I done this in T89c51rd2 and in AT89c51rd2 datasheet seems like the same.

You must make an image in the ram
Put one dptr pointing at the start of ram datas
Put the other dptr pointing at the start of flash
If i'm not wrong put in acc the number of bytes to program and in R0 one code.
And.. Call FFF0 that's all!!!

I think in AT device you must change a bit flag too but i'm not sure.

Lets go! seems more difficult that really it is!
I done in T89c51 and works fine