uboot启动流程分析3- lowlevel_init
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arch/arm/cpu/armv7/lowlevel_init.S，
22行，初始化sp，指向地址CONFIG_SYS_INIT_SP_ADDR，是多少？？？- -include/configs/mx6ullevk.h -
arch/arm/include/asm/arch-mx6/imx-regs.h
IRAM_BASE_ADDR 是6ULL内部OCRAM地址，可查看参考手册，大小为 0x00020000= 128k -
include/generated/generic-asm-offsets.h -
综上
CONFIG_SYS_INIT_SP_ADDR
= IRAM_BASE_ADDR + IRAM_SIZE - GENERATED_GBL_DATA_SIZE
= 0X00900000 + 0X00020000 - 256
= 0X0091FF00， -
34行，sp减去GD_SIZE GD_SIZE = 248 ， sp = 0X0091FF00 - 248 =0X0091FE08
35行，八字节对齐
36行，将sp地址赋值给r9寄存器 -
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42行，将ip和lr压栈
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57行，调用s_init
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58行，出栈，lr赋值给pc