20230826 step03 获取当前异常等级 CurrentEL
armV8-A 架构文档的 C5.2.1
在EL0不可用，EL1时。。。。。，返回值前60bits 和后2bits都为0
修改一下boot.S
// 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
make
make run-gdb
new teminal: make gdb
读取内存
反汇编
si step instruction， i r 查看寄存器；这里可以看到EL等于2，当前为EL2