There is a need in the world for boot loaders. Not all microcontrollers work like the two part number series that you quoted. There are 100's of types to choose from and many of those may require a boot-loader function to allow in-application re-programming. The choice of part may often be made on many other factors than available serial port ISP.

So it is a legitimate question to ask about making a boot loader function!!

I made one such boot loader for the Cygnal 'F12x series parts that may be easily adapted to other Flash based parts. Here is a link to the code for that loader. There is companion VB6 code that goes with the loader for the host end. Please email me if you want to see that,

Michael Karas