMAGE FILE LARGE ADDRESS AWARE bit set  it then has a limit of 4 GB  not 2Gb - see https   msdn microsoft com en-us library aa366778 VS 85  aspx  The things you hear about special boot flags  3 GB   3GB switches  or  userva are all about 32-bit operating systems and do not apply on 64-bit Windows   See https   msdn microsoft com en-us library aa366778 v vs 85  aspx for more details   As about the 32-bit operating systems  contrary to the belief  there is no physical limit of 4GB for 32-bit operating systems  For example  32-bit Server Operating Systems like Microsoft Windows Server 2008 32-bit can access up to 64 GB  Windows Server 2008 Enterprise and Datacenter editions      by means of Physical Address Extension  PAE   which was first introduced by Intel in the Pentium Pro  and later by AMD in the Athlon processor - it defines a page table hierarchy of three levels  with table entries of 64 bits each instead of 32  allowing these CPUs to directly access a physical address space larger than 4 gigabytes     so theoretically  a 32-bit OS can access 2 64 bytes theoretically  or 17 179 869 184 gigabytes  but the segment is limited by 4GB  However  due to marketing reasons  Microsoft have limited maximum accessible memory on non-server operating systems to just 4GB  or  even  3GB effectively  Thus  a single process can access more than 4GB on a 32-bit OS - and Microsoft SQL server is an example   32-bit processes under 64-bit Windows do not have any disadvantage comparing to 64-bit processes in using shared kernel s virtual address space  also called system space   All processes  be it 64-bit or 32-bit  under 64-bit Windows share the same 64-bit system space   Given the fact that the system space is shared across all processes  on 32-bit Windows  processes that create large amount of handles  like threads  semaphores  files  etc   consume system space by kernel objects and can run out of memory even if you have lot of memory available in total  In contrast  on 64-bit Windows  the kernel space is 64-bit and is not limited by 4 GB  All system calls made by 32-bit applications are converted to native 64-bit calls in the user mode

User · Answer

2 GB by default   If the application is large address space aware  linked with  LARGEADDRESSAWARE   it gets 4 GB  not 3 GB  see http   msdn microsoft com en-us library aa366778 aspx   They re still limited to 2 GB since many application depends on the top bit of pointers to be zero

User · Answer

A 32-bit process is still limited to the same constraints in a 64-bit OS  The issue is that memory pointers are only 32-bits wide  so the program can t assign resolve any memory address larger than 32 bits

User · Answer

You ve got the same basic restriction when running a 32bit process under Win64  Your app runs in a 32 but subsystem which does its best to look like Win32  and this will include the memory restrictions for your process  lower 2GB for you  upper 2GB for the OS

User · Answer

Nobody seems to touch upon the fact that if you have many different 32-bit applications  the wow64 subsystem can map them anywhere in memory above 4G  so on a 64-bit windows with sufficient memory  you can run many more 32-bit applications than on a native 32-bit system

[windows] How much memory can a 32 bit process access on a 64 bit operating system?

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