old_contents: import esp
  
  class FlashBdev:
  
      SEC_SIZE = 4096
      START_SEC = esp.flash_user_start() // SEC_SIZE
      NUM_BLK = 0x6b
  
      def __init__(self, blocks=NUM_BLK):
          self.blocks = blocks
  
      def readblocks(self, n, buf):
          #print("readblocks(%s, %x(%d))" % (n, id(buf), len(buf)))
          esp.flash_read((n + self.START_SEC) * self.SEC_SIZE, buf)
  
      def writeblocks(self, n, buf):
          #print("writeblocks(%s, %x(%d))" % (n, id(buf), len(buf)))
          #assert len(buf) <= self.SEC_SIZE, len(buf)
          esp.flash_erase(n + self.START_SEC)
          esp.flash_write((n + self.START_SEC) * self.SEC_SIZE, buf)
  
      def ioctl(self, op, arg):
          #print("ioctl(%d, %r)" % (op, arg))
          if op == 4:  # BP_IOCTL_SEC_COUNT
              return self.blocks
          if op == 5:  # BP_IOCTL_SEC_SIZE
              return self.SEC_SIZE
  
  def set_bl_flash_size(real_size):
      if real_size == 256*1024:
          code = 1
      elif real_size == 512*1024:
          code = 0
      elif real_size == 1024*1024:
          code = 2
      elif real_size == 2048*1024:
          code = 3
      elif real_size == 4096*1024:
          code = 4
      else:
          code = 2
      buf = bytearray(4096)
      esp.flash_read(0, buf)
      buf[3] = (buf[3] & 0xf) | (code << 4)
      esp.flash_erase(0)
      esp.flash_write(0, buf)
  
  # If bootloader size ID doesn't correspond to real Flash size,
  # fix bootloader value and reboot.
  size = esp.flash_id() >> 16
  # Check that it looks like realistic power of 2 for flash sizes
  # commonly used with esp8266
  if 22 >= size >= 18:
      size = 1 << size
      if size != esp.flash_size():
          import machine
          import time
          print("Bootloader Flash size appear to have been set incorrectly, trying to fix")
          set_bl_flash_size(size)
          machine.reset()
          while 1: time.sleep(1)
  
  size = esp.flash_size()
  if size < 1024*1024:
      bdev = None
  else:
      # 20K at the flash end is reserved for SDK params storage
      bdev = FlashBdev((size - 20480) // FlashBdev.SEC_SIZE - FlashBdev.START_SEC)
  
new_contents: import esp
  
  class FlashBdev:
  
      SEC_SIZE = 4096
      RESERVED_SECS = 1
      START_SEC = esp.flash_user_start() // SEC_SIZE + RESERVED_SECS
      NUM_BLK = 0x6b - RESERVED_SECS
  
      def __init__(self, blocks=NUM_BLK):
          self.blocks = blocks
  
      def readblocks(self, n, buf):
          #print("readblocks(%s, %x(%d))" % (n, id(buf), len(buf)))
          esp.flash_read((n + self.START_SEC) * self.SEC_SIZE, buf)
  
      def writeblocks(self, n, buf):
          #print("writeblocks(%s, %x(%d))" % (n, id(buf), len(buf)))
          #assert len(buf) <= self.SEC_SIZE, len(buf)
          esp.flash_erase(n + self.START_SEC)
          esp.flash_write((n + self.START_SEC) * self.SEC_SIZE, buf)
  
      def ioctl(self, op, arg):
          #print("ioctl(%d, %r)" % (op, arg))
          if op == 4:  # BP_IOCTL_SEC_COUNT
              return self.blocks
          if op == 5:  # BP_IOCTL_SEC_SIZE
              return self.SEC_SIZE
  
  def set_bl_flash_size(real_size):
      if real_size == 256*1024:
          code = 1
      elif real_size == 512*1024:
          code = 0
      elif real_size == 1024*1024:
          code = 2
      elif real_size == 2048*1024:
          code = 3
      elif real_size == 4096*1024:
          code = 4
      else:
          code = 2
      buf = bytearray(4096)
      esp.flash_read(0, buf)
      buf[3] = (buf[3] & 0xf) | (code << 4)
      esp.flash_erase(0)
      esp.flash_write(0, buf)
  
  # If bootloader size ID doesn't correspond to real Flash size,
  # fix bootloader value and reboot.
  size = esp.flash_id() >> 16
  # Check that it looks like realistic power of 2 for flash sizes
  # commonly used with esp8266
  if 22 >= size >= 18:
      size = 1 << size
      if size != esp.flash_size():
          import machine
          import time
          print("Bootloader Flash size appear to have been set incorrectly, trying to fix")
          set_bl_flash_size(size)
          machine.reset()
          while 1: time.sleep(1)
  
  size = esp.flash_size()
  if size < 1024*1024:
      bdev = None
  else:
      # 20K at the flash end is reserved for SDK params storage
      bdev = FlashBdev((size - 20480) // FlashBdev.SEC_SIZE - FlashBdev.START_SEC)
  
subject: Add RESERVED_SECS before the filesystem.