Detecting Memory (x86): Difference between revisions

 

Usage:

; Clear carry flag

clc

 

; AX = amount of continuous memory in KB starting from 0.

 

Note: this function is supposed to be always present, and may not modify the carry flag. If an emulator doesn't support it, the carry flag will be set, indicating error.

In reality, this function returns an unsorted list that may contain unused entries and (in rare/dodgy cases) may return overlapping areas.

Each list entry is stored in memory at ES:DI, and DI is <b>not</b> incremented for you. The format of an entry is 2 uint64_t's and a uint32_t in the 20 byte version,

It is probably best to always store the list entries as 24 byte quantities -- to preserve uint64_t alignments, if nothing else. (Make sure to set

that last uint64_t to 1 before each call, to make your map compatible with ACPI).

 

Linux Usage:

XOR CX, CX

XOR DX, DX

; AX = number of contiguous Kb, 1M to 16M

; BX = contiguous 64Kb pages above 16M

 

====BIOS Function: INT 0x15, AX = 0xDA88====

 

Usage:

CLC ; CF bug workaround

MOV AH, 0x88

JE SHORT .ERR

; AX = number of contiguous KB above 1M

 

====BIOS Function: INT 0x15, AH = 0x8A====

 

Usage:

unsigned short total;

unsigned char lowmem, highmem;

total = lowmem | highmem << 8;

return total;

 

====E820h====

 

{{Quotation

}}

 

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.

Alternatively, you can modify the multiboot header, specifically the multiboot_mmap_entry struct, to the following to get the correct values:

 

struct multiboot_mmap_entry

{

} __attribute__((packed));

typedef struct multiboot_mmap_entry multiboot_memory_map_t;

 

Each multiboot mmap entry is stored as the following:

== What about on UEFI? ==

On UEFI, you have 'BootServices->GetMemoryMap'. This function is similar to E820 and is the only solution on new UEFI machines. Basically, to use, first you call it once to get the size of the memory map. Then you allocate a buffer of that size, and then call again to get the map itself. Watch out, by allocating memory you could increase the size of the memory map. Considering that a new allocation can split a free memory area into two, you should add space for 2 additional memory descriptors. It returns an array of EFI_MEMORY_DESCRIPTORs. They have the following format (taken from GNU EFI):

typedef struct {

UINT32 Type; // EFI_MEMORY_TYPE, Field size is 32 bits followed by 32 bit pad

UINT64 Attribute; // Field size is 64 bits

} EFI_MEMORY_DESCRIPTOR;

To traverse them, you can use the NEXT_MEMORY_DESCRITOR macro.

 

Memory types are different to the E820 codes. For converting, see [https://github.com/tianocore/edk2/blob/70d5086c3274b1a5b099d642d546581070374e6e/OvmfPkg/Csm/LegacyBiosDxe/LegacyBootSupport.c#L1601 CSM E820 compatibility].

typedef enum {

EfiReservedMemoryType,

EfiMaxMemoryType

} EFI_MEMORY_TYPE;

 

==Code Examples==

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===

 

; use the INT 0x15, eax= 0xE820 BIOS function to get a memory map

; note: initially di is 0, be sure to set it to a value so that the BIOS code will not be overwritten.

stc ; "function unsupported" error exit

ret

 

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===

* it's better to assume that memory holes are present (and risk skipping some RAM) than to assume that memory holes aren't present (and risk crashing). This means assuming that the area from 0x00F00000 to 0x00FFFFFF can't be used and not probing this area at all (it's possible that some sort of ISA device is in this area, and that any write to this area can cause problems).

 

;Probe to see if there's RAM at a certain address

;

pop eax

ret

 

Further Notes:

 

[[Category:X86]]