INT 15h, AX=E820h - Query System Address Map

Real mode only.

This call returns a memory map of all the installed RAM, and of physical memory ranges reserved by the BIOS. The address map is returned by making successive calls to this API, each returning one "run" of physical address information. Each run has a type which dictates how this run of physical address range should be treated by the operating system.

If the information returned from INT 15h, AX=E820h in some way differs from INT 15h, AX=E801h or INT 15h AH=88h, then the information returned from E820h supersedes what is returned from these older interfaces. This allows the BIOS to return whatever information it wishes to for compatibility reasons.


	EAX	Function Code	E820h
	EBX	Continuation	Contains the "continuation value" to get the
				next run of physical memory.  This is the
				value returned by a previous call to this
				routine.  If this is the first call, EBX
				must contain zero.
	ES:DI	Buffer Pointer	Pointer to an  Address Range Descriptor
				structure which the BIOS is to fill in.
	ECX	Buffer Size	The length in bytes of the structure passed
				to the BIOS.  The BIOS will fill in at most
				ECX bytes of the structure or however much
				of the structure the BIOS implements.  The
				minimum size which must be supported by both
				the BIOS and the caller is 20 bytes.  Future
				implementations may extend this structure.
	EDX	Signature	'SMAP' -  Used by the BIOS to verify the
				caller is requesting the system map
				information to be returned in ES:DI.

	CF	Carry Flag	Non-Carry - indicates no error
	EAX	Signature	'SMAP' - Signature to verify correct BIOS
	ES:DI	Buffer Pointer	Returned Address Range Descriptor pointer.
				Same value as on input.
	ECX	Buffer Size	Number of bytes returned by the BIOS in the
				address range descriptor.  The minimum size
				structure returned by the BIOS is 20 bytes.
	EBX	Continuation	Contains the continuation value to get the
				next address descriptor.  The actual
				significance of the continuation value is up
				to the discretion of the BIOS.  The caller
				must pass the continuation value unchanged
				as input to the next iteration of the E820
				call in order to get the next Address Range
				Descriptor.  A return value of zero means that
				this is the last descriptor.  Note that the
				BIOS indicate that the last valid descriptor
				has been returned by either returning a zero
				as the continuation value, or by returning

Address Range Descriptor Structure

Offset in Bytes		Name		Description
	0	    BaseAddrLow		Low 32 Bits of Base Address
	4	    BaseAddrHigh	High 32 Bits of Base Address
	8	    LengthLow		Low 32 Bits of Length in Bytes
	12	    LengthHigh		High 32 Bits of Length in Bytes
	16	    Type		Address type of  this range.
The BaseAddrLow and BaseAddrHigh together are the 64 bit BaseAddress of this range. The BaseAddress is the physical address of the start of the range being specified.

The LengthLow and LengthHigh together are the 64 bit Length of this range. The Length is the physical contiguous length in bytes of a range being specified.

The Type field describes the usage of the described address range as defined in the table below.

Value	Pneumonic		Description
1	AddressRangeMemory	This run is available RAM usable by the
				operating system.
2	AddressRangeReserved	This run of addresses is in use or reserved
				by the system, and must not be used by the
				operating system.
Other	Undefined		Undefined - Reserved for future use.  Any
				range of this type must be treated by the
				OS as if the type returned was
The BIOS can use the AddressRangeReserved address range type to block out various addresses as "not suitable" for use by a programmable device.

Some of the reasons a BIOS would do this are:

Assumptions and Limitations

Example address map

This sample address map describes a machine which has 128 MB RAM, 640K of base memory and 127 MB extended. The base memory has 639K available for the user and 1K for an extended BIOS data area. There is a 4 MB Linear Frame Buffer (LFB) based at 12 MB. The memory hole created by the chipset is from 8 M to 16 M. There are memory mapped APIC devices in the system. The IO Unit is at FEC00000 and the Local Unit is at FEE00000. The system BIOS is remapped to 4G - 64K.

Note that the 639K endpoint of the first memory range is also the base memory size reported in the BIOS data segment at 40:13.

Key to types: "ARM" is AddressRangeMemory, "ARR" is AddressRangeReserved.

Base (Hex)	Length	Type	Description
0000 0000	639K	ARM	Available Base memory - typically the same
				value as is returned via the INT 12 function.
0009 FC00	1K	ARR	Memory reserved for use by the BIOS(s).
				This area typically includes the Extended
				BIOS data area.
000F 0000	64K	ARR	System BIOS
0010 0000	7M	ARM	Extended memory, this is not limited to
				the 64 MB address range.
0080 0000	8M	ARR	Chipset memory hole required to support the
				LFB mapping at 12 MB.
0100 0000	120M	ARM	Base board RAM relocated above a chipset
				memory hole.
FEC0 0000	4K	ARR	IO APIC memory mapped I/O at FEC00000.  Note
				the range of addresses required for an APIC
				device may vary from base OEM to OEM.
FEE0 0000	4K	ARR	Local APIC memory mapped I/O at FEE00000.
FFFF 0000	64K	ARR	Remapped System BIOS at end of address space.

Sample operating system usage

The following code segment is intended to describe the algorithm needed when calling the Query System Address Map function. It is an implementation example and uses non standard mechanisms.

    E820Present = FALSE;
    Regs.ebx = 0;
    do {
        Regs.eax = 0xE820;  = SEGMENT (&Descriptor);
        Regs.di  = OFFSET  (&Descriptor);
        Regs.ecx = sizeof  (Descriptor);
	Regs.edx = 'SMAP';

        _int( 0x15, Regs );

        if ((Regs.eflags & EFLAG_CARRY)  ||  Regs.eax != 'SMAP') {

        if (Regs.ecx < 20  ||  Regs.ecx > sizeof (Descriptor) ) {
            // bug in bios - all returned descriptors must be
            // at least 20 bytes long, and can not be larger then
            // the input buffer.


	E820Present = TRUE;
        Add address range Descriptor.BaseAddress through
        Descriptor.BaseAddress + Descriptor.Length
        as type Descriptor.Type

    } while (Regs.ebx != 0);

    if (!E820Present) {
	call INT 15h, AX=E801h and/or INT 15h, AH=88h to obtain old style
	memory information

INT 15h, AX=E801h - Get Memory Size for Large Configurations

Real mode only (as far as I know).

Originally defined for EISA servers, this interface is capable of reporting up to 4 GB of RAM. While not nearly as flexible as E820h, it is present in many more systems.


	AX	Function Code	E801h

	CF	Carry Flag	Non-Carry - indicates no error
	AX	Extended 1	Number of contiguous KB between 1 and 16 MB,
				maximum 0x3C00 = 15 MB.
	BX	Extended 2	Number of contiguous 64 KB blocks between
				16 MB and 4 GB.
	CX	Configured 1	Number of contiguous KB between 1 and 16 MB,
				maximum 0x3C00 = 15 MB.
	DX	Configured 2	Number of contiguous 64 KB blocks between
				16 MB and 4 GB.
Not sure what this difference between the "Extended" and "Configured" numbers are, but they appear to be identical, as reported from the BIOS.

NOTE: It is possible for a machine using this interface to report a memory hole just under 16 MB (Count 1 is less than 15 MB, but Count 2 is non-zero).

INT 15h, AH=88h - Get Extended Memory Size

Real mode only.

This interface is quite primitive. It returns a single value for contiguous memory above 1 MB. The biggest limitation is that the value returned is a 16-bit value, in KB, so it has a maximum saturation of just under 64 MB even presuming it returns as much as it can. On some systems, it won't return anything above the 16 MB boundary.

The one useful point is that it works on every PC available.


	AH	Function Code	88h

	CF	Carry Flag	Non-Carry - indicates no error
	AX	Memory Count	Number of contiguous KB above 1 MB.