The Mainframe is a processing unit within the central control tower on Talvania. It was presumably a creation of the machine race that formerly inhabited the planet. The Mainframe is the penultimate boss in Metroid Prime: Federation Force, and is fought during M21: The Core.
The Mainframe is suspended in the center of a large shaft, and consists of four constantly rotating blocks, similar to a Rubik's cube. A larger machine that seems to control it is situated above and makes erratic movements. Every few seconds, the cubes on the Mainframe will open on all four sides and reveal small laser cannons inside of it, fire them, close up, spin and repeat. They can be destroyed, and all of them must be to reveal the Mainframe's weak point. An easy tactic to destroy four cannons in one go is to fire a Shock Shot at them, which will reverberate and destroy the three closest to the cannon that was shot.
When most of the cannons have all been destroyed, the Mainframe will begin to spin and fire lasers erratically. As the final cannons are destroyed, the cubes will disappear and the Mainframe's core will be exposed. Shooting this is the only way to damage it. During this time, it will fire rotating laser beams at the core's base that deal severe damage to any Federation Force soldiers that touch it.
After the core's Energy Meter has been depleted, the Mainframe will regenerate the cubes. The battle is fought similarly to the first phase, but this time some of the cubes will be closed on one side, or they will shut soon after they open. Again, the cannons must be destroyed to advance the battle.
Once the cannons have been destroyed and the core has been damaged, the Mainframe will restore its cubes once again, but this time larger and sturdier. When its cubes have been destroyed, the core will now be defended with a laser above the previous ones, which stop the Federation Force from jumping to avoid the lower lasers. After the core has been damaged a final time, the Mainframe explodes, and leaves a crater in its wake that the Force can descend. Through here, the final Transfer Device is found. The Force destroys it and thus reveals the location of the Doomseye.
The mission's bonus objective is to take no damage from the Mainframe's lasers. The Slow Beam causes the lasers to freeze in place and a deployed Shield Generator will allow the lasers to pass through the Federation Force without any damage, and they usually last long enough for the Force to damage the Mainframe enough for it to regenerate its protective cubes.