The prior structure did not enforce separation of concerns and so had many inter-program dependencies which led to bugs unless exhaustive code analysis was done. The new structure was more modular and so resilient because it was easier to change without impact. The first domains were often built with the name of the prior program but without the trailing "P". For example, Program Control Domain (DFHPC) or Transient Data Domain (DFHTD). The kernel operated as a switcher for inter-domain requests initially this proved expensive for frequently called domains (such as Trace) but by utilizing PL/AS macros these calls were in-lined without compromising on the separate domain design.