optiLoader.ino

Copyright (c) 2011 by Bill Westfield ("WestfW")
Distributed under the terms of the "MIT OSSW License."

Optiloader is a sketch designed for bulk upgrading of Arduino board
bootloaders using another Arduino as a device programmer.  It stores
multiple copies of the optiboot bootloader in program flash memory.  When
run (ie by hitting reset), it probes the target device, figures out the type
of CPU (ATmega8, ATmega168, ATmega168P, ATmega168PB, ATmega328, ATmega328P,
ATmega328PB) and initiates upload of the bootloader and appropriate fuse
programming.  Since optiLoader runs entirely with the Arduino and there is
no communications with a host PC required, this can procede very rapidly,
and is not subject to interference of (for example) avrdude with auto-reset.

The internal copies of the bootloader are prepared manually from the .hex
files compiled in the bootloader directories.  (There's an easy editor macro
process, but it is manual.)

While communication with a host PC is not required, the sketch does send
status information to the serial port at 19200bps.  Here's a sample:

OptiLoader Bootstrap programmer.
2011 by Bill Westfield (WestfW)

Target power on! ...
Starting Program Mode [OK]

Reading signature:950F
Searching for image...
  Found "optiboot_atmega328.hex" for atmega328P
  Start address at 7E00
  Total bytes read: 502

Setting fuses for programming
  Lock: 3F FFE000  Low: FF FFA000  High: DE FFA800  Ext: 5 FFA400

Programming bootloader: 512 bytes at 0x3F00
  Commit Page: 3F00:3F00
  Commit Page: 3F40:3F40
  Commit Page: 3F80:3F80
  Commit Page: 3FC0:3FC0

Restoring normal fuses
  Lock: 2F FFE000

Target power OFF!

Type 'G' or hit RESET for next chip

~~~
Apparently, this was a pretty neat idea, because several other people have taken it and (substantially!) enhanced, or re-written it.  (and done much more complete documentation as well.)

https://learn.adafruit.com/standalone-avr-chip-programmer?view=all
http://www.gammon.com.au/bootloader