32bit Linux vs 64 bit Linux Post: 302318958

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Operating Systems Linux Red Hat 32bit Linux vs 64 bit Linux Post 302318958 by robsonde on Saturday 23rd of May 2009 12:51:23 AM

05-23-2009

Registered User

Any 32 bit OS will only see / use 3.5GB of RAM.

Remember that in the absence of the 64 bit support, the OS memory manager is limited to a 4GB physical address space. Most of that address space is filled with RAM, but not all of it. Memory-mapped devices (such as your video card) will use some of that physical address space, as will the BIOS ROMs. After all the non-memory devices have had their say, there will be less than 4GB of address space available for RAM below the 4GB physical address boundary.

the motherboard assigned the ROMs and the hardware devices to the physical address space between 3.5GB and 4GB (occupying about 0.5GB of address space). When you start plugging in your memory chips, then, they are assigned physical addresses starting at the bottom, and then skip over the address space that has already been assigned to the hardware and ROM, then resume.

On this imaginary system, then, the 0.5GB of address space used for hardware and ROMs causes that much memory to get shoved upwards, and it ends up above the 4GB boundary. Without 64 bit support, the processor is capable only of addressing memory below the 4GB boundary, which means that the memory above that boundary is inaccessible. It's consuming electricity but isn't doing anything.

The solution is to go to 64-bit OS so that the processor can access the physical address space above the 4GB boundary.

So why don't we just map the ROMs and the hardware devices to space above 4GB??
then the CPU can't access the IO devices so you have system with 4GB of RAM and no video card......

any questions??

robsonde

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dma32_pool_size

dma32_pool_size(5)						File Formats Manual						dma32_pool_size(5)

NAME

       dma32_pool_size - the amount of memory to reserve for the 32-bit DMA pool

VALUES

   Minimum
   Maximum
   Default
       Specify an integer value.

DESCRIPTION

       This  tunable  specifies  the amount of memory in the first 4GB of the physical address space to be reserved for DMA for 32-bit cards.  The
       memory is set aside at boot time and cannot be used for other purposes.	This is required only on some  Itanium	platforms.   On  platforms
       where the 32-bit pool is not needed, this value is ignored.

       The value of this tunable should be set based on the number of 32-bit cards in the system and the type of cards.

   Related Parameters
       None

WARNINGS

       All  HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases of
       HP-UX.

       Installation of optional kernel software, from HP or other vendors, may cause changes to tunable  parameter  values.   After  installation,
       some  tunable  parameters may no longer be at the default or recommended values.  For information about the effects of installation on tun-
       able values, consult the documentation for the kernel software being installed.	For information about optional kernel  software  that  was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

SEE ALSO

       None

Itanium(R)-based System Only				     Tunable Kernel Parameters						dma32_pool_size(5)