Although I never made an EPROM programmer or 8052 basic board, but I have few queries before anyone can give you the correct answer.

1) Which device you are trying to program? EPROM or NVRAM?

2) If it is an EPROM, and the loaction 8000 contains some data (other than 255) then programmer can't write 255 to 8000H

Write operation to EPROM can make a non zero bit to Zero but not vice versa. You can program a previously programmed byte e.g 11001100 to 00001100 but not to 11111111.

May be this can help you