Once application image is loaded into memory, it needs to be run on the respective cores.

Bootloaders are responsible for this. It is a software that runs as soon as an SoC is powered on. It performs SOC initializations, configures clocks, registers. It then communicates with an external media to receive the application.

In MCU+SDK, multistage bootloading is done where first the ROM bootloader boots a secondary bootloader, and the secondary bootloader, or SBL boots up the application.

The ROM bootloader is stored in RAM. It expects a x509 signed image of the SBL.

Then the SBL needs to be placed at 0x70020000 in RAM, and the application offset needs to be specified to the SBL.

The SBL then reads this application, parses it, loads to respective CPUs, releases core from reset. The application starts running now.

For detailed information on bootloaders and bootflow, refer Understanding the bootflow and bootloaders

SBLs in SDK

SBL	Reference
SBL NULL	SBL NULL
SBL OSPI	SBL OSPI
SBL UART	SBL UART
SBL CAN	SBL CAN
SBL SD	SBL SD

The SBL is like any other example of the SDK. They use the bootloader library APIs to carry out the bootloading process.

Depending on the boot media from which we load the application binary, we have multiple SBLs like sbl_ospi,sbl_uart etc.

A bare minimum SBL called the sbl_null is also included which aids the users to load their applications via CCS. Here are some details regarding those.

SBL NULL

The sbl_null is a secondary bootloader which doesn't load any application binary, but just does the SOC initialization and puts all the cores in WFI (Wait For Interrupt) mode.
This is supposed to be a "development form" bootloader which should be used only during initial development.
The other method is using NO-BOOT/DEV-BOOT boot modes of the devices and using GEL scripts to initialize the SoC via debugger. The application binaries can then be side-loaded. ROM is not involved in this case. The sbl_null is an alternative to this process.
This is referred to as the SOC initialization binary, refer SOC Initialization using the Binary Flashed in OSPI memory for more on this.

SBL OSPI

The sbl_ospi is a secondary bootloader which reads and parses the application image from a location in the OSPI flash and then moves on to core initialization and other steps
To boot an application using the sbl_ospi, the application image needs to be flashed at a particular location in the OSPI flash memory.
This location or offset is specified in the SysConfig of the sbl_ospi application. Currently this is 0x81000. This offset is chosen under the assumption that the sbl_ospi application takes at max 512 KB from the start of the flash. If a custom bootloader is used, make sure that this offset is chosen in such a way that it is greater than the size of the bootloader which is being flashed and also aligns with the block size of the flash.
To flash an application (or any file in fact) to a location in the OSPI flash memory, follow the steps mentioned in Basic steps to flash files

SBL UART

The sbl_uart is a secondary bootloader which receives the multicore application via UART, stores it in memory and then does the parsing, core initialization etc.
To boot an application using the sbl_uart, you can refer to UART Bootloader Python Script subsection. Detailed steps on the usage is mentioned in the same subsection.

SBL CAN

The sbl_can is a secondary bootloader which needs to be flashed in QSPI Flash.
The sbl_can receives the multicore application via CAN, stores it in memory and then does the parsing, core initialization etc.
To boot an application using the sbl_can, you can refer to CAN Bootloader Python Script subsection.

SBL SD

The sbl_sd is a secondary bootloader which reads the application image file from the SD card and then moves on to core initialization and other steps
To boot an application using the sbl_sd, the application image needs to be copied to the SD card as a file named "app"
Similarly you can copy any application file to the SD card and rename in the SD card as "app" so that the SBL can pick it up.
Currently the sbl_sd reads the full application file into an MSRAM buffer and then parses the multicore application. Because of this reason application images higher than ~512 KB in size can't be booted by sbl_sd as of now.
To boot an application using sbl_sd, you can refer to SBL SD subsection.

SBL Based bootflow on HSFS device

For non secure boot, the SBL just reads the application image from a boot media, parses it, and boots it.

HSFS Application Run

SBL Based bootflow on HSSE device

In secure boot, the SBL reads the signed application image from a boot media, authenticates it, decrypts it, and boots it.

For detailed information refer Enabling Secure Boot

Debugging support

For information on various debugging tools and techniques, refer Debugging Sitara AM2x Microcontrollers