The 'data segment' is an executable segment that contains all of the data used by the rest of the game's code. It is the last and largest segment in the executable and stores all of the text and variables the game uses. The start of the data segment in various games is as follows:
Keen 1: $13050 Keen 2: $17780 Keen 3: $19820 Keen 4: $2EE70 Keen 5: $30340 Keen 6: $30D30 Keen D: $23A70
Data in the data segment is referenced by its location (or offset) from the start of the segment, as an example: Text located at $14051 in the Keen 1 data segment would have a reference (or 'pointer' to it) of $1001W ($14051 - $13050 = $1001). This includes game variables such as the number of lives Keen has, which are two-byte variables in the segment.