old_contents: import sys
  
  SEGS_MAX_SIZE = 0x9000
  
  assert len(sys.argv) == 4
  
  with open(sys.argv[3], 'wb') as fout:
  
      with open(sys.argv[1], 'rb') as f:
          data_flash = f.read()
          fout.write(data_flash)
          print('flash    ', len(data_flash))
  
      pad = b'\xff' * (SEGS_MAX_SIZE - len(data_flash))
      fout.write(pad)
      print('padding  ', len(pad))
  
      with open(sys.argv[2], 'rb') as f:
          data_rom = f.read()
          fout.write(data_rom)
          print('irom0text', len(data_rom))
  
      print('total    ', SEGS_MAX_SIZE + len(data_rom))
  
new_contents: import sys
  import struct
  
  SEGS_MAX_SIZE = 0x9000
  
  assert len(sys.argv) == 4
  
  with open(sys.argv[3], 'wb') as fout:
  
      with open(sys.argv[1], 'rb') as f:
          data_flash = f.read()
          fout.write(data_flash)
          print('flash    ', len(data_flash))
  
      with open(sys.argv[2], 'rb') as f:
          data_rom = f.read()
  
      pad = b'\xff' * (SEGS_MAX_SIZE - len(data_flash))
      assert len(pad) >= 4
      fout.write(pad[:-4])
      fout.write(struct.pack("I", SEGS_MAX_SIZE + len(data_rom)))
      print('padding  ', len(pad))
  
      fout.write(data_rom)
      print('irom0text', len(data_rom))
  
      print('total    ', SEGS_MAX_SIZE + len(data_rom))
  
subject: Store firmware size as last 4 bytes of padding area.