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My laptop shows me only 4091 Mb of RAM instead of 4096Mb that it should. When I remove one piece of memory (it doesn not matter which one), it then shows 2046Mb.

I do not remember when it started, but when it was new, it showed the full 4096Mb. After Windows has been re-installed it starts to shows 4094Mb, and then 4091Mb.

What could the cause of the issue be?

The Nvidia 210 has separate 512 Mb (well, as it says on laptop: "4GB RAM, 512MB video" - hope if it will be shared memory it will be printed "4GB for RAM and video").

I was using many strange OS on it - like Backtrack Linux Live DVD, Kolibri OS (that is 1.44 Mb). Could it be some kind of loop of RAM-disk or evil zero-level hypervisor? I tried to boot the system with another HDD and even with SATA-to-CF adapter and CF card - no matter, the lack of memory stays on :( .

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Can you provide a screenshot of your task manager? – Ramhound Jul 17 '13 at 11:15

The BIOS needs some memory for things like ACPI tables, power management, and so on.

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Source? According to this post the BIOS does not access anything over the 1MB mark. – BlueRaja - Danny Pflughoeft Jul 17 '13 at 16:27
Plus, if this was the case, when he removed any single stick of memory, the BIOS would still need to "claim" the same amount of memory, which it does not. – NRGdallas Jul 17 '13 at 17:34
Look at the memory map on a modern system. I just measured on one of my machines and the BIOS reserved 3MB of NVS memory. – David Schwartz Jul 17 '13 at 19:48

The memory address range is not only used for RAM, VRAM but also for memory access IO devices such as LAN cards, BIOS, etc. So it will never show the real RAM size

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