Function handle_sret

crates/virtual-machine/src/cpu/traps.rs:171–200

Handle SRET - restore SIE from SPIE, privilege from SPP, return PC from SEPC. Err if illegal.

(regs: &mut Registers, csrs: &mut CsrFile)

    // Clear MPRV (not returning to M-mode).
    csrs.mstatus &= !(1u64 << 17);

    Ok(csrs.sepc)
}

// --- Unit tests for trap behavior ---
#[cfg(test)]
mod tests {
    use super::*;
    use crate::cpu::csr::CsrFile;
    use crate::cpu::registers::{PrivilegeMode, Registers};

    #[test]
    fn test_mret_restores_privilege_and_pc() {
        let mut regs = Registers::new();
        let mut csrs = CsrFile::new();

        csrs.mstatus = 1u64 << 11; // MPP = 1 (Supervisor)
        csrs.mepc = 0x1000;

        regs.priv_mode = PrivilegeMode::Machine;
        let new_pc = handle_mret(&mut regs, &mut csrs);

        assert_eq!(new_pc, 0x1000);
        assert_eq!(regs.priv_mode, PrivilegeMode::Supervisor);
        assert_eq!((csrs.mstatus >> 11) & 0x3, 0);
    }

    #[test]
    fn test_sret_restores_privilege_and_pc() {
        let mut regs = Registers::new();
        let mut csrs = CsrFile::new();

        regs.priv_mode = PrivilegeMode::Supervisor;