20230826 step03 获取当前异常等级 CurrentEL

// The entry point is in its own section ".text.boot".
.section ".text.boot"

// The symbol "_start" is the kernel's entry point, make it global.
.global _start

// Entry point for the main core.
_start:
        MRS X5,CurrentEL        //read current  Exception level.EL, bits [3:2]
        LSR X5,X5,2     //Shift out two 0 at the tail

// Hang forever in a loop.
hang_forever:
        wfe                   // Allow the CPU to go to low-power mode.
        b hang_forever