2.7. Image Terminology
This page contains the current name, abbreviated name and purpose of the various images referred to in the Trusted Firmware project.
2.7.1. Common Image Features
Some of the names and abbreviated names have changed to accommodate new requirements. The changed names are as backward compatible as possible to minimize confusion. Where applicable, the previous names are indicated. Some code, documentation and build artefacts may still refer to the previous names; these will inevitably take time to catch up.
The main name change is to prefix each image with the processor it corresponds to (for example
SCP_, …). In situations where there is no ambiguity (for example, within AP specific code/documentation), it is permitted to omit the processor prefix (for example, just BL1 instead of
Previously, the format for 3rd level images had 2 forms;
BL3was either suffixed with a dash (“-”) followed by a number (for example,
BL3-1) or a subscript number, depending on whether rich text formatting was available. This was confusing and often the dash gets omitted in practice. Therefore the new form is to just omit the dash and not use subscript formatting.
The names no longer contain dash (“-”) characters at all. In some places (for example, function names) it’s not possible to use this character. All dashes are either removed or replaced by underscores (“_”).
The abbreviation BL stands for BootLoader. This is a historical anomaly. Clearly, many of these images are not BootLoaders, they are simply firmware images. However, the BL abbreviation is now widely used and is retained for backwards compatibility.
The image names are not case sensitive. For example,
bl1is interchangeable with
BL1, although mixed case should be avoided.
2.7.2. Trusted Firmware Images
188.8.131.52. Firmware Image Package:
This is a packaging format used by TF-A to package firmware images in a single binary. The number and type of images that should be packed in a FIP is platform-specific and may include TF-A images and other firmware images required by the platform. For example, most platforms require a BL33 image which corresponds to the normal world bootloader (e.g. UEFI or U-Boot).
184.108.40.206. AP Boot ROM:
Typically, this is the first code to execute on the AP and cannot be modified. Its primary purpose is to perform the minimum initialization necessary to load and authenticate an updateable AP firmware image into an executable RAM location, then hand-off control to that image.
220.127.116.11. AP RAM Firmware:
This is the 2nd stage AP firmware. It is currently also known as the “Trusted Boot Firmware”. Its primary purpose is to perform any additional initialization required to load and authenticate all 3rd level firmware images into their executable RAM locations, then hand-off control to the EL3 Runtime Firmware.
18.104.22.168. EL3 Runtime Firmware:
Also known as “SoC AP firmware” or “EL3 monitor firmware”. Its primary purpose is to handle transitions between the normal and secure world.
22.214.171.124. Secure-EL1 Payload (SP):
Typically this is a TEE or Trusted OS, providing runtime secure services to the normal world. However, it may refer to a more abstract Secure-EL1 Payload (SP). Note that this abbreviation should only be used in systems where there is a single or primary image executing at Secure-EL1. In systems where there are potentially multiple SPs and there is no concept of a primary SP, this abbreviation should be avoided; use the recommended Other AP 3rd level images abbreviation instead.
126.96.36.199. AP Normal World Firmware:
For example, UEFI or uboot. Its primary purpose is to boot a normal world OS.
188.8.131.52. Other AP 3rd level images:
The abbreviated names of the existing 3rd level images imply a load/execution
ordering (for example,
AP_BL31 -> AP_BL32 -> AP_BL33). Some systems may
have additional images and/or a different load/execution ordering. The
abbreviated names of the existing images are retained for backward compatibility
but new 3rd level images should be suffixed with an underscore followed by text
identifier, not a number.
In systems where 3rd level images are provided by different vendors, the
abbreviated name should identify the vendor as well as the image
function. For example,
184.108.40.206. Realm Monitor Management Firmware:
This is the Realm-EL2 firmware. It is required if Realm Management Extension (RME) feature is enabled. If a path to RMM image is not provided, TF-A builds Test Realm Payload (TRP) image by default and uses it as the RMM image.
220.127.116.11. SCP Boot ROM:
Typically, this is the first code to execute on the SCP and cannot be modified.
Its primary purpose is to perform the minimum initialization necessary to load
and authenticate an updateable SCP firmware image into an executable RAM
location, then hand-off control to that image. This may be performed in
conjunction with other processor firmware (for example,
This image was previously abbreviated as
BL0 but in some systems, the SCP
may directly load/authenticate its own firmware. In these systems, it doesn’t
make sense to interleave the image terminology for AP and SCP; both AP and SCP
Boot ROMs are
BL1 from their own point of view.
18.104.22.168. SCP RAM Firmware:
This is the 2nd stage SCP firmware. It is currently also known as the “SCP runtime firmware” but it could potentially be an intermediate firmware if the SCP needs to load/authenticate multiple 3rd level images in future.
This image was previously abbreviated as BL3-0 but from the SCP’s point of view, this has always been the 2nd stage firmware. The previous name is too AP-centric.
2.7.3. Firmware Update (FWU) Images
The terminology for these images has not been widely adopted yet but they have to be considered in a production Trusted Board Boot solution.
22.214.171.124. AP Firmware Update Boot ROM:
Typically, this is the first normal world code to execute on the AP during a
firmware update operation, and cannot be modified. Its primary purpose is to
load subsequent firmware update images from an external interface and communicate
AP_BL1 to authenticate those images.
During firmware update, there are (potentially) multiple transitions between the secure and normal world. The “level” of the BL image is relative to the world it’s in so it makes sense to encode “NS” in the normal world images. The absence of “NS” implies a secure world image.
126.96.36.199. AP Firmware Update Config:
This image does the minimum necessary AP secure world configuration required to
complete the firmware update operation. It is potentially a subset of
188.8.131.52. SCP Firmware Update Config:
This image does the minimum necessary SCP secure world configuration required to
complete the firmware update operation. It is potentially a subset of
184.108.40.206. AP Firmware Updater:
This is the 2nd stage AP normal world firmware updater. Its primary purpose is to load a new set of firmware images from an external interface and write them into non-volatile storage.
2.7.4. Other Processor Firmware Images
Some systems may have additional processors to the AP and SCP. For example, a Management Control Processor (MCP). Images for these processors should follow the same terminology, with the processor abbreviation prefix, followed by underscore and the level of the firmware image.