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In SP7350 software architecture, BL1 is the i-boot. BL2 is the x-boot. BL31 is the secure monitor. BL32 is the OP-TEE. BL33 is the U-Boot.
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BL31 is a specific stage or component within Trusted Firmware-A (TF-A). When we talk about image of FT-A, it means BL31 image because only BL31 code of TF-A is used.
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