[Ham-Computers] linux and ram

[Hsu, Aaron (NBC Universal)]

Hi Jonathan,
 
The 4GB memory limit is a physical limitation of 32-bit architecture.  In binary, each bit is a power of 2, so 2^32=4GB.  So the 4GB limit applies to any 32-bit OS, not just Windows.  For reference, previous well known x86 memory addressing limits were 1MB (original IBM PC architecture @ 20-bits) and 16MB (IBM PC AT architecture @ 24-bits) - early versions of DOS could only access 1MB total (640K RAM space) and later versions only 16MB RAM.
 
Program compatibility in 64-bit OS highly depends on how well the 64-bit OS can isolate the 32-bit app.  The problem is that the 32-bit app will not "see" the upper 32-bits of the memory address.  Here are some simple memory addresses as examples (in binary, 8-bit groupings):
 
32-bit:                                     00000000 00000000 00000000 00000001
64-bit: 00000000 00000000 00000000 00000001 00000000 00000000 00000000 00000001

Hopefully, the e-mail system won't mangle those long lines.  Basically, these are the binary representations of the values "1" (32-bit), and 8GB+1 (or 8,589,934,593) (64-bit).

As you can see, in a full 64-bit environment, the 32-bit address only contains the "lower" 32-bits.  Programs witten in 64-bit code also include the "upper" 32-bits.  If you run a 32-bit app and pass it a 64-bit address, it won't "see" the "upper" 32-bits - all it will see are the lower 32-bits.  So using these two examples, instead of using the address 8GB+1, a 32-bit app will use the address "1" (because the upper 32-bits are not taken into consideration).  Can you see how this might cause a problem?

To mitigate this, good operating systems will attempt to run the application in some sort of 32-bit compatibility mode -the application is given a 32-bit environment to work in and is isolated from the 64-bit environment - Windows did this with 16-bit apps.  However, depending on how the application interacts with the OS, there are certain times when the app may want to access something in memory directly rather than via an OS call.  In this type of situation, the 32-bit app, not aware of the "upper" 32-bits of the 64-bit address space, might inadvertantly access an incorrect location and possibly cause corruption of the incorrect address.  The OS should catch this and terminate the error with a fault, but this doesn't always happen.

I can't speak directly for Linux, but the same rules basically apply to any OS.  How well an older app runs in a 64-bit environment depends on how well mannered it is with memory addressing (all memory address calls within it's own address space or via OS calls) and how well the OS "manages" the older app.  Some OS' (and/or apps) are better at this as they were written (or updated) with 64-bit computing in mind.

Hope this helps answer your questions!

73,

  - Aaron, NN6O


BTW, there are some "tricks" available to make 32-bit apps/OS' "see" more than 4GB, but that's beyond the scope of this message.  


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Sent: Wednesday, February 11, 2009 2:56 PM
Subject: [Ham-Computers] linux and ram


what is the max amount of ram that linux will see on 32-bit vs 64-bit?

are there many compatibility issues with programs running on 64-bit linux as compared to running 64-bit windose, etc?


I know to recognize over 4GB RAM in windose you have to be running a 64-bit version... is this the same in linux?


Thanks in advance...

Jonathan