Hi Mohamed,

You need to check the data sheets for the differences between the 89C51 and the 89C52. The main differences are the extra timer(Timer2) in the 89C52 that the 89C51 does not have, the size of the flash rom is only 4K bytes in the 89C51 where the 89C52 has 8K bytes and the 89C51 only has 128 bytes of internal RAM where the 89C52 has 256 bytes of internal RAM.

If you are using external memory then the RAM or ROM size may not be important but it depends on the code and how it uses the internal RAM/ROM.

Why do you need to make the code run on a 89C51 if the code works OK on the 89C52?

Regards,

Bert van den Berg