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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.
Input:
EAX Function Code E820hOutput:
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
revision.
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
carry.
Address Range Descriptor Structure
Offset in Bytes Name DescriptionThe 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.
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 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 DescriptionThe BIOS can use the AddressRangeReserved address range type to block out various addresses as "not suitable" for use by a programmable device.
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
AddressRangeReserved.
Some of the reasons a BIOS would do this are:
- The address range contains system ROM.
- The address range contains RAM in use by the ROM.
- The address range is in use by a memory mapped system device.
- The address range is for whatever reason are unsuitable for a standard device to use as a device memory space.
Assumptions and Limitations
- 1. The BIOS will return address ranges describing base board memory and ISA or PCI memory that is contiguous with that baseboard memory.
- 2. The BIOS WILL NOT return a range description for the memory mapping of PCI devices, ISA Option ROM's, and ISA plug & play cards. This is because the OS has mechanisms available to detect them.
- 3. The BIOS will return chipset defined address holes that are not being used by devices as reserved.
- 4. Address ranges defined for base board memory mapped I/O devices (for example APICs) will be returned as reserved.
- 5. All occurrences of the system BIOS will be mapped as reserved. This includes the area below 1 MB, at 16 MB (if present) and at end of the address space (4 gig).
- 6. Standard PC address ranges will not be reported. Example video memory at A0000 to BFFFF physical will not be described by this function. The range from E0000 to EFFFF is base board specific and will be reported as suits the bas board.
- 7. All of lower memory is reported as normal memory. It is OS's responsibility to handle standard RAM locations reserved for specific uses, for example: the interrupt vector table(0:0) and the BIOS data area(40:0).
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;
Regs.es = SEGMENT (&Descriptor);
Regs.di = OFFSET (&Descriptor);
Regs.ecx = sizeof (Descriptor);
Regs.edx = 'SMAP';
_int( 0x15, Regs );
if ((Regs.eflags & EFLAG_CARRY) || Regs.eax != 'SMAP') {
break;
}
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.
break;
}
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.
Input:
AX Function Code E801hOutput:
CF Carry Flag Non-Carry - indicates no errorNot sure what this difference between the "Extended" and "Configured" numbers are, but they appear to be identical, as reported from the BIOS.
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.
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.
Input:
AH Function Code 88hOutput:
CF Carry Flag Non-Carry - indicates no error
AX Memory Count Number of contiguous KB above 1 MB.