I was looking at the memory tab of Window's "Resource Monitor," it listed 258mb of "Hardware Reserved" memory. What is "Hardware Reserved" memory really used for? Is Window's catering to users who it knows would never understand what it's really used for(something complex/confusing, but OS decided), or is there really memory that CANNOT EVER be used by the OS?

Maybe video card...

Yes there are memory locations that should never be used. Some BIOS interrupts (don't remember which) can map and identify these areas.

int 15, eax=e820 is the most common for mapping them, but AFAIK, the info provided as to the reason of their reservation is very general.

Lots of hardware on the PCI bus use memory-mapped IO instead of ports, the video frame buffer has to sit somewhere, even CPU's talk to each other via memory addresses. They are usually at the top of the address space.

I think it is a Microsoft conspiracy. They are deliberately hiding memory from all users. They use this memory for their own purposes, to further their plans for world domination.

Either that or what sinsi said.

Actually, come to think of it, I think that sinsi's explanation is the more likely of the two.

I didn't mean it like that, Windows tends to simplify things. Sometimes to the extent that what they say is too ambiguous for interested people like myself to decipher what they're really talking about. But now that you mention it... I bet it ties into Bill Gate's plan to replace all world currency with MS points. They're everywhare, Xbox, Zune, Hotmail, they all require a Windows Live account. It's a global conspiracy!!!

Well, it would be an Intel conspiracy then, since I'm pretty sure that Linux will be hiding memory too...omg, it just grows!!!

Linus is just Bill's pawn to keep him from getting sued again. I bet Linux will even report the same amount, it's all orchestrated behind the scenes to make them look competitive. I wonder: Does Windows use the e820 function?

Anyway, if you are running windows you can look at device manager to see what sort of memory addresses are used by hardware.
Writing to those addresses could do strange things to your hardware, similar to sending bytes to random ports would.

win95 uses the e820 function iirc

Now that you mention the Device Manager, how many of the IO ranges and such can be used by me(as in used in my OS)? Are most of them custom configured by Windows/Drivers to the ports/irqs listed or are most of them at their default ranges?

Interesting: High Definition Audio Controller - (C000000h - C0003FFFh) What would this memory be used for, code, data, IO?

http://board.flatassembler.net/topic.php?p=43144#43144

Hardware reserved memory is several regions taken by specific devices/mechanisms present on the system. This includes things like ACPI, APIC, VGA/VESA LFB, PCI, BIOS ROM and then devices like graphics/sounds cards. Some of this memory is used by the OS for MMIO interaction with the hardware, other may be reserved by the hardware for its own use (as defined by their own specs) or simply be a buffer that is used by the OS and hardware.

Detection is a little more complicated since E820 only seems to map physical memory, ACPI and ROM areas; other devices have predefined locations or can be looked up easily using other methods. In the case of PCI I think you have to probe it to find the other devices and their ranges. I don't know about other chipset features.

If Windows doesn't use e820, and I assume it doesn't use other BIOS memory functions, what does it use?