diff --git a/src/register.rs b/src/register.rs
index 3fb33f5..1803e5c 100644
--- a/src/register.rs
+++ b/src/register.rs
@@ -1,6 +1,14 @@
 //! The register module Holds the basic [`Registers`] type and functions needed
 //! to access/manipulate them
 
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("Invalid value for the flags register, {0:x}. The lower 4 bits must be zero.")]
+    InvalidFRegisterValue(u8)
+}
+
 const INITIAL_PROGRAM_COUNTER_VALUE: u16 = 0x100;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -57,7 +65,7 @@ pub struct Registers {
     pub stack_pointer: u16,
     pub program_counter: u16,
     // the "F" register and the "flags" register are the same register
-    pub flags: u8,
+    flags: u8,
 }
 
 impl Flag {
@@ -112,6 +120,25 @@ impl Registers {
         (self.flags & (1 << pos)) >> pos
     }
 
+    /// Get the raw value of the flags register, ignore bit meanings
+    #[must_use]
+    pub fn get_raw_flag_bits(&self) -> u8 {
+        self.flags
+    }
+
+    /// Set the bits of the flags register.
+    ///
+    /// # Errors
+    /// Returns an error if the lower 4 bits of the register are set.
+    pub fn set_raw_flag_bits(&mut self, value: u8) -> Result<(), Error> {
+        if value & 0xF0 != value {
+            return Err(Error::InvalidFRegisterValue(value));
+        }
+
+        self.flags = value;
+        Ok(())
+    }
+
     /// Set a flag register bit (at position `pos`) to the given value.
     /// This will only pay attention to the first bit, so only zero or one should be passed
     ///
diff --git a/tests/cpu/jsmoo/tests.rs b/tests/cpu/jsmoo/tests.rs
index 18a3e0d..bb27bb5 100644
--- a/tests/cpu/jsmoo/tests.rs
+++ b/tests/cpu/jsmoo/tests.rs
@@ -20,7 +20,7 @@ fn test_jsmoo_test(filename: &str) {
         processor.registers.c = test_case.initial.c;
         processor.registers.d = test_case.initial.d;
         processor.registers.e = test_case.initial.e;
-        processor.registers.flags = test_case.initial.f;
+        processor.registers.set_raw_flag_bits(test_case.initial.f).unwrap();
         processor.registers.h = test_case.initial.h;
         processor.registers.l = test_case.initial.l;
         processor.registers.program_counter = test_case.initial.pc;
@@ -42,7 +42,7 @@ fn test_jsmoo_test(filename: &str) {
         assert_eq!(processor.registers.c, test_case.r#final.c);
         assert_eq!(processor.registers.d, test_case.r#final.d);
         assert_eq!(processor.registers.e, test_case.r#final.e);
-        assert_eq!(processor.registers.flags, test_case.r#final.f);
+        assert_eq!(processor.registers.get_raw_flag_bits(), test_case.r#final.f);
         assert_eq!(processor.registers.h, test_case.r#final.h);
         assert_eq!(processor.registers.l, test_case.r#final.l);
         assert_eq!(processor.registers.program_counter, test_case.r#final.pc);