Memory Map Viewer

Overview

The Memory Map Viewer is a UEFI application that retrieves and displays the firmware memory map.

It demonstrates how UEFI exposes the current system memory layout before an operating system takes control.

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Goal

The goal of this application is to understand how firmware manages memory and how an operating system loader can retrieve this information before calling ExitBootServices().

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What it demonstrates

• Calling GetMemoryMap()
• Allocating memory using AllocatePool()
• Handling EFI memory descriptors
• Iterating descriptors using DescriptorSize
• Converting memory types into readable names
• Grouping contiguous memory regions
• Displaying formatted output in the UEFI console

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UEFI Memory Map

The UEFI memory map describes how physical memory is currently used by the firmware.

Each entry is an EFI_MEMORY_DESCRIPTOR containing information such as:

• Memory type
• Physical start address
• Number of pages
• Memory attributes

A simplified example:

Memory Type        Start Address        Size

Loader Code       0x00000000A0000000   64 KB
Boot Services     0x00000000B0000000   2 MB
Conventional RAM  0x00000001C0000000   512 MB

GetMemoryMap Workflow

GetMemoryMap() requires a specific workflow.

The application must first query the required buffer size:

Call GetMemoryMap()

        |
        v

EFI_BUFFER_TOO_SMALL

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        v

Allocate buffer

        |
        v

Call GetMemoryMap() again

        |
        v

Process descriptors

Memory Allocation Considerations

An important detail is that allocating the memory map buffer changes the memory map itself.

This means the typical workflow is:

1. Request the required size
2. Add additional space for changes
3. Allocate the buffer
4. Retrieve the final memory map

Example:

MemoryMapSize += 2 * DescriptorSize;

The additional space prevents allocation changes from invalidating the second call.

Descriptor Iteration

EFI memory descriptors must not be iterated using normal pointer arithmetic:

Descriptor++;

The descriptor size is firmware-defined.

The correct approach is:

Descriptor =
    (EFI_MEMORY_DESCRIPTOR*)
    (
        (UINT8*)Descriptor + DescriptorSize
    );

This allows the application to work with different firmware implementations.

Output

The application displays:

• Memory type
• Physical address
• Page count
• Calculated size

Contiguous regions of the same memory type are grouped together to improve readability.

Example output:

Related Concepts

• Memory Map
• Boot Services
• GetMemoryMap() Findings

Next Steps

The next application explores filesystem access through UEFI protocols:

Filesystem Explorer