GDB consists of three major subsystems: user interface, symbol handling (the symbol side), and target system handling (the target side).
The user interface consists of several actual interfaces, plus supporting code.
The symbol side consists of object file readers, debugging info interpreters, symbol table management, source language expression parsing, type and value printing.
The target side consists of execution control, stack frame analysis, and physical target manipulation.
The target side/symbol side division is not formal, and there are a number of exceptions. For instance, core file support involves symbolic elements (the basic core file reader is in BFD) and target elements (it supplies the contents of memory and the values of registers). Instead, this division is useful for understanding how the minor subsystems should fit together.