Thus, the total memory demand is demonstrated by a two-dimensional matrix 40 (i.e., a memory map) as shown in FIG. 4, where rows 41, 42, 43, 44 and 45 represent phases and each column 46 represents a virtual memory block 39 corresponding to a source text module 12, or a data structure such as a table or list generated by one of the other parts of the environment (such as cleanlines tables, token t