diff --git a/libcortex_a9/src/fpu.rs b/libcortex_a9/src/fpu.rs
index c4dd568..6daa144 100644
--- a/libcortex_a9/src/fpu.rs
+++ b/libcortex_a9/src/fpu.rs
@@ -1,12 +1,14 @@
 use core::arch::asm;
 /// Enable FPU in the current core.
+/// You must execute an ISB immediately after an update of the CPACR.
+/// https://developer.arm.com/documentation/ddi0388/i/system-control/register-descriptions/coprocessor-access-control-register
 pub fn enable_fpu() {
     unsafe {
         asm!("
             mrc p15, 0, r1, c1, c0, 2
             orr r1, r1, (0b1111<<20)
             mcr p15, 0, r1, c1, c0, 2
-
+            isb
             vmrs r1, fpexc
             orr r1, r1, (1<<30)
             vmsr fpexc, r1