UEFI Shell Filesystem Mapping
A practical guide to map, fs0, removable media, ESP visibility, and why Shell filesystem aliases can change.
A USB stick worked yesterday. Today Shell shows fs1: instead of fs0:. A boot script fails even though the EFI file is still there.
This is not random. It is a symptom of how UEFI Shell maps devices.
First command
Shell> map -rmap -r refreshes mappings. In debug sessions, I run it before trusting any fsX: name.
Typical output may look like:
FS0: Alias(s):HD0a65535a1:;BLK1:
PciRoot(0x0)/Pci(0x17,0x0)/Sata(0x0,0xFFFF,0x0)/HD(1,GPT,...)
FS1: Alias(s):HD1a65535a1:;BLK3:
PciRoot(0x0)/Pci(0x14,0x0)/USB(0x2,0x0)/HD(1,MBR,...)The important part is not only FS0. The important part is the device path under it.
Real world example: startup.nsh cannot find tool
A script says:
fs0:\tools\flash.efiIt works on the engineer’s desk, then fails in the factory.
Why?
Because the factory jig has an internal ESP, a recovery USB, and sometimes a diagnostic USB. The tool USB becomes fs2: instead of fs0:.
A safer script searches:
for %a run (fs0 fs1 fs2 fs3 fs4)
if exist %a:\tools\flash.efi then %a:\tools\flash.efi
endforDebug checklist
When an EFI file is not found
Use this before blaming the file path.
Firmware Engineer Notes
When debugging boot media, I record three things together:
fsX alias
Device Path
Directory listingOnly recording fs0: is not enough. It hides the reason why the mapping changed.
Related notes
- UEFI Shell for Firmware Engineers
- What is FilePath Device Path Node?
- Removable Media Boot
- Fallback Boot Path
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