Go to the first, previous, next, last section, table of contents.


How to detect all installed RAM

There are three BIOS calls which return the information of installed RAM. GRUB uses these calls to detect all installed RAM and which address range should be treated by operating systems.

INT 15H, AX=E820h interrupt call

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 (see section INT 15H, AX=E801h interrupt call) or INT 15h AH=88h (see section INT 15H, AX=88h interrupt call), 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: 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. Descriptor structure which the BIOS is to fill in. 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. verify the caller is requesting the system map information to be returned in ES:DI.
EAX Function Code E820h
EBX Continuation Contains the continuation
ES:DI Buffer Pointer Pointer to an Address Range
ECX Buffer Size The length in bytes of the
EDX Signature `SMAP' - Used by the BIOS to

Output: correct BIOS revision. Descriptor pointer. Same value as on input. BIOS in the address range descriptor. The minimum size structure returned by the BIOS is 20 bytes. 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 E820h 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 indicates that the last valid descriptor has been returned by either returning a zero as the continuation value, or by returning carry.
CF Carry Flag Non-Carry - indicates no error
EAX Signature `SMAP' - Signature to verify
ES:DI Buffer Pointer Returned Address Range
ECX Buffer Size Number of bytes returned by the
EBX Continuation Contains the continuation value

The Address Range Descriptor Structure is:
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: RAM usable by the operating system. use or reserved by the system, and must not be used by the operating system. use. Any range of this type must be treated by the OS as if the type returned was AddressRangeReserved.
Value Mnemonic Description
1 AddressRangeMemory This run is available
2 AddressRangeReserved This run of addresses is in
Other Undefined Undefined - Reserved for future

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:

Here is the list of assumptions and limitations:

  1. The BIOS will return address ranges describing system memory and ISA or PCI memory that is contiguous with that system 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 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 GB).
  6. Standard I/O 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 motherboard-specific and will be reported differently on different computers.
  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).

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

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 & EFLAGS_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 than
           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 interrupt call

Real mode only.

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 E801h.

Output: and 16 MB, maximum 0x3C00 = 15 MB. between 16 MB and 4GB. and 16 MB, maximum 0x3c00 = 15 MB. between 16 MB and 4 GB.
CF Carry Flag Non-Carry - indicates no error.
AX Extended 1 Number of contiguous KB between 1
BX Extended 2 Number of contiguous 64KB blocks
CX Configured 1 Number of contiguous KB between 1
DX Configured 2 Number of contiguous 64KB blocks

Not sure what this difference between the Extended and Configured numbers are, but they appear to be identical, as reported from the BIOS.

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, AX=88h interrupt call

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 88h

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


Go to the first, previous, next, last section, table of contents.