Please elaborate on the nature of your confusion.

The "data" space is the region of 8051 internal RAM that is addressable using the direct address method. Locations 0x00-0x7F are directly addressable, with direct addressing of 0x80-0xFF accessing the SFR's. Nominally, the C51 compiler wants to use addresses 0x00-0x07 (R0-R7 in register bank 0) for it's working registers. This leaves 120 bytes for all of the data objects in your program, plus the call/return stack. By allocating a data object that is (presumably) 100 bytes in size, this only leaves 20 bytes for all your other data objects and the stack. Is this enough? Possibly not, and that's why your link is failing -- you have more data allocated than you have room for in the "data" section. You'll have to either decrease the size of the offending data object(s) or move them to another memory space (e.g., "idata", "xdata", or "pdata").

To go further than that explanation, we'll need more specific questions.

Regards,

--Dan Henry