The Core



The Core is the name given by 343 Guilty Spark to the massive repository of knowledge found in the Control Room of every Halo, and, more practically, the computer system which stores this knowledge.

Background
The Core is, in many ways, the most important part of a Halo: it directs and controls every other aspect of the ring world. It is from here that the rings may be activated by way of the Index. The manner in which this is done is not entirely clear, but it seems that the Index must be physically brought into contact with the Core by a Reclaimer. Although it is not certain that only a Reclaimer can perform this action, it seems likely. Guilty Spark said that he was not allowed to (although not that he couldn't) and Tartarus seemed to think that it was impossible for him to do so. Once the Index has been 'reunified' with the Core, Halo's firing sequence begins.

The Core seems to be restricted-access according to Guilty Spark's "protocol". Certainly, an AI inside it has a surprising amount of control: Cortana was able to make Guilty Spark fall to the floor just by waving her hand, indicating the ability to override some of the Monitor's powers. It is possible that the Core can control the ring's gravity and, by extrapolation, its atmosphere, spin and other functions as well.

Installation 04
Installation 04's Core was, at one point, inhabited by Cortana. While inside, she obtained a tremendous quantity of knowledge, which some speculate caused damage and possibly even rampancy. However, this position enabled her to learn of the Flood and Captain Keyes's plight against them. More importantly, it enabled her to stop the Installation from firing and steal the Index when the Master Chief was tricked into starting Halo's firing process.

Installation 05
Installation 05's Core has not been visible for more than a few seconds thus far. However, it seemed to function in the same manner as its Alpha Halo counterpart.