diff --git a/src/asm.rs b/src/asm.rs
index e85684f..9ae1281 100644
--- a/src/asm.rs
+++ b/src/asm.rs
@@ -3,17 +3,23 @@
 use crate::asm;
 
 /// A no-operation. Useful to prevent delay loops from being optimized away.
+///
+/// Unlike [barrier], this does not prevent reordering of memory access.
 #[inline(always)]
 pub fn nop() {
     unsafe {
-        asm!("nop");
+        // Do not use pure because prevent nop from being removed.
+        asm!("nop", options(nomem, nostack, preserves_flags));
     }
 }
 
 /// A compiler fence, prevents instruction reordering.
+///
+/// Unlike [nop], this does not emit machine code.
 #[inline(always)]
 pub fn barrier() {
     unsafe {
-        asm!("");
+        // Do not use `nomem` and `readonly` because prevent preceding and subsequent memory accesses from being reordered.
+        asm!("", options(nostack, preserves_flags));
     }
 }
diff --git a/src/interrupt.rs b/src/interrupt.rs
index d67bde9..f896464 100644
--- a/src/interrupt.rs
+++ b/src/interrupt.rs
@@ -10,7 +10,9 @@ pub fn disable() {
     match () {
         #[cfg(target_arch = "msp430")]
         () => unsafe {
-            asm!("dint {{ nop");
+            // Do not use `nomem` and `readonly` because prevent subsequent memory accesses from being reordered before interrupts are disabled.
+            // Do not use `preserves_flags` because DINT modifies the GIE (global interrupt enable) bit of the status register.
+            asm!("dint {{ nop", options(nostack));
         },
         #[cfg(not(target_arch = "msp430"))]
         () => {}
@@ -29,7 +31,9 @@ pub unsafe fn enable() {
     match () {
         #[cfg(target_arch = "msp430")]
         () => {
-            asm!("nop {{ eint {{ nop");
+            // Do not use `nomem` and `readonly` because prevent preceding memory accesses from being reordered after interrupts are enabled.
+            // Do not use `preserves_flags` because EINT modifies the GIE (global interrupt enable) bit of the status register.
+            asm!("nop {{ eint {{ nop", options(nostack));
         }
         #[cfg(not(target_arch = "msp430"))]
         () => {}
diff --git a/src/register/pc.rs b/src/register/pc.rs
index d84fe02..c1ae13f 100644
--- a/src/register/pc.rs
+++ b/src/register/pc.rs
@@ -7,7 +7,7 @@ use crate::asm;
 pub fn read() -> u16 {
     let r;
     unsafe {
-        asm!("mov R0, {0}", out(reg) r);
+        asm!("mov R0, {0}", out(reg) r, options(nomem, nostack, preserves_flags));
     }
     r
 }
diff --git a/src/register/sp.rs b/src/register/sp.rs
index 9cc818e..49430ab 100644
--- a/src/register/sp.rs
+++ b/src/register/sp.rs
@@ -7,7 +7,7 @@ use crate::asm;
 pub fn read() -> u16 {
     let r;
     unsafe {
-        asm!("mov R1, {0}", out(reg) r);
+        asm!("mov R1, {0}", out(reg) r, options(nomem, nostack, preserves_flags));
     }
     r
 }
diff --git a/src/register/sr.rs b/src/register/sr.rs
index 22a3471..58d4d8e 100644
--- a/src/register/sr.rs
+++ b/src/register/sr.rs
@@ -79,7 +79,7 @@ impl Sr {
 pub fn read() -> Sr {
     let r: u16;
     unsafe {
-        asm!("mov R2, {0}", out(reg) r);
+        asm!("mov R2, {0}", out(reg) r, options(nomem, nostack, preserves_flags));
     }
     Sr { bits: r }
 }