Missing 1GB RAM, where is it?

Haojian Zhuang haojian.zhuang at gmail.com
Thu Apr 14 12:12:36 EDT 2011

On Thu, Apr 14, 2011 at 11:43 PM, Andrzej Kardas <andrzej-kardas at o2.pl> wrote:
> On 14.04.2011 17:09, Adrian Cornish wrote:
> Just a thought - lower end graphic cards use system RAM as their own
> dont they
> You right, devices like graphic card, ethernet cards etc. are mapped into
> memory, what cause some amount RAM is not available for system use. But as
> i've wrote before, i'm wondering why addresses in /proc/iomem (where the map
> of memory is) are not contiguous. I think, there should be info about what
> region of memory is mapped where (BIOS, PCI bus, for system use etc) for
> whole 4GB (Am I right??) address space (for 32bit OS). Instead of that,
> there are small "holes" in addresses and the total size of this holes (in my
> case) is about 400MB. So i wonder where that memory is.
What you see from /proc/iomem is virtual address. For the 32-bit
system, it's 4GB size.
Virtual address needn't be sorted one by one.

You have 4GB physical memory. But only a part is mapped linearly in
virtual address.
They're called lowmem. The rest are used as highmem what can be
allocated by device
driver or applications. Kernel needs to map it first before accessing highmem.

CPU's MMU manages the mapping table between virtual address and
physical address.

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