hi,
so even if you had the source code it would only be educational and not otherwise useful since you wouldn't be able to change it
Yes and no. Although in AT89C51RD2 it is not possible to replace the bootloader code physically however user is able to create own bootloader, place it into program flash area and then redefine the boot-startup address to own loader.
The question what is better - have bootloader in Flash memory (T89C51RD2) or ROM (AT89C51RD2), may be answered such way:
- pure T89C51RD2 provides 1kb bootloader placed inside of 64kb E2PROM Flash code memory, so out of factory chip has 63kb of user program memory only. User may erase firmware bootloader and utilize all the 64kb space for own purposes; indeed he may place own bootloader inside 64kb space as well. Bootloader sector may be protected from accidental API software erasure by a lock bit. The bootloader must be erased and should be reprogrammed in parallel mode if chip erase operation was performed due security/lock bits cleaning.
- pure AT89C51RD2/ED2 provides 2kb bootloader placed in separate BOOT ROM. User may create own bootloader and put its vector instead factory one. Nevertheless, there is no special lock bit which protects custom bootloader area from accidental erasure in case when an user application calls BOOT ROM API due malfunction.
Factory bootloader may be neither erased nor reprogrammed with ISP/API/parallel programming because it is placed in ROM.

Maybe am I wrong? Please correct...

Regards,
Oleg