Verified answer

Don't read FCOM before reading FCTM first.

You do have expert assistance in learning A320 systems, right?

FMGS is not comparable with MCDU, because MCDU is part of FMGS.

As you know, FMGS is a whole system which contains:

     - Two FMGCs

     - Two MCDUs

     - Two FACs

     - One FCU

and comes in two types of guidances: managed and selected.

Each FMGC have two main parts:

     - FM (navigation, management, prediction, optimisation)

     - FG (FD, AP, A/THR)

In addition each FMGC have its own set of databases, which are:

     - Navigation, performance and MagVar databases

     - AMI

Next we have MCDU. Purpose of MCDU is loading and displaying flight data. You communicate with FMGC via MCDU. I believe you know that so I'll skip further explanations.

FAC units control the rudder, rudder trim and yaw damper units. The FACs compute data for flight envelope and speed functions including flight envelope warnings.

Lastly the FCU, which is a short-term interface between you and FMGC. You use it to select any flight parameter or modify those selected in MCDU and also to select different guidance modes.

Here's the architecture for you - http://img13.imageshack.us/img13/7724/eis.png

Simple, huh?

The MCDU is the pilot interface to the FMGC. The FMGC is directly inaccessible to the pilot.

A320 Fcom