What is UEFI Runtime Driver?
Quick note explaining UEFI Runtime Driver for BIOS/UEFI and embedded firmware readers.
A UEFI runtime driver keeps code or data available after ExitBootServices().
Why it matters
- Explains how UEFI drivers are structured and connected.
- Helps debug driver binding, controller connection, and resource cleanup.
- Useful when reading Driver Binding Protocol code.
Practical example
Example: if Start() fails halfway, the driver should close protocols and free resources it already allocated before returning the error.
Quick checklist
Quick takeaway
UEFI Runtime Driver is a small concept, but it often becomes important when reading logs or debugging real firmware.
Put it into the system flow
I try not to treat UEFI Runtime Driver as a dictionary entry. I read it as part of a firmware path: who produces it, who consumes it, and what symptom appears when it is wrong. That habit makes the note useful during debugging, not only during study.
A practical picture
Most firmware concepts become clearer when placed in the right phase. PEI, DXE, BDS, and runtime code have different responsibilities. For UEFI Runtime Driver, I would ask which module creates it, which protocol/PPI/HOB or variable carries it, and which later component depends on it.
In a real debugging session
For UEFI Runtime Driver, I usually ask four questions: which firmware phase sees it, which module produces it, which module consumes it, and where the symptom appears when it is wrong. That turns a BIOS/UEFI definition into a useful debug checkpoint.
When a DXE driver does not bind or BDS stops at an unexpected point, avoid staring at one error line only. Walk backward through protocols, handles, device paths, variables, and policy decisions. Firmware failures are usually chained.
Related notes
- What is Supported() in UEFI Driver Model?
- What is Start() in UEFI Driver Model?
- What is Stop() in UEFI Driver Model?
- What is UEFI Device Driver?
- What is UEFI Service Driver?
Public references
- UEFI Specification 2.11 - Runtime Services
- UEFI Driver Writer’s Guide
- UEFI Specification 2.11 - UEFI Driver Model
- EDK II Driver sample
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