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    //! Implementation of [`TrapContext`]
    use riscv::register::sstatus::{self, Sstatus, SPP};

    #[repr(C)]
    ///trap context structure containing sstatus, sepc and registers
    pub struct TrapContext {
        /// general regs[0..31]
        pub x: [usize; 32],
        /// CSR sstatus
        pub sstatus: Sstatus,
        /// CSR sepc
        pub sepc: usize,
        /// Addr of Page Table
        pub kernel_satp: usize,
        /// kernel stack
        pub kernel_sp: usize,
        /// Addr of trap_handler function
        pub trap_handler: usize,
    }

    impl TrapContext {
        ///set stack pointer to x_2 reg (sp)
        pub fn set_sp(&mut self, sp: usize) {
            self.x[2] = sp;
        }
        ///init app context
        pub fn app_init_context(
            entry: usize,
            sp: usize,
            kernel_satp: usize,
            kernel_sp: usize,
            trap_handler: usize,
        ) -> Self {
            let mut sstatus = sstatus::read();
            // set CPU privilege to User after trapping back
            sstatus.set_spp(SPP::User);
            let mut cx = Self {
                x: [0; 32],
                sstatus,
                sepc: entry,
                kernel_satp,
                kernel_sp,
                trap_handler,
            };
            cx.set_sp(sp);
            cx
        }
    }