Can someone expain how I use this C data structure that comes from grub? I don't understand hi mem and lo mem

Question

Grub is a multiboot compliant boot loader. When it boots an operating system it creates a structure defining the available memory and leaves a pointer to that structure in memory.

I got that information here:

http://wiki.osdev.org/Detecting_Memory_(x86)#Memory_Map_Via_GRUB

This is the structure that I think I'm interested in:

 typedef struct memory_map
 {
   unsigned long size;
   unsigned long base_addr_low;
   unsigned long base_addr_high;
   unsigned long length_low;
   unsigned long length_high;
   unsigned long type;
 } memory_map_t;

So I've got a collection of memory map structures. As the above page mentions, you can see the memory map by typing "displaymem" at the grub prompt. This is my output

But I don't fully understand the structure....

Why are the lengths set to 0 (0x0)? Do I have to combine low memory and high memory?

It says the values are in 64 bit so did it push together "low mem and high mem" together like this:

__int64 full_address = (low_mem_addr + high_mem_addr);

or am I getting 1 list containing both low and high addresses exclusively in them?

and since I'm using a 32bit machine do I basically refer to each unique address with both values?

I was expecting one single list of addresses, like displaymem shows but with actual length fields populated but I'm not seeing that. Is there something I don't understand?

Answer 1

Ok, basically it's just two variables...that are 64 bit numbers, so what is above and what is below are IDENTICAL!

typedef struct memory_map
 {
   unsigned long size;
   //unsigned long base_addr_low;
   //unsigned long base_addr_high;
   unsigned long long base_addr;
   // unsigned long length_low;
   // unsigned long length_high;
   unsigned long long length; //holds both halves.
   unsigned long type;
 } memory_map_t;

You can get the two halves out like this:

unsigned long base_addr_low = base_addr
unsigned long base_addr_high = base_addr >> 32

The question was just that simple. :-s