This function checks for identifier that should be already declared.

Storing identifiers into symbol table is simple because of the way how Turbo Pascal organizes symbol tables.

This function case insensitive compares two identifiers and returns false if they are not equal. Otherwise returns true and Id2 points to byte after its end. This function must be as fast as possible! It is called whenever Turbo Pascal needs to compare two identifiers.

Turbo Pascal calculates a checksum of public identifiers for every unit to quickly check if something has been changed.