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.Output:
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.
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 AddressRangeReserved.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:
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.
E820Present = FALSE;
Reg.ebx = 0;
do {
Reg.eax = 0xE820;
Reg.es = SEGMENT (&Descriptor);
Reg.di = OFFSET (&Descriptor);
Reg.ecx = sizeof (Descriptor);
Reg.edx = 'SMAP';
_int( 0x15, regs );
if ((Regs.eflags & EFLAG_CARRY) || Regs.eax != 'SMAP') {
break;
}
if (Regs.ecx < 20 || Reg.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;
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Add address range Descriptor.BaseAddress through
Descriptor.BaseAddress + Descriptor.Length
as type Descriptor.Type
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} while (Regs.ebx != 0);
if (!E820Present) {
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call INT 15h, AX=E801h and/or INT 15h, AH=88h to obtain old style
memory information
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}
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 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).
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.