diff --git a/src/main.rs b/src/main.rs
index b58db88..d970561 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -13,36 +13,73 @@ const BIT_X: u8 = 8;
 // (Not used) - user mode accessible
 const BIT_U: u8 = 16;
 
+/*
 // (Not used) - global mapping
 const BIT_G: u8 = 32;
 
 // (Not presently used) accessed -- set if we've touched this since last time A cleared
 const BIT_A: u8 = 64;
 
-// (Will be used for JIT on writes to executable pags) dirty -- written since last time D cleared
+// (Not presently used)dirty -- written since last time D cleared
+// Note each core/hart has its own dirty state for local JIT
 const BIT_D: u8 = 128;
 
-/*
-// (Not used) reserved for supervisor
+// (Not used or room for it) reserved for supervisor
 const BITS_RSW_LO: i64 = 256;
 const BITS_RSW_HI: i64 = 512;
 const BITS_RSW: i64 = BITS_RSW_LO | BITS_RSW_HI;
 */
 
+
+#[repr(C)]
+#[derive(Clone, Copy)]
+struct PageTableEntry {
+    flags: u8
+}
+
+impl PageTableEntry {
+    fn new(flags: u8) -> Self {
+        return Self {
+            flags
+        }
+    }
+
+    fn as_u8(&self) -> u8 {
+        return self.flags;
+    }
+
+    fn is_valid(&self) -> bool {
+        return self.flags & BIT_V == BIT_V;
+    }
+
+    fn is_readable(&self) -> bool {
+        return self.flags & (BIT_V | BIT_R) == (BIT_V | BIT_R);
+    }
+
+    fn is_writable(&self) -> bool {
+        return self.flags & (BIT_V | BIT_R | BIT_W) == (BIT_V | BIT_R | BIT_W);
+    }
+
+    fn is_executable(&self) -> bool {
+        return self.flags & (BIT_V | BIT_R | BIT_X) == (BIT_V | BIT_R | BIT_X);
+    }
+
+}
+
 type ExecutorFunc = fn(i64, &mut CoreState, &mut MachineState) -> i64;
 
 #[repr(C)]
 struct MachineState {
     memory: Vec<u8>,
-    pages: Vec<u8>,
+    pages: Vec<PageTableEntry>,
 }
 
 /**
  * Note that physical memory accessors can traverse page boundaries;
  * we lay out linear memory from 0 to +4 gigabytes and will allocate
  * as many page table entries as are needed to cover RAM. These eat
- * up an extra 1 byte per 4 kilobytes of address space used, initially
- * allocating enough for all physical memory initially allocated.
+ * up an extra 1 byte per 4 KiB of address space used (1 MiB per 4 GiB),
+ * initially allocating enough for all physical memory allocated.
  *
  * This will be relatively space-inefficient for sparse address spaces
  * in the range of several gigabytes and more but requires only one
@@ -52,7 +89,7 @@ struct MachineState {
  * page tables, even if running on different threads.
  */
 impl MachineState {
-    fn restore(memory: Vec<u8>, pages: Vec<u8>) -> Self {
+    fn new_with_state(memory: Vec<u8>, pages: Vec<PageTableEntry>) -> Self {
         if ((memory.len() >> PAGE_BITS) << PAGE_BITS) != memory.len() {
             panic!("memory size must be a multiple of 4096 bytes");
         }
@@ -67,23 +104,23 @@ impl MachineState {
 
     fn new(memory_size: usize) -> Self {
         let memory = vec![0u8; memory_size];
-        let pages = vec![0u8; memory_size >> PAGE_BITS];
-        return Self::restore(memory, pages);
+        let pages = vec![PageTableEntry::new(0); memory_size >> PAGE_BITS];
+        return Self::new_with_state(memory, pages);
     }
 
-    fn get_page_table_entry(&mut self, address: usize) -> u32 {
+    fn get_page_table_entry(&mut self, address: usize) -> PageTableEntry {
         let page = address >> PAGE_BITS;
         if page < self.pages.len() {
-            return self.pages[page] as u32;
+            return self.pages[page];
         } else {
-            return 0;
+            return PageTableEntry::new(0);
         }
     }
 
-    fn set_page_table_entry(&mut self, address: usize, bits: u32) {
+    fn set_page_table_entry(&mut self, address: usize, entry: PageTableEntry) {
         let page = address >> PAGE_BITS;
         if page < self.pages.len() {
-            self.pages[address >> 12] = bits as u8;
+            self.pages[address >> 12] = entry;
         } else {
             panic!("@fixme: handle attempts to expand address space");
         }
@@ -188,9 +225,6 @@ struct CoreState {
     // bytes separately
     f: [f64; 32],
 
-    // 4096 csrs? no we're not gonna store them all
-    satp: i64,
-
     // * fflags, accrued exceptions: bits 0-4
     //   * nx: bit 0
     //   * uf: bit 1
@@ -206,11 +240,18 @@ struct CoreState {
     // Because function references are linked separately on
     // each thread in WebAssembly, this has to live in each
     // core's state separately.
-    executors: HashMap<i64, ExecutorFunc>
+    executors: HashMap<i64, ExecutorFunc>,
+
+    // Local dirty flags for JIT pages.
+    // When we get a fence.i instructrion, look for all dirty
+    // pages and invalidate any functions including them
+    // Takes up to 1 byte per 4 KiB (1 MiB per 4 GiB) per thread.
+    // Could be made more compact if only 1 bit is needed.
+    dirty: Vec<u8>
 }
 
 impl CoreState {
-    fn new() -> Self {
+    fn new(machine: &MachineState) -> Self {
         return Self {
             x: [
                 0, 0, 0, 0, 0, 0, 0, 0,
@@ -218,7 +259,6 @@ impl CoreState {
                 0, 0, 0, 0, 0, 0, 0, 0,
                 0, 0, 0, 0, 0, 0, 0, 0
             ],
-            satp: 0,
             f: [
                 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,
                 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0,
@@ -226,7 +266,8 @@ impl CoreState {
                 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0
             ],
             fcsr: 0,
-            executors: HashMap::new()
+            executors: HashMap::new(),
+            dirty: vec![0u8; machine.memory.len()]
         }
     }
 }
@@ -300,7 +341,7 @@ extern "C" fn interpreter(
 fn main() {
     let size = 8 * 1024 * 1024;
     let mut machine = MachineState::new(size);
-    let mut core = CoreState::new();
+    let mut core = CoreState::new(&machine);
     let pc = interpreter(&mut machine, &mut core, 0);
     println!("Ended with PC {}", pc);
 }