What is Device Path?

Quick note explaining Device Path for BIOS/UEFI and embedded firmware readers.

May 9, 2024 3 min read

Đọc bằng English 日本語

A UEFI Device Path describes the route to a device, controller, partition, or boot file. It is not a string path like C:\Windows; it is a binary sequence of Device Path Nodes. Each node has a Type, SubType, Length, and node-specific data.

In simple terms, a filesystem path tells you where a file lives inside a volume. A Device Path tells firmware how to reach that volume and then the file.

Node structure

Every Device Path node starts with the same header:

typedef struct {
  UINT8 Type;
  UINT8 SubType;
  UINT8 Length[2];
} EFI_DEVICE_PATH_PROTOCOL;

Field	Meaning	What to check while debugging
`Type`	Major node group: hardware, ACPI, messaging, media…	Which layer of the path is this node describing?
`SubType`	Specific node kind	PCI, USB, SATA, HD, FilePath, and so on
`Length`	Total node length including the header	A bad length can make the parser walk into garbage
Data	Node-specific payload	Device/function, partition GUID, file path, etc.

Quick checklist

Does the path end with an End Device Path Node?
Is each node Length valid?
Does the HD node still point to the current ESP or partition GUID?
Does the FilePath node point to the expected .efi file?
After disk replacement, cloning, or CMOS clear, is the Boot#### path stale?

Real Boot#### example

A Windows boot option may contain a path like this:

PciRoot(0x0)
  /Pci(0x17,0x0)
  /Sata(0x0,0xFFFF,0x0)
  /HD(1,GPT,xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx,0x800,0x100000)
  /\EFI\Microsoft\Boot\bootmgfw.efi

Node	Meaning	Values worth checking
`PciRoot(0x0)`	PCI root bridge 0	Usually the main root complex
`Pci(0x17,0x0)`	PCI device/function	`0x17` is the device number, `0x0` is the function
`Sata(...)`	SATA controller/port path	Port or controller changes can invalidate the path
`HD(...)`	Partition that contains the loader	Partition number, GPT GUID/signature, start LBA, size
`\EFI\...\bootmgfw.efi`	FilePath node	The `.efi` file firmware must open

The key point: a Device Path does not merely say “open bootmgfw.efi”. It also describes the controller and partition that must be reached first.

Common failure pattern

After cloning a disk, recreating the EFI System Partition, updating BIOS, or clearing CMOS, a Boot#### variable may still exist while its embedded Device Path is stale. Boot Manager can read the variable, but it cannot open the referenced partition or file. The visible symptom may be “missing boot option”, “boots the wrong Windows Boot Manager”, or “No bootable device”.

Takeaway

A Device Path is firmware’s binary map. DevicePathToText() only renders that map for humans; firmware still walks the structured nodes.

Public references

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Node structure

Quick checklist

Real Boot#### example

Common failure pattern

Takeaway

Related notes

Public references

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