diff --git a/bdevice.py b/bdevice.py
index 5ea4bf0..748eb3d 100644
--- a/bdevice.py
+++ b/bdevice.py
@@ -63,7 +63,10 @@ class BlockDevice:
         return
 
     def readblocks(self, blocknum, buf, offset=0):
-        self.readwrite(offset + (blocknum << self._nbits), buf, True)
+        if blocknum < 0 or (offset + (blocknum << self._nbits)) > self._a_bytes:
+            print('rb', hex(blocknum), offset, hex(offset + (blocknum << self._nbits)))
+        else:
+            self.readwrite(offset + (blocknum << self._nbits), buf, True)
 
     def writeblocks(self, blocknum, buf, offset=0):
         self.readwrite(offset + (blocknum << self._nbits), buf, False)
diff --git a/flash/FLASH.md b/flash/FLASH.md
index 021d72b..04ec201 100644
--- a/flash/FLASH.md
+++ b/flash/FLASH.md
@@ -133,20 +133,26 @@ each chip select line a flash array is instantiated. A `RuntimeError` will be
 raised if a device is not detected on a CS line. The test has no effect on
 the array contents.
 
-Arguments:  
- 1. `spi` Mandatory. An initialised SPI bus created by `machine`.
+Arguments. In most cases only the first two mandatory args are required:  
+ 1. `spi` An initialised SPI bus created by `machine`.
  2. `cspins` A list or tuple of `Pin` instances. Each `Pin` must be initialised
  as an output (`Pin.OUT`) and with `value=1` and be created by `machine`.
  3. `size=None` Chip size in KiB. The size is read from the chip. If a value
  is passed, the actual size is compared with the passed value: a mismatch will
- raise a `ValueError`. Optionally set to 32768 for the S25FL256L chip or 16384
- for the S25FL128L.
+ raise a `ValueError`. A `ValueError` will also occur if chips in the array
+ have differing sizes. See table below for values.
  4. `verbose=True` If `True`, the constructor issues information on the flash
  devices it has detected.
  5. `sec_size=4096` Chip sector size.
  6. `block_size=9` The block size reported to the filesystem. The size in bytes
  is `2**block_size` so is 512 bytes by default.
 
+Size values:  
+| Chip      | Size  |
+|:---------:|:-----:|
+| S25FL256L | 32768 |
+| S25FL128L | 16384 |
+
 ### 4.1.2 Methods providing byte level access
 
 It is possible to read and write individual bytes or arrays of arbitrary size.
diff --git a/flash/flash_spi.py b/flash/flash_spi.py
index 1f8f3d0..599bc94 100644
--- a/flash/flash_spi.py
+++ b/flash/flash_spi.py
@@ -63,8 +63,8 @@ class FLASH(FlashDevice):
             cs(1)
             scansize = 1 << (mvp[3] - 10)
             if size is None:
-                size = scansize
-            if size != scansize:
+                size = scansize  # Save size of 1st chip
+            if size != scansize:  # Mismatch passed size or 1st chip.
                 raise ValueError('Flash size mismatch: expected {}KiB, found {}KiB'.format(size, scansize))
         if verbose:
             s = '{} chips detected. Total flash size {}MiB.'
@@ -152,10 +152,16 @@ class FLASH(FlashDevice):
     # Given an address, set current chip select and address buffer.
     # Return the number of bytes that can be processed in the current chip.
     def _getaddr(self, addr, nbytes):
+#        print(hex(addr), hex(self._a_bytes), hex(self._c_bytes), self._nbits, self._block_size, divmod(addr, self._c_bytes))
         if addr >= self._a_bytes:
+#            print(hex(addr), hex(self._a_bytes), hex(self._c_bytes), self._nbits, self._block_size, divmod(addr, self._c_bytes))
             raise RuntimeError("Flash Address is out of range")
         ca, la = divmod(addr, self._c_bytes)  # ca == chip no, la == offset into chip
-        self._ccs = self._cspins[ca]  # Current chip select
+        try:
+            self._ccs = self._cspins[ca]  # Current chip select
+        except:
+            print(ca, la)
+            raise
         cmdlen = self._cmdlen
         mvp = self._mvp[:cmdlen]
         if cmdlen > 3: