Whereabouts in memory are you trying to write your program to (i.e. what address range)? The program expects RAM in the lower part of memory (<32k) and flash in the upper part of memory. If you have RAM where it is expecting flash or vice versa it will not work since the two write differently.

Try editing memory using the "E" command. Edit both as RAM and as program memory and in each case make sure your updates are there when you exit and return to the screen. If they are not it sounds like a hardware problem (such as circuit not being wired correctly). The memory has to be connected such that it appears in both RAM space and program space simultaneously. A "generic" 8051 circuit likely will not do this. There is a schematic showing an example circuit on the pjrc.com website.

Frank Kienast