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Opposite question about missing memory

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July 16, 2012 6:14:32 AM

I understand very well why people (such as me) with 4 GB of ram cannot see it all when using a 32-bit operating system such as Windows XP (32b). And, I have grown accustomed to seeing, IIRC, about 3.4? (can't quite recall) gigabytes of ram when looking at system properties. (I had a gts 250 with 512 mb of ram.)

Recently, however, I changed to a 6850 with a full gigabyte of ram.

But ... Windows XP system properties reports that I now have 3.25 GB of ram, which, too, is what vSuite Ramdisk reports as the amount of 'invisible os memory'.

What's up with that? Seems like it should be some number at least 1 GB less than 4 GB; that is to say, Windows XP should be seeing something more around 2.75 GB (or about 512 MB less).

(BTW, those with questions about using all their memory with a 32-bit operating system might give some thought to a ramdisk with their page file and/or temp directories there.)

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a c 90 } Memory
July 16, 2012 6:27:51 AM

you have video ram and system ram mixed up. the ram on your video card is not used as system ram and does not have the 4g system cap. it when people use onboard video chipsets..the cpu with the gpu built in that the cpu/gpu has to use system ram as video ram. so those mb shaddow x amount of ram for video. when you have as gpu and a cpu the os is going to use the gpu video ram for all the image needs of the system.
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July 16, 2012 6:44:25 AM

smorizio said:
you have video ram and system ram mixed up. the ram on your video card is not used as system ram and does not have the 4g system cap. it when people use onboard video chipsets..the cpu with the gpu built in that the cpu/gpu has to use system ram as video ram. so those mb shaddow x amount of ram for video. when you have as gpu and a cpu the os is going to use the gpu video ram for all the image needs of the system.


Thanks for your prompt reply. Guess I never really thought this through before, or misunderstood what others said about address space being used by video cards (onboard only, i guess?). So, I could install a couple of HD 7970s, each with 3GB of ram in CrossFire, and I'd still see 3.25 GB of ram?
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July 16, 2012 6:46:48 AM

Well, then, wait a second. If my gts 250 wasn't using 512 MB of the missing ram, what the heck is using 750 MB of ram? I have a pretty plain jane system. e8400, g31, onboard sound and nic. Nothing special.
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a c 90 } Memory
July 16, 2012 6:52:39 AM



Due to an architectural decision made long ago, if you have 4GB of physical RAM installed, Windows is only able to report a portion of the physical 4GB of RAM (ranges from ~2.75GB to 3.5GB depending on the devices installed, motherboard's chipset & BIOS).

This behavior is due to "memory mapped IO reservations". Those reservations overlay the physical address space and mask out those physical addresses so that they cannot be used for working memory. This is independent of the OS running on the machine.

Significant chunks of address space below 4GB (the highest address accessible via 32-bit) get reserved for use by system hardware:

BIOS – including ACPI and legacy video support
PCI bus including bridges etc.
PCI Express support will reserve at least 256MB, up to 768MB depending on graphics card installed memory

What this means is a typical system may see between ~256MB and 1GB of address space below 4GB reserved for hardware use that the OS cannot access. Intel chipset specs are pretty good at explaining what address ranges gets reserved by default and in some cases call out that 1.5GB is always reserved and thus inaccessible to Windows.
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July 16, 2012 7:37:54 AM

smorizio said:
Due to an architectural decision made long ago, if you have 4GB of physical RAM installed, Windows is only able to report a portion of the physical 4GB of RAM (ranges from ~2.75GB to 3.5GB depending on the devices installed, motherboard's chipset & BIOS).

This behavior is due to "memory mapped IO reservations". Those reservations overlay the physical address space and mask out those physical addresses so that they cannot be used for working memory. This is independent of the OS running on the machine.

Significant chunks of address space below 4GB (the highest address accessible via 32-bit) get reserved for use by system hardware:

BIOS – including ACPI and legacy video support
PCI bus including bridges etc.
PCI Express support will reserve at least 256MB, up to 768MB depending on graphics card installed memory

What this means is a typical system may see between ~256MB and 1GB of address space below 4GB reserved for hardware use that the OS cannot access. Intel chipset specs are pretty good at explaining what address ranges gets reserved by default and in some cases call out that 1.5GB is always reserved and thus inaccessible to Windows.


Again, thank you for a prompt, thoughtful, and very well written response.

I was about to shrug my shoulders and go, "Okay," when it occurred to me: I'd bet you dollars to donuts that if I rooted around in the bone yard for a 2 x 512 MB kit of ddr2 800, I could boot and run this system using just one stick. Yeah, sure, no dual channel memory or geeky ramdrive, and my frame rates are gonna drop in L4D, but I'm sure it'd boot and run well enough for my wife's use.

Too, what about all the systems I've built and used using just 256 or 512 MB of ram. Sure, they didn't have PCI-e or a 775 socket, but they all had bios, pci buses, and agp ports.

Not doing a good job of formulating my question, but you sound like someone who would understand what I'm trying to say (ask).

Thanking you in advance, Your Humble Servant.
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a c 146 } Memory
July 16, 2012 8:26:55 AM

Go in BIOS and set the iGpu to use smallest amount possible.
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a b } Memory
July 16, 2012 9:00:14 AM

nikorr said:
Go in BIOS and set the iGpu to use smallest amount possible.


He's not using an integrated gpu, he's using dedicated and asking why the memory available didn't drop as much as he thought it would...

Mistermouse, what smorizio is saying is that the OS is reserving memory ADDRESSES to use to "talk" to other parts of the system hardware. It's NOT using the physical memory.

So if you put in 512MB you will be able to use 512MB. If you put in 2GB you will have 2 GB. It's only when you put in more memory than the available remaining address space that the system can't access that portion of memory. Nothing is using that physical memory, the system just doesn't have enough addresses left to assign to and use it.
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a c 146 } Memory
July 16, 2012 9:13:17 AM

unksol said:
He's not using an integrated gpu, he's using dedicated and asking why the memory available didn't drop as much as he thought it would...

Mistermouse, what smorizio is saying is that the OS is reserving memory ADDRESSES to use to "talk" to other parts of the system hardware. It's NOT using the physical memory.

So if you put in 512MB you will be able to use 512MB. If you put in 2GB you will have 2 GB. It's only when you put in more memory than the available remaining address space that the system can't access that portion of memory. Nothing is using that physical memory, the system just doesn't have enough addresses left to assign to and use it.

I know, but sometimes it is reserved and he don't want that.

By making it min. size, it releases the rest of it.
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July 16, 2012 9:18:00 AM

unksol said:
He's not using an integrated gpu, he's using dedicated and asking why the memory available didn't drop as much as he thought it would...

Mistermouse, what smorizio is saying is that the OS is reserving memory ADDRESSES to use to "talk" to other parts of the system hardware. It's NOT using the physical memory.

So if you put in 512MB you will be able to use 512MB. If you put in 2GB you will have 2 GB. It's only when you put in more memory than the available remaining address space that the system can't access that portion of memory. Nothing is using that physical memory, the system just doesn't have enough addresses left to assign to and use it.


I think I understand, especially since it would seem that I would have some trouble with my 750 MB ramdrive if something else (hardware or os) was using that memory, wouldn't I?

In short, the system/os is 'using' (reserving), not the memory itself, but the memory addresses that would be needed to use (address) that memory.

Being a tedious person, I have to ask: Why? Why so many? I mean, three quarters of a billion anything is a lot. And, too, if they're not gonna use memory, then, well, why reserve memory addresses? Why not make reservations at Butterfield's instead?
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a b } Memory
July 16, 2012 9:32:30 AM

mistermousepotato said:
I think I understand, especially since it would seem that I would have some trouble with my 750 MB ramdrive if something else (hardware or os) was using that memory, wouldn't I?

In short, the system/os is 'using' (reserving), not the memory itself, but the memory addresses that would be needed to use (address) that memory.

Being a tedious person, I have to ask: Why? Why so many? I mean, three quarters of a billion anything is a lot. And, too, if they're not gonna use memory, then, well, why reserve memory addresses? Why not make reservations at Butterfield's instead?


Lol... This is a good read, particularly the memory mapped IO section

http://en.wikipedia.org/wiki/3_GB_barrier
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July 16, 2012 9:58:37 AM

unksol said:
Lol... This is a good read, particularly the memory mapped IO section

http://en.wikipedia.org/wiki/3_GB_barrier


That actually made sense. Thanks.

Interesting to contrast the response to this issue by Apple and Microsoft. Even recognizing the different paradigms in the ecostructures of the two; nevertheless, Microsoft's response seems to have been ham fisted, or perhaps they had an agenda? Either way, they didn't address the problem; in fact, they managed to institutionalize the problem for nigh on two decades now.

Hm. I can't help but wonder whose drivers they were that caused "driver compatibility issues."
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July 28, 2012 5:30:03 AM

Best answer selected by mistermousepotato.
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a c 146 } Memory
July 28, 2012 5:59:24 AM

This topic has been closed by Nikorr
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