M4 field disk
From Halopedia, the Halo wiki
M4 Field Disks (also known as recorder chips or helmet chips) are used in Helmet Recorders to store visual and audio files created by the recorder's video and sound sensors. They can be plugged into nearly any information terminal or display unit that can play optical and audio data.
During the Battle of Sigma Octanus IV, the Spartan-IIs employed these, and the Master Chief's Recorder chip contained the record of his mission to Côte d'Azur, as well as intelligence on Covenant Engineers and Hunters. This was much appreciated by HIGHCOM.The ONI Brass didn't appreciate his mission orders.
Private Jenkins' helmet was equipped with one, and the Master Chief used a playback system in his helmet to view the squad's progress as he tried to find Captain Keyes. It was using this chip's recording that he first saw the Flood.
