Viewing the Firmware Memory Map

A possibly useful demonstration of the x86 BIOS emulator that’s implemented in the HAL in version 6.0 and higher is to have a driver execute int 15h function E820h on a running system in order to discover the memory map that the loader will have worked from in deciding what memory Windows can use. This memory map is very different from what can be seen in the Device Manager when viewing resources by type. As helpful as this latter map can be, it shows only what Windows has ended up using, not what it might have used.

The value of retrieving the map that Windows started with is increased beyond mere curiosity now that computers are typically sold with more RAM than can fit in what’s left of the first 4GB of physical address space after an increasing number of increasingly greedy peripheral devices each take their cut. Some chipsets, perhaps many, deal with this reasonably elegantly by remapping much of the excess RAM to the end of all other RAM that is anyway addressable above 4GB. Some don’t. Either way, since Microsoft does not license 32-bit Windows Vista to use physical memory above 4GB, most users just end up wondering where the missing part of their 4GB has gone, manufacturers have to warn that not all the 4GB will be usable, and the Internet gets to show how very good it is at propagating all manner of supposed explanations.

If you’re using 32-bit Windows Vista (or higher) on a computer that has 4GB or more of RAM fitted, and you wonder what your options are for getting all that memory into use, then you might benefit from knowing whether any RAM that doesn’t show below 4GB is remapped above 4GB. The Firmware Memory Map Tool presented on this page will give you this information. If you find that no RAM is reported above 4GB, then the RAM that is overridden below 4GB is truly lost. Even installing 64-bit Windows will not help, and you are clearly better off to know before trying. If you do have RAM above 4GB, then you can know how much you stand to gain from getting a Windows that isn’t limited to 4GB.

The Firmware Memory Map Tool

The FWMEMMAP.SYS driver exists just to reproduce the way that the loader obtained the memory map that Windows started from. Windows does not retain this memory map, but a driver may call int 15h function E820h enough times to retrieve the whole memory map and then make the result accessible from user mode via a Device I/O Control interface. The FWMEMMAP.EXE utility is a console application that exists just to load the FWMEMMAP.SYS driver, ask it for the map, show the map to you, and then unload the driver.

The driver calls the BIOS by using the HAL’s real-mode emulator, which is conveniently new for Windows Vista. One of the points to this introduction of BIOS support through an emulator is presumably that it should work for both the x86 and x64 architectures, without Microsoft having to build into 64-bit Windows all the support needed for getting the processor to execute 16-bit real-mode code in a virtual-8086 task. So that you may see this works, I provide x64 builds of the driver and program, even though I do not imagine they are much use in practice. After all, if you have 64-bit Windows, you will typically not need this utility to tell you whether any RAM is remapped to be addressable above 4GB: you’ll just see that Windows is using it.

You should be aware that the driver uses five undocumented kernel-mode functions:

If you are troubled about software using features that the manufacturer does not disclose, then do not use this driver.


Now, I should point out that there are some risks with calling the BIOS emulator on a running system. The HAL interprets the real-mode code that would execute if an interrupt actually were called in real mode, e.g., by a DOS program running on a machine that has booted DOS, or indeed by the Windows loaders (both BOOTMGR and WINLOAD) before the kernel runs. Wherever the real-mode code would execute port I/O instructions, the HAL actually may read from or write to the port. There is at least some potential that port I/O for the emulator interferes with port I/O being performed concurrently by drivers for purposes that have nothing to do with the emulator. Except for CMOS and PCI ports, there seems to be no means of synchronisation. Though conflict over port I/O seems highly improbable for int 15h function E820h, it perhaps is not impossible.

Also unlikely but not impossible when attempting to use the BIOS emulator on a running system is that the emulator gets called concurrently from different threads. The implementation is not designed for this, presumably because Microsoft intends only a very specific use in highly particular circumstances. Re-entering the emulator in version 6.0 will confuse it, but except in one special case version 6.1 defends by raising an assertion (giving you a bug check’s blue screen of death).

Attempting to retrieve the memory map through the BIOS emulator may anyway come to nothing because although the int 15h function E820h is successfully emulated, it returns an error. A notable cause is that the BIOS expects this function to be called only in real mode and insists on it. Since the emulator has the smsw instruction always produce 0x2D, a BIOS that checks will perceive that it is running in protected mode—yes, even though it is being executed with real-mode addressing. If your BIOS happens to be one that will not return the map unless called in real mode, then there there is not much that can sensibly be done about it.


The following example should be typical of the tool’s output. The computer in question is fitted with 4GB of RAM as four 1GB chips. Much of the address space immediately below 4GB is not usable for RAM, but 768MB is remapped to be usable above 4GB.

Map of firmware memory ranges (from int 15h function E820h)

       Address               Size              Type
=================== =================== =================
0x00000000`00000000 0x00000000`0009E800  1 (memory)
0x00000000`0009E800 0x00000000`00001800  2 (reserved)
0x00000000`000F0000 0x00000000`00010000  2 (reserved)
0x00000000`00100000 0x00000000`CFD90000  1 (memory)
0x00000000`CFE90000 0x00000000`00053000  4 (ACPI NVS)
0x00000000`CFEE3000 0x00000000`0000D000  3 (ACPI Reclaim)
0x00000000`CFEF0000 0x00000000`00010000  2 (reserved)
0x00000000`E0000000 0x00000000`10000000  2 (reserved)
0x00000000`FEC00000 0x00000000`01400000  2 (reserved)
0x00000001`00000000 0x00000000`30000000  1 (memory)

Summary (in MB, ignoring partial MB):

Total memory:               4094
Memory above 4GB:            768

Ranges that are addressable as RAM are marked “memory”. To interpret other values in the Type column, refer to the definition of int 15h function E820h in the Advanced Configuration and Power Interface (ACPI) Specification, Section 14, System Address Map Interfaces. Note that the map has holes: int 15h function E820h does not report “standard PC address ranges” or areas of address space that are used “for the memory mapping of PCI devices, ISA Option ROMs, and ISA Plug and Play cards”.

The example given above has three such holes:

The first is expected for compatibility going all the way back to the original IBM PC. The others will vary from one computer to another, depending on what peripherals are fitted. To learn how Windows uses memory in these holes, start the Device Manager, then from its View menu select “Resources by type” and expand Memory. For the example shown, almost all of the first hole is given over to the video card, which has 256MB at 0xD0000000, but the second hole is highly fragmented, from another 32MB at 0xF8000000 for the video card down to a network card’s use of just 256 bytes at 0xFDCFE000. Conspicuously, since the computer happens to run 32-bit Windows Vista, the Device Manager knows nothing of the 768MB above 4GB.


For distribution, the Firmware Memory Map Tool is compressed into zip files:


Please note that the tool requires Windows version 6.0 or higher. Earlier versions of Windows will decline to run the program or load the driver.

The driver and the program must be in the same directory. Just run the program from a Command Prompt. You will need administrative privilege. On an x64 system, you will need to start Windows in Test Mode, i.e., with testsigning enabled in the Boot Configuration Data (BCD). For 32-bit Windows, Test Mode is merely optional but it does spare you from a warning in the event log (specifically in Microsoft-Windows-CodeIntegrity/Operational). On both architectures, loading the driver will typically cause warnings in the System log from the Windows Defender.

The restriction to Test Mode on x64 systems is because the executables are signed with a root certificate only. I am not a software manufacturer and do not have a Software Publishing Certificate (SPC) for signing kernel-mode drivers. Yes, I am a specialist in kernel-mode drivers, but I write them for other people to publish, and the release-signing of the products they sell is done by them, not by me. Getting an SPC is a lot of trouble to go to for a simple utility that will likely be run only once on any one machine. Test Mode seems perfectly reasonable to me. If you don’t like Test Mode but do have an SPC, then re-sign the driver as your own.

Source Code

Source code is provided as a root directory and three subdirectories:

The code is written for use with the Windows Driver Kit (WDK). To build the executables, open one of the WDK’s build environments for Windows Vista, change to the root directory of this source tree, and run the WDK’s BUILD utility.

To have the binaries and symbol files be collected in a tree beneath a subdirectory named BIN, undefine the environment variable NO_BINPLACE before running BUILD. For details, refer to the PROJECT.MK file in the root directory of the source tree.

The executables are not signed unless you define one or more environment variables before building. If you are happy for test-signing with a root certificate named My Own Testing Authority which you have already created and imported into your personal certificate store, then define the environment variable SIGNCODE. Otherwise, describe your certificate by defining environment variables SIGNCODE_CERTIFICATE_STORE and SIGNCODE_CERTIFICATE_NAME, and optionally SIGNCODE_CROSS_CERTIFICATE and SIGNCODE_TIME_STAMPER. For details, refer to the PROJECT.MK file in the root directory of the source tree.