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Memory Usage On 64 bit machines

Posted on 2015-02-08
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Last Modified: 2015-02-11
Suppose I have 12 Gigs of memory in a 64 bit PC.
When I run a 64 bit program, does it use the first 4 gig that is available to 32 bit programs, or the extra 8 gigs?
In other words, is having 12 gigs available of indirect benefit to 32 bit programs?
Is it first come, first served for the first 4 gigs?

Does windows dynamically switch memory usage of 64 bit programs, as 32 bit programs start?
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Question by:Eirman
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9 Comments
 
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by:Scobber
Scobber earned 200 total points
ID: 40596725
No
The 3-4gb limitation on 32 bit windows is fiction. It is a licensing limitation not a hardware limitation. All modern processors that support PAE (physical address extension) are capable of using large amounts of ram

Read more about PAE here
http://en.m.wikipedia.org/wiki/Physical_Address_Extension
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by:Seth Simmons
Seth Simmons earned 400 total points
ID: 40596857
does it use the first 4 gig that is available to 32 bit programs

it uses what is available regardless of the architecture of the application

is having 12 gigs available of indirect benefit to 32 bit programs?

the only benefit is allowing for more processes to run
32bit processes are still limited to using 4gb regardless of how much is in the system

Does windows dynamically switch memory usage of 64 bit programs, as 32 bit programs start?

no; it will use whatever is available.  it doesn't segregate memory for specific applications (32bit vs. 64bit)
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by:Eirman
ID: 40596938
Does that mean that if 8 gigs is in use by 64bit programs,
there is always 4 gigs available for 32 programs?
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by:Qlemo
Qlemo earned 400 total points
ID: 40597080
The 4g limit is enforced for each single 32bit process. 3 such processes can allocate the 12g. And all processes are sharing the complete memory area.
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by:Eirman
ID: 40597163
The 4g limit is enforced for each single 32bit process.
Wow - I never knew that
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by:John Hurst
John Hurst earned 360 total points
ID: 40597255
In the 7 years I have been using 64-bit machines with 8Gb of memory and all 32-bit Application software, my machines rarely use more than 3.5GB total no matter how many 32-bit Applications I run. It takes running a second machine to use up more memory. I have over 2 dozen 32-bit processes running right now and total memory consumption is 3GB out of 8GB total.
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garycase earned 640 total points
ID: 40597260
"... When I run a 64 bit program, does it use the first 4 gig that is available to 32 bit programs, or the extra 8 gigs?  "  ....
"...  Is it first come, first served for the first 4 gigs? "

These reflect a bit of misunderstanding about the architecture of your PC.    Your PC's memory is "mapped" into an address space for the current process ... so it doesn't matter which specific memory is assigned to a 32-bit process ... the memory it's using will always be addressable with 32-bits.

Note that it doesn't even matter if there is memory available when a process needs it -- the OS will simply swap out some memory from an inactive process and then map that memory to the requesting process [That's what "page faults" are -- a memory page has to be written to disk so the physical RAM can be assigned to another process].

You could, for example, have 5 32-bit processes running, each with 4GB of RAM assigned ... even though you've only got 12GB of RAM :-)     [Depending on how active they were, however, the system might get VERY sluggish due to the exceptionally high number of page faults]
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Expert Comment

by:garycase
ID: 40597262
Forgot to add this:

"... In other words, is having 12 gigs available of indirect benefit to 32 bit programs? " ==>  Actually, it's of DIRECT benefit to 32-bit programs.     You could be running nothing but 32-bit programs and all 12GB would still be available for use by those programs ... the memory pages would simply be mapped to the first 4GB when they were in use.
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Author Closing Comment

by:Eirman
ID: 40603696
Things are a lot clearer now .... I feel less ignorant.
garycase's explanation was especially enlightening.
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