Hello,

thanks to Jan et Joe for their explanation.

I disassemble the HEX file for a better understanding (shame on me). I do not like "black boxes":-(.

After having uploaded the HEX (firmware update) in the microcontroller, when booting, the program jump to 0x2000, then, when autobaud is done and a 'U' character is received, the first 8K of block1 is erased (previous ISP firmware), the "ljmp 0x2000" instruction is replaced by NOPs (fortunaly, flash bits can be switched from 1 to 0 without erasing the whole sector) and copy in block1 the 1st 8Ko of code contained in block0, plus other little things.
So, at the next reboot, it do not jump to 0x2000 but directly to the firmware in block1.

Am I right, or really wrong ?

Little curiosity, which (non documented) register corresponds to address 21H ?
And what means the bit0 of this register ?

Regards,

Phil.