You can try how many bytes is it eating up from the stack: fill the stack with some "interesting" constant (e.g. 5A or so), set SP to 20h, run the IAP command, and then check how far start the 5As again (or dump the content of RAM to the serial port). It should be around 18-20 bytes.

Also, you can try to set frequency (r0) to 0, rather than 11. In this way, the erase/programming algorithm would read a hardware "programming finished" flag rather than rely on simple timing - see http://www.8052.com/forum/read.phtml?id=99075 . I would like to hear whether this made any difference.

Also, isn't there a chance that the hardware is faulty? Do you have a powerful enough power source, with sufficient decoupling very close to the chip?

Jan Waclawek

PS. How do you know that it will return if the block is erased - you can't have block0 erased before running the program, can you?