Good Question,.... I fear,.....

I guess I don't know the meanig of the word "flash".

I thought I could program any byte to any value any time. But the C test program I just made, to play around with the PGM_MTP 02 call indicates I can only program a byte once, and only if it's FF from the start. (Or acording to your question)

And the pages in P89C51RD2 is too large for mee to
copy, erase, and re-flash I guess. Unless I can use XRAM for it. .... Hmmmmmmmmmm....

I'm aiming at doing this because I want to use the Keil ISD51 debugger with flash breakpoints (running in real time between breakpoints).

Thanks a lot Erik !