Detecting Memory (x86): Difference between revisions

 

Usage:

unsigned short total;

unsigned char lowmem, highmem;

total = lowmem | highmem << 8;

return total;

 

====E820h====

Taking this into account, our example code would look like the following. Note that if you prefer a version that does not require the multiboot.h header downloaded from the link above, there is a version listed in the code examples section of this article.

 

 

 

}

}

 

'''WARNING:''' If you downloaded the multiboot header from gnu.org (linked above) you probably got a version which defines the base address and length fields as one 64-bit unsigned integer each, rather than two 32-bit unsigned integers each. [https://forum.osdev.org/viewtopic.php?t=30318 This may cause gcc to pack the structure incorrectly] which can lead to nonsensical values when you try to read it.

Declare the appropriate structure, get the pointer to the first instance, grab whatever address and length information you want, and finally skip to the next memory map instance by adding size+sizeof(mmap->size) to the pointer, because mmap->size does not take itself into account and because GRUB treating base_addr_low as offset 0 in the structure. You must also use mmap_length to make sure you don't overshoot the entire buffer.

 

typedef struct multiboot_memory_map {

unsigned int size;

...

}

 

===Getting an E820 Memory Map===

 

Sample in C (assuming we are in a bootloader environment, real mode, DS and CS = 0000):

// running in real mode may require:

__asm__(".code16gcc\n");

}

}

 

===Getting an UEFI Memory Map===

EFI_STATUS Status;

EFI_MEMORY_DESCRIPTOR *EfiMemoryMap;

EfiEntry = NEXT_MEMORY_DESCRIPTOR (EfiEntry, EfiDescriptorSize);

} while((UINT8*)EfiEntry < (UINT8*)EfiMemoryMap + EfiMemoryMapSize);

 

===Manual Probing in C===

* the assembly language manual probing code that follows this example is better

 

/*

* void count_memory (void)

outb(0xA1, irq2);

}

 

===Manual Probing in ASM===