188.8.131.52.1. Typical Boot Flow (Non-Secure)
A non-secure second-stage boot process typically follows a flow as in the following figure.
Low-level initialization steps include reconfiguring or disabling the L4 watchdog 0 timer, invalidating the instruction cache and branch predictor, remapping the on-chip RAM to the lowest memory region, and setting up the data area.
Upon entering the second-stage boot loader, the L4 watchdog 0 timer is active. The second stage boot loader can disable, reconfigure, or leave the watchdog timer unchanged. Once enabled after reset, the watchdog timer cannot be disabled, only paused or reset.
The instruction cache and branch predictor, which were previously enabled by the boot ROM code, must be invalidated. If the second-stage boot loader enables and uses the data cache, it must initialize all levels of the data cache before enabling.
The second-stage boot loader must remap the exception vector table because the exception vectors are still pointing to the exception handler in the boot ROM when it starts executing. By setting the system interconnect remap bit 0 to one, the on-chip RAM mirrors to the lowest region of the memory map. After this remap, the exception vectors use the exception handlers in the boot loader image.
The figure below shows the memory map before and after remap. The first mapping depicts boot ROM execution. The second mapping shows the remap of the on-chip RAM when the remap bit is set during second-stage boot loader execution.
The second-stage boot loader can reconfigure all HPS clocks. During clock reconfiguration, the boot loader asserts reset to the peripherals in the HPS affected by the clock changes.
The I/O assignments for the HPS are configured as part of the IOCSR configuration in the second-stage boot loader. Effectively, a bitstream containing the I/O assignments is sent to the device as part of the initialization code in the second-stage boot loader. If the FPGA fabric and I/O have not yet been configured through the FPGA and the HPS needs to access SDRAM, you should program the HPS to use the full or early I/O release configuration method to configure the shared and hard memory controller I/O. Refer to "FPGA Configuration" section of the "Booting and Configuration" appendix in the Arria 10 Hard Processor System Technical Reference Manual for details on full and early I/O release configuration.
The second-stage boot loader looks for a valid next-stage boot image in the next-stage boot device by checking the boot image validation data and checksum in the mirror image. Once validated, the second-stage boot loader copies the next-stage boot image (OS or application image) from the next-stage boot device to the SDRAM.
Before software passes control to the next-stage boot software, the second-stage boot loader can write a valid value (0x49535756) to the romcode_initswstate register in the System Manager. This value indicates that there is a valid boot image in the on-chip RAM. The romcode_initiswlastld register holds the index of the last second-stage boot loader software image loaded by the Boot ROM from the boot device. When a warm reset occurs, the Boot ROM loads the image indicated by the romcode_initswlastld register if the BSEL value is the same as the last boot.
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