Reporting low memory memory map

[lopidas]

I read here in http://www.uruk.org/orig-grub/mem64mb.html in Assumptions and limitations that e820 does report whole low memory as free (type 1). Is it true (I am running on emulators)? How can I get reliable map of this space (without making assumptions)? Does SMBIOS returns reliable map?

[Combuster]

e820 returns the memory available for protected mode OSes - which includes the IVT and BDA as free space, but not the part the BIOS needs for it's 32-bit functions like the EBDA which is reserved. Which means it does not technically return all of lower memory, just what you're allowed to use which is the more interesting figure in the end.

Don't use the SMBIOS. it's designed to tell you what the system actually contains, rather than what's safe and available for you to use.


Bottom line, E820 is the designated method for querying the memory map, so you should use it when possible.

[lopidas]

I am not sure, if it can help, but I have found in Table. 7-9 in SMBIOS specification. Which looks like mapping all non-RAM related devices. Does it do it?

[tom9876543]

The low memory map is fixed.

0:0 - 0:03FF Interrupt Descriptor Table
40:0 - 40:FF Bios Data Area http://www.bioscentral.com/misc/bda.htm

As you know, you need to find the address of the EBDA via BIOS Interrupt.
This is explained at http://wiki.osdev.org/Detecting_Memory_%28x86%29

[lopidas]

I want to clarify one thing: are these parts coppied to RAM, which are mapped to that range or mapped on the addresses of that range?

[tom9876543]

It is normal RAM, not memory mapped I/O.