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Add enter_unprivileged() function #594

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Add enter_unprivileged() function #594

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28 changes: 28 additions & 0 deletions cortex-m/src/asm.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -244,6 +244,34 @@ pub unsafe fn semihosting_syscall(mut nr: u32, arg: u32) -> u32 {
nr
}

/// Switch to unprivileged mode.
///
/// Sets CONTROL.SPSEL (setting the program stack to be the active
/// stack) and CONTROL.nPRIV (setting unprivileged mode), updates the
/// program stack pointer to the address in `psp`, then jumps to the
/// address in `entry`.
///
/// # Safety
///
/// `psp` and `entry` must point to valid stack memory and executable
/// code, respectively. `psp` must be 8 bytes aligned.
#[cfg(cortex_m)]
#[inline(always)]
pub unsafe fn enter_unprivileged(psp: *const u32, entry: fn() -> !) -> ! {
asm!(
"mrs {tmp}, CONTROL",
"orr {tmp}, #2",
"msr PSP, {psp}",
"msr CONTROL, {tmp}",
"isb",
"bx {ent}",
tmp = in(reg) 0,
psp = in(reg) psp,
ent = in(reg) entry,
options(noreturn, nomem, nostack)
);
}

/// Bootstrap.
///
/// Clears CONTROL.SPSEL (setting the main stack to be the active stack),
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