When the --debug option is specified the compiler generates extra symbol information some of which are put into the assembler source and some are put into the .adb file. Then the linker creates the .cdb file from the individual .adb files with the address information for the symbols. The debugger reads the symbolic information generated by the compiler & the address information generated by the linker. It uses the SIMULATOR (Daniel's S51) to execute the program, the program execution is controlled by the debugger. When a command is issued for the debugger, it translates it into appropriate commands for the simulator. (Currently SDCDM only connects to the simulator but newcdb at http://ec2drv.sf.net/ is an effort to connect directly to the hardware.)