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x-boot (external boot code) is a first-stage boot-loader placed in eMMC, NAND flash, or SD cards, loaded into system SRAM by i-boot. As it runs on SRAM, its size (including code, data, and stack) must not exceed the system SRAM's capacity. The primary task of x-boot is to initialize the DRAM controller and PHY, perform calibration on DRAM PHY and signals, and once calibrated, the DRAM is ready for use. x-boot then switches the CPU from 32-bit mode to 64-bit mode and loads TF-A and U-Boot from external storage into DRAM, executing TF-A.
x-boot (external boot code) is the first-stage boot loader. It is loaded into internal SRAM and run by internal ROM code of SP7350. Note that before X-Boot initializes and trains DRAM controller, DRAM is not available.
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