Data, idata confusion?

Hi,

I have 2 pieces of software that both independently work correctly. When combined, however, they don't seem to get on. I think my problem is concerned with memory space corruption. ie. one module's data overwriting the other's.

Here are some excerpts from the appropriate .LST files:

MODULE INFORMATION:   STATIC OVERLAYABLE
   CODE SIZE        =   2404    ----
   CONSTANT SIZE    =   ----    ----
   XDATA SIZE       =   1024    ----
   PDATA SIZE       =   ----    ----
   DATA SIZE        =      2     114
   IDATA SIZE       =     37    ----
   BIT SIZE         =      1       2
END OF MODULE INFORMATION.

MODULE INFORMATION:   STATIC OVERLAYABLE
   CODE SIZE        =    894    ----
   CONSTANT SIZE    =   ----    ----
   XDATA SIZE       =   ----    ----
   PDATA SIZE       =   ----    ----
   DATA SIZE        =   ----      23
   IDATA SIZE       =   ----      44
   BIT SIZE         =   ----    ----
END OF MODULE INFORMATION.

MODULE INFORMATION:   STATIC OVERLAYABLE
   CODE SIZE        =    326    ----
   CONSTANT SIZE    =    328    ----
   XDATA SIZE       =   ----    ----
   PDATA SIZE       =   ----    ----
   DATA SIZE        =   ----      18
   IDATA SIZE       =   ----      36
   BIT SIZE         =   ----    ----
END OF MODULE INFORMATION.

Can anybody please explain for me the difference between static and overlayable memory allocations? I'd like to know from the above how I can tell if I have filled either the DATA or IDATA memory spaces. How would I be certain that I am experiencing memory space corruption? My processor is an AT89C51AC2.

Regards,
Murray R. Van Luyn.