diff --git a/artiq/firmware/runtime/config.rs b/artiq/firmware/runtime/config.rs
index d8304f8b9..c4b45ee6c 100644
--- a/artiq/firmware/runtime/config.rs
+++ b/artiq/firmware/runtime/config.rs
@@ -1,192 +1,220 @@
-use core::str;
-use std::btree_map::BTreeMap;
-use byteorder::{ByteOrder, BigEndian};
-use board::{mem, csr, cache, spiflash};
+#[cfg(has_spiflash)]
+mod imp {
+    use core::str;
+    use std::btree_map::BTreeMap;
+    use byteorder::{ByteOrder, BigEndian};
+    use board::{mem, csr, cache, spiflash};
 
-const ADDR: usize = mem::FLASH_BOOT_ADDRESS + 0x80000 /* max runtime size */;
-const SIZE: usize = csr::CONFIG_SPIFLASH_SECTOR_SIZE as usize;
+    const ADDR: usize = mem::FLASH_BOOT_ADDRESS + 0x80000 /* max runtime size */;
+    const SIZE: usize = csr::CONFIG_SPIFLASH_SECTOR_SIZE as usize;
 
-mod lock {
-    use core::slice;
-    use core::sync::atomic::{AtomicUsize, Ordering};
+    mod lock {
+        use core::slice;
+        use core::sync::atomic::{AtomicUsize, Ordering};
 
-    static LOCKED: AtomicUsize = AtomicUsize::new(0);
+        static LOCKED: AtomicUsize = AtomicUsize::new(0);
 
-    pub struct Lock;
+        pub struct Lock;
 
-    impl Lock {
-        pub fn take() -> Result<Lock, ()> {
-            if LOCKED.swap(1, Ordering::SeqCst) != 0 {
-                Err(()) // already locked
-            } else {
-                Ok(Lock) // locked now
+        impl Lock {
+            pub fn take() -> Result<Lock, ()> {
+                if LOCKED.swap(1, Ordering::SeqCst) != 0 {
+                    Err(()) // already locked
+                } else {
+                    Ok(Lock) // locked now
+                }
+            }
+
+            pub fn data(&self) -> &'static [u8] {
+                unsafe { slice::from_raw_parts(super::ADDR as *const u8, super::SIZE) }
             }
         }
 
-        pub fn data(&self) -> &'static [u8] {
-            unsafe { slice::from_raw_parts(super::ADDR as *const u8, super::SIZE) }
-        }
-    }
-
-    impl Drop for Lock {
-        fn drop(&mut self) {
-            LOCKED.store(0, Ordering::SeqCst)
-        }
-    }
-}
-
-pub use self::lock::Lock;
-
-struct Iter<'a> {
-    data:   &'a [u8],
-    offset: usize
-}
-
-impl<'a> Iter<'a> {
-    fn new(data: &'a [u8]) -> Iter<'a> {
-        Iter { data: data, offset: 0 }
-    }
-}
-
-impl<'a> Iterator for Iter<'a> {
-    type Item = Result<(&'a [u8], &'a [u8]), ()>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        let data = &self.data[self.offset..];
-
-        if data.len() < 4 {
-            error!("offset {}: truncated record", self.offset);
-            return Some(Err(()))
-        }
-
-        let record_size = BigEndian::read_u32(data) as usize;
-        if record_size < 4 {
-            error!("offset {}: invalid record size", self.offset);
-            return Some(Err(()))
-        }
-        if record_size == !0 /* all ones; erased flash */ {
-            return None
-        }
-
-        let record_body = &data[4..record_size];
-        match record_body.iter().position(|&x| x == 0) {
-            None => {
-                error!("offset {}: missing separator", self.offset);
-                Some(Err(()))
-            }
-            Some(pos) => {
-                self.offset += record_size;
-
-                let (key, zero_and_value) = record_body.split_at(pos);
-                Some(Ok((key, &zero_and_value[1..])))
+        impl Drop for Lock {
+            fn drop(&mut self) {
+                LOCKED.store(0, Ordering::SeqCst)
             }
         }
     }
-}
 
-pub fn read<F: FnOnce(Result<&[u8], ()>) -> R, R>(key: &str, f: F) -> R {
-    f(Lock::take().and_then(|lock| {
-        let mut iter = Iter::new(lock.data());
-        let mut value = &[][..];
-        while let Some(result) = iter.next() {
-            let (record_key, record_value) = result?;
-            if key.as_bytes() == record_key {
-                // last write wins
-                value = record_value
+    use self::lock::Lock;
+
+    struct Iter<'a> {
+        data:   &'a [u8],
+        offset: usize
+    }
+
+    impl<'a> Iter<'a> {
+        fn new(data: &'a [u8]) -> Iter<'a> {
+            Iter { data: data, offset: 0 }
+        }
+    }
+
+    impl<'a> Iterator for Iter<'a> {
+        type Item = Result<(&'a [u8], &'a [u8]), ()>;
+
+        fn next(&mut self) -> Option<Self::Item> {
+            let data = &self.data[self.offset..];
+
+            if data.len() < 4 {
+                error!("offset {}: truncated record", self.offset);
+                return Some(Err(()))
+            }
+
+            let record_size = BigEndian::read_u32(data) as usize;
+            if record_size < 4 {
+                error!("offset {}: invalid record size", self.offset);
+                return Some(Err(()))
+            }
+            if record_size == !0 /* all ones; erased flash */ {
+                return None
+            }
+
+            let record_body = &data[4..record_size];
+            match record_body.iter().position(|&x| x == 0) {
+                None => {
+                    error!("offset {}: missing separator", self.offset);
+                    Some(Err(()))
+                }
+                Some(pos) => {
+                    self.offset += record_size;
+
+                    let (key, zero_and_value) = record_body.split_at(pos);
+                    Some(Ok((key, &zero_and_value[1..])))
+                }
             }
         }
-        Ok(value)
-    }))
-}
-
-pub fn read_str<F: FnOnce(Result<&str, ()>) -> R, R>(key: &str, f: F) -> R {
-    read(key, |result| {
-        f(result.and_then(|value| str::from_utf8(value).map_err(|_| ())))
-    })
-}
-
-fn append_at(mut offset: usize, key: &[u8], value: &[u8]) -> Result<usize, ()> {
-    let record_size = 4 + key.len() + 1 + value.len();
-    if offset + record_size > SIZE {
-        return Err(())
     }
 
-    let mut record_size_bytes = [0u8; 4];
-    BigEndian::write_u32(&mut record_size_bytes[..], record_size as u32);
+    pub fn read<F: FnOnce(Result<&[u8], ()>) -> R, R>(key: &str, f: F) -> R {
+        f(Lock::take().and_then(|lock| {
+            let mut iter = Iter::new(lock.data());
+            let mut value = &[][..];
+            while let Some(result) = iter.next() {
+                let (record_key, record_value) = result?;
+                if key.as_bytes() == record_key {
+                    // last write wins
+                    value = record_value
+                }
+            }
+            Ok(value)
+        }))
+    }
 
-    spiflash::write(ADDR + offset, &record_size_bytes[..]);
-    offset += record_size_bytes.len();
+    pub fn read_str<F: FnOnce(Result<&str, ()>) -> R, R>(key: &str, f: F) -> R {
+        read(key, |result| {
+            f(result.and_then(|value| str::from_utf8(value).map_err(|_| ())))
+        })
+    }
 
-    spiflash::write(ADDR + offset, key);
-    offset += key.len();
-
-    spiflash::write(ADDR + offset, &[0]);
-    offset += 1;
-
-    spiflash::write(ADDR + offset, value);
-    offset += value.len();
-
-    cache::flush_l2_cache();
-    Ok(offset)
-}
-
-fn compact() -> Result<(), ()> {
-    let lock = Lock::take()?;
-
-    let mut items = BTreeMap::new();
-    {
-        let mut iter = Iter::new(lock.data());
-        while let Some(result) = iter.next() {
-            let (key, value) = result?;
-            items.insert(key, value);
+    fn append_at(mut offset: usize, key: &[u8], value: &[u8]) -> Result<usize, ()> {
+        let record_size = 4 + key.len() + 1 + value.len();
+        if offset + record_size > SIZE {
+            return Err(())
         }
+
+        let mut record_size_bytes = [0u8; 4];
+        BigEndian::write_u32(&mut record_size_bytes[..], record_size as u32);
+
+        spiflash::write(ADDR + offset, &record_size_bytes[..]);
+        offset += record_size_bytes.len();
+
+        spiflash::write(ADDR + offset, key);
+        offset += key.len();
+
+        spiflash::write(ADDR + offset, &[0]);
+        offset += 1;
+
+        spiflash::write(ADDR + offset, value);
+        offset += value.len();
+
+        cache::flush_l2_cache();
+        Ok(offset)
     }
 
-    spiflash::erase_sector(ADDR);
-    cache::flush_l2_cache();
+    fn compact() -> Result<(), ()> {
+        let lock = Lock::take()?;
 
-    let mut offset = 0;
-    for (key, value) in items {
-        offset = append_at(offset, key, value)?;
-    }
-    Ok(())
-}
-
-fn append(key: &str, value: &[u8]) -> Result<(), ()> {
-    let lock = Lock::take()?;
-
-    let free_offset = {
-        let mut iter = Iter::new(lock.data());
-        while let Some(result) = iter.next() {
-            let _ = result?;
+        let mut items = BTreeMap::new();
+        {
+            let mut iter = Iter::new(lock.data());
+            while let Some(result) = iter.next() {
+                let (key, value) = result?;
+                items.insert(key, value);
+            }
         }
-        iter.offset
-    };
 
-    append_at(free_offset, key.as_bytes(), value)?;
-    Ok(())
-}
+        spiflash::erase_sector(ADDR);
+        cache::flush_l2_cache();
 
-pub fn write(key: &str, value: &[u8]) -> Result<(), ()> {
-    match append(key, value) {
-        Ok(()) => (),
-        Err(()) => {
-            compact()?;
-            append(key, value)?;
+        let mut offset = 0;
+        for (key, value) in items {
+            offset = append_at(offset, key, value)?;
         }
+        Ok(())
+    }
+
+    fn append(key: &str, value: &[u8]) -> Result<(), ()> {
+        let lock = Lock::take()?;
+
+        let free_offset = {
+            let mut iter = Iter::new(lock.data());
+            while let Some(result) = iter.next() {
+                let _ = result?;
+            }
+            iter.offset
+        };
+
+        append_at(free_offset, key.as_bytes(), value)?;
+        Ok(())
+    }
+
+    pub fn write(key: &str, value: &[u8]) -> Result<(), ()> {
+        match append(key, value) {
+            Ok(()) => (),
+            Err(()) => {
+                compact()?;
+                append(key, value)?;
+            }
+        }
+        Ok(())
+    }
+
+    pub fn remove(key: &str) -> Result<(), ()> {
+        write(key, &[])
+    }
+
+    pub fn erase() -> Result<(), ()> {
+        let _lock = Lock::take()?;
+
+        spiflash::erase_sector(ADDR);
+        cache::flush_l2_cache();
+
+        Ok(())
     }
-    Ok(())
 }
 
-pub fn remove(key: &str) -> Result<(), ()> {
-    write(key, &[])
+#[cfg(not(has_spiflash))]
+mod imp {
+    pub fn read<F: FnOnce(Result<&[u8], ()>) -> R, R>(_key: &str, f: F) -> R {
+        f(Err(()))
+    }
+
+    pub fn read_str<F: FnOnce(Result<&str, ()>) -> R, R>(_key: &str, f: F) -> R {
+        f(Err(()))
+    }
+
+    pub fn write(_key: &str, _value: &[u8]) -> Result<(), ()> {
+        Err(())
+    }
+
+    pub fn remove(_key: &str) -> Result<(), ()> {
+        Err(())
+    }
+
+    pub fn erase() -> Result<(), ()> {
+        Err(())
+    }
 }
 
-pub fn erase() -> Result<(), ()> {
-    let _lock = Lock::take()?;
-
-    spiflash::erase_sector(ADDR);
-    cache::flush_l2_cache();
-
-    Ok(())
-}
+pub use self::imp::*;
diff --git a/artiq/firmware/runtime/config_dummy.rs b/artiq/firmware/runtime/config_dummy.rs
deleted file mode 100644
index 2680c9ab6..000000000
--- a/artiq/firmware/runtime/config_dummy.rs
+++ /dev/null
@@ -1,19 +0,0 @@
-pub fn read<F: FnOnce(Result<&[u8], ()>) -> R, R>(_key: &str, f: F) -> R {
-    f(Err(()))
-}
-
-pub fn read_str<F: FnOnce(Result<&str, ()>) -> R, R>(_key: &str, f: F) -> R {
-    f(Err(()))
-}
-
-pub fn write(_key: &str, _value: &[u8]) -> Result<(), ()> {
-    Err(())
-}
-
-pub fn remove(_key: &str) -> Result<(), ()> {
-    Err(())
-}
-
-pub fn erase() -> Result<(), ()> {
-    Err(())
-}
diff --git a/artiq/firmware/runtime/lib.rs b/artiq/firmware/runtime/lib.rs
index 57f098d9e..0ac8c493f 100644
--- a/artiq/firmware/runtime/lib.rs
+++ b/artiq/firmware/runtime/lib.rs
@@ -34,10 +34,7 @@ macro_rules! borrow_mut {
     })
 }
 
-#[cfg(has_spiflash)]
 mod config;
-#[cfg(not(has_spiflash))]
-#[path="config_dummy.rs"] mod config;
 mod ethmac;
 mod rtio_mgt;