Reply to Steven,
On Sun, 09 Aug 2009 15:51:46 -0400, bearophile
<bearophileh...@lycos.com> wrote:
grauzone:
Then what is there to complain?<
I have paid for 2 GB RAM, so I am allowed to desire a 1800 MB array
:-)
hehe. Not necessarily.
There are often hardware/OS limitations. For example, many 32-bit
Intel chipsets support 4GB of memory but only allow you to use 3GB
because 1GB of *address space* is used for PCI registers. What ends
up happening is you waste 1GB of memory. However, it's advantageous
to use 4GB instead of 3GB because of the memory parity -- each
channel should have the same amount of memory. I worked for a
company that built such systems for some customers. It was a pain
because we wanted to ensure all the memory was tested, but there was
no way to physically test it...
In other cases, there may be video hardware that shares memory with
your OS, leaving you less available space.
My point is, don't count on having 2GB of usable space even if you
physically have 2GB of RAM, it may not be the case. Better off to not
desire than to complain about edge conditions based on hardware
limitations. It's like complaining that your car doesn't work
properly at 150Mph even though the spedometer goes that high.
-Steve
With virtual memory, the amount of actual RAM you have is only a speed consideration.
You can allocate 1.5GB of ram on a system with only 512 MB. It just ends
up swapping when you go and use it. The only constraint is the address space
and the OS. And with things like me memory mapped files you can even treat
the CPU address space as a window on the hard drive and allocate more RAM
than you have address space for.
This blog post and it's link are an interesting read on this topic:
http://blogs.msdn.com/oldnewthing/archive/2009/07/06/9818299.aspx
