diff --git a/sim/simflash/src/lib.rs b/sim/simflash/src/lib.rs
new file mode 100644
index 0000000..991bbe4
--- /dev/null
+++ b/sim/simflash/src/lib.rs
@@ -0,0 +1,289 @@
+//! A flash simulator
+//!
+//! This module is capable of simulating the type of NOR flash commonly used in microcontrollers.
+//! These generally can be written as individual bytes, but must be erased in larger units.
+
+#[macro_use] extern crate error_chain;
+mod pdump;
+
+use std::fs::File;
+use std::io::Write;
+use std::iter::Enumerate;
+use std::path::Path;
+use std::slice;
+use pdump::HexDump;
+
+error_chain! {
+    errors {
+        OutOfBounds(t: String) {
+            description("Offset is out of bounds")
+            display("Offset out of bounds: {}", t)
+        }
+        Write(t: String) {
+            description("Invalid write")
+            display("Invalid write: {}", t)
+        }
+    }
+}
+
+pub trait Flash {
+    fn erase(&mut self, offset: usize, len: usize) -> Result<()>;
+    fn write(&mut self, offset: usize, payload: &[u8]) -> Result<()>;
+    fn read(&self, offset: usize, data: &mut [u8]) -> Result<()>;
+
+    fn sector_iter(&self) -> SectorIter;
+    fn device_size(&self) -> usize;
+}
+
+fn ebounds<T: AsRef<str>>(message: T) -> ErrorKind {
+    ErrorKind::OutOfBounds(message.as_ref().to_owned())
+}
+
+fn ewrite<T: AsRef<str>>(message: T) -> ErrorKind {
+    ErrorKind::Write(message.as_ref().to_owned())
+}
+
+/// An emulated flash device.  It is represented as a block of bytes, and a list of the sector
+/// mapings.
+#[derive(Clone)]
+pub struct SimFlash {
+    data: Vec<u8>,
+    write_safe: Vec<bool>,
+    sectors: Vec<usize>,
+    // Alignment required for writes.
+    align: usize,
+}
+
+impl SimFlash {
+    /// Given a sector size map, construct a flash device for that.
+    pub fn new(sectors: Vec<usize>, align: usize) -> SimFlash {
+        // Verify that the alignment is a positive power of two.
+        assert!(align > 0);
+        assert!(align & (align - 1) == 0);
+
+        let total = sectors.iter().sum();
+        SimFlash {
+            data: vec![0xffu8; total],
+            write_safe: vec![true; total],
+            sectors: sectors,
+            align: align,
+        }
+    }
+
+    #[allow(dead_code)]
+    pub fn dump(&self) {
+        self.data.dump();
+    }
+
+    /// Dump this image to the given file.
+    #[allow(dead_code)]
+    pub fn write_file<P: AsRef<Path>>(&self, path: P) -> Result<()> {
+        let mut fd = File::create(path).chain_err(|| "Unable to write image file")?;
+        fd.write_all(&self.data).chain_err(|| "Unable to write to image file")?;
+        Ok(())
+    }
+
+    // Scan the sector map, and return the base and offset within a sector for this given byte.
+    // Returns None if the value is outside of the device.
+    fn get_sector(&self, offset: usize) -> Option<(usize, usize)> {
+        let mut offset = offset;
+        for (sector, &size) in self.sectors.iter().enumerate() {
+            if offset < size {
+                return Some((sector, offset));
+            }
+            offset -= size;
+        }
+        return None;
+    }
+
+}
+
+impl Flash for SimFlash {
+    /// The flash drivers tend to erase beyond the bounds of the given range.  Instead, we'll be
+    /// strict, and make sure that the passed arguments are exactly at a sector boundary, otherwise
+    /// return an error.
+    fn erase(&mut self, offset: usize, len: usize) -> Result<()> {
+        let (_start, slen) = self.get_sector(offset).ok_or_else(|| ebounds("start"))?;
+        let (end, elen) = self.get_sector(offset + len - 1).ok_or_else(|| ebounds("end"))?;
+
+        if slen != 0 {
+            bail!(ebounds("offset not at start of sector"));
+        }
+        if elen != self.sectors[end] - 1 {
+            bail!(ebounds("end not at start of sector"));
+        }
+
+        for x in &mut self.data[offset .. offset + len] {
+            *x = 0xff;
+        }
+
+        for x in &mut self.write_safe[offset .. offset + len] {
+            *x = true;
+        }
+
+        Ok(())
+    }
+
+    /// We restrict to only allowing writes of values that are:
+    ///
+    /// 1. being written to for the first time
+    /// 2. being written to after being erased
+    ///
+    /// This emulates a flash device which starts out erased, with the
+    /// added restriction that repeated writes to the same location
+    /// are disallowed, even if they would be safe to do.
+    fn write(&mut self, offset: usize, payload: &[u8]) -> Result<()> {
+        if offset + payload.len() > self.data.len() {
+            panic!("Write outside of device");
+        }
+
+        // Verify the alignment (which must be a power of two).
+        if offset & (self.align - 1) != 0 {
+            panic!("Misaligned write address");
+        }
+
+        if payload.len() & (self.align - 1) != 0 {
+            panic!("Write length not multiple of alignment");
+        }
+
+        for (i, x) in &mut self.write_safe[offset .. offset + payload.len()].iter_mut().enumerate() {
+            if !(*x) {
+                bail!(ewrite(format!("Write to unerased location at 0x{:x}",
+                                     offset + i)));
+            }
+            *x = false;
+        }
+
+        let mut sub = &mut self.data[offset .. offset + payload.len()];
+        sub.copy_from_slice(payload);
+        Ok(())
+    }
+
+    /// Read is simple.
+    fn read(&self, offset: usize, data: &mut [u8]) -> Result<()> {
+        if offset + data.len() > self.data.len() {
+            bail!(ebounds("Read outside of device"));
+        }
+
+        let sub = &self.data[offset .. offset + data.len()];
+        data.copy_from_slice(sub);
+        Ok(())
+    }
+
+    /// An iterator over each sector in the device.
+    fn sector_iter(&self) -> SectorIter {
+        SectorIter {
+            iter: self.sectors.iter().enumerate(),
+            base: 0,
+        }
+    }
+
+    fn device_size(&self) -> usize {
+        self.data.len()
+    }
+}
+
+/// It is possible to iterate over the sectors in the device, each element returning this.
+#[derive(Debug)]
+pub struct Sector {
+    /// Which sector is this, starting from 0.
+    pub num: usize,
+    /// The offset, in bytes, of the start of this sector.
+    pub base: usize,
+    /// The length, in bytes, of this sector.
+    pub size: usize,
+}
+
+pub struct SectorIter<'a> {
+    iter: Enumerate<slice::Iter<'a, usize>>,
+    base: usize,
+}
+
+impl<'a> Iterator for SectorIter<'a> {
+    type Item = Sector;
+
+    fn next(&mut self) -> Option<Sector> {
+        match self.iter.next() {
+            None => None,
+            Some((num, &size)) => {
+                let base = self.base;
+                self.base += size;
+                Some(Sector {
+                    num: num,
+                    base: base,
+                    size: size,
+                })
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{Flash, SimFlash, Error, ErrorKind, Result, Sector};
+
+    #[test]
+    fn test_flash() {
+        // NXP-style, uniform sectors.
+        let mut f1 = SimFlash::new(vec![4096usize; 256], 1);
+        test_device(&mut f1);
+
+        // STM style, non-uniform sectors
+        let mut f2 = SimFlash::new(vec![16 * 1024, 16 * 1024, 16 * 1024, 64 * 1024,
+                                128 * 1024, 128 * 1024, 128 * 1024], 1);
+        test_device(&mut f2);
+    }
+
+    fn test_device(flash: &mut Flash) {
+        let sectors: Vec<Sector> = flash.sector_iter().collect();
+
+        flash.erase(0, sectors[0].size).unwrap();
+        let flash_size = flash.device_size();
+        flash.erase(0, flash_size).unwrap();
+        assert!(flash.erase(0, sectors[0].size - 1).is_bounds());
+
+        // Verify that write and erase do something.
+        flash.write(0, &[0]).unwrap();
+        let mut buf = [0; 4];
+        flash.read(0, &mut buf).unwrap();
+        assert_eq!(buf, [0, 0xff, 0xff, 0xff]);
+
+        flash.erase(0, sectors[0].size).unwrap();
+        flash.read(0, &mut buf).unwrap();
+        assert_eq!(buf, [0xff; 4]);
+
+        // Program the first and last byte of each sector, verify that has been done, and then
+        // erase to verify the erase boundaries.
+        for sector in &sectors {
+            let byte = [(sector.num & 127) as u8];
+            flash.write(sector.base, &byte).unwrap();
+            flash.write(sector.base + sector.size - 1, &byte).unwrap();
+        }
+
+        // Verify the above
+        let mut buf = Vec::new();
+        for sector in &sectors {
+            let byte = (sector.num & 127) as u8;
+            buf.resize(sector.size, 0);
+            flash.read(sector.base, &mut buf).unwrap();
+            assert_eq!(buf.first(), Some(&byte));
+            assert_eq!(buf.last(), Some(&byte));
+            assert!(buf[1..buf.len()-1].iter().all(|&x| x == 0xff));
+        }
+    }
+
+    // Helper checks for the result type.
+    trait EChecker {
+        fn is_bounds(&self) -> bool;
+    }
+
+    impl<T> EChecker for Result<T> {
+
+        fn is_bounds(&self) -> bool {
+            match *self {
+                Err(Error(ErrorKind::OutOfBounds(_), _)) => true,
+                _ => false,
+            }
+        }
+    }
+}
diff --git a/sim/simflash/src/pdump.rs b/sim/simflash/src/pdump.rs
new file mode 100644
index 0000000..dbb42d5
--- /dev/null
+++ b/sim/simflash/src/pdump.rs
@@ -0,0 +1,76 @@
+// Printable hexdump.
+
+pub trait HexDump {
+    // Output the data value in hex.
+    fn dump(&self);
+}
+
+struct Dumper {
+    hex: String,
+    ascii: String,
+    count: usize,
+    total_count: usize,
+}
+
+impl Dumper {
+    fn new() -> Dumper {
+        Dumper {
+            hex: String::with_capacity(49),
+            ascii: String::with_capacity(16),
+            count: 0,
+            total_count: 0,
+        }
+    }
+
+    fn add_byte(&mut self, ch: u8) {
+        if self.count == 16 {
+            self.ship();
+        }
+        if self.count == 8 {
+            self.hex.push(' ');
+        }
+        self.hex.push_str(&format!(" {:02x}", ch)[..]);
+        self.ascii.push(if ch >= ' ' as u8 && ch <= '~' as u8 {
+            ch as char
+        } else {
+            '.'
+        });
+        self.count += 1;
+    }
+
+    fn ship(&mut self) {
+        if self.count == 0 {
+            return;
+        }
+
+        println!("{:06x} {:-49} |{}|", self.total_count, self.hex, self.ascii);
+
+        self.hex.clear();
+        self.ascii.clear();
+        self.total_count += 16;
+        self.count = 0;
+    }
+}
+
+impl<'a> HexDump for &'a [u8] {
+    fn dump(&self) {
+        let mut dump = Dumper::new();
+        for ch in self.iter() {
+            dump.add_byte(*ch);
+        }
+        dump.ship();
+    }
+}
+
+impl HexDump for Vec<u8> {
+    fn dump(&self) {
+        (&self[..]).dump()
+    }
+}
+
+#[test]
+fn samples() {
+    "Hello".as_bytes().dump();
+    "This is a much longer string".as_bytes().dump();
+    "\x00\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f".as_bytes().dump();
+}