How to write bootloader ?

Hi,

Can you explain me how to write a boot loader firmware?,
The boot loader is design for 18f2455 chip, with USB communication.

I'm using this chip for about a year, and until now I use microchip boot loader, but now I want to write my own boot loader.

The main problems are:
1. at the start of each line in the hex file is the address to write, how should I use it ?
2. As far as I know read/Erase/write can be done only in blocks, is it true ?

Also, if you have any more info , I will be glad to hear.

Thanks
Shamir