SBL payload stage is to boot a target OS. It is a modular design to extend bootloader to perform a purpose-driven functionality that is generally independent of platform hardware.


Os Loader (Built-in)

A versatile linux loader implementation that boots Linux, ELF, UEFI-PI FV, or PE executables. It also supports launching OSes compliant with the MultiBoot specification.

When built as an FV formatted payload OS Loader permits the inclusion and launching of a pre-OS payload binary that will hand-off control to an OS after the pre-OS payload finishes execution.

See Enable Pre-OS Payload Support for more details.

Firmware Update Payload (Built-in)

A special purpose payload to update full boot flash in a secure and fault-tolerant flow.

UEFI Payload (External)

A EDK II based payload implementation to boot Windows. It provides secure boot, SMM, and UEFI runtime services.

Custom Payload

SBL supports launching customized payloads that provides purpose-driven functionality.

Payload Types

A payload can be Tightly Coupled or Loosely Coupled with Slim Bootloader depending on if the payload has code/data dependency or data-only dependency on the bootloader.

  • Tightly Coupled

    • Implementation depends on SBL source code to build. It requires platform specific libraries.

    • Can leverage code interfaces and libraries provided by the bootloader.

    • SBL provides a set of helper libraries like file system support, serial port debug message support, etc. to simplify the development of the payload.

    Examples: OsLoader, FirmwareUpdate

  • Loosely Coupled

    • A loosely coupled payload on the other hand, depends on platform initialization information passed by the bootloader to continue their initialization and boot process.

    • The platform abstraction is provided through Hand-off-Blocks (HOB) data structures.

    • These payloads are platform agnostic.

    Examples: UEFI Payload

Multiple Payload Support

In some use cases, more than one payload is necessary in SBL. However, it is difficult to fit all payloads into limited flash device. For example, UEFI payload image can be over 1MB in size. This feature is designed to support multiple payloads, allowing built-in payload (OsLoader) reside in redundant region, while the external payloads in non-redundant region.

See Integrate Multiple Payloads for more details.

See Create New Payload for more details.