Atmel AT89C51 does not have upper data memory for it has only 128 bytes of RAM, but AT89C52 does.

I don't know about the compiler. I'm using Keil development tools for 51 and there I have to declare upper data memory as indirect (for 8051 does not support direct memory addressing in this RAM area). I declare this memory as IDATA. Maybe this could be a problem.

Hope I've helped you a bit. Franc