It begins by acknowledging the problem. Head-mountable displays (HMDs) are developed to be comfortably worn for extended periods of time. The ever increasing desire to improve performance, functionality, and battery life of HMDs results in a device that includes several electrical components, each of which add weight to the system. There is a constant balance between maximizing user comfort while maintaining or excelling the performance and functionality of the device. Unbalanced weight distribution experienced by a user while wearing the headmountable display, particularly weight distributed in front of the user’s face, can negatively impact the user’s experience. Thus, there is a need for a light-weight HMD assembly that does not sacrifice performance or functionality. Apple also admits the heat issue, stating that there needs to be a way to more efficiently dissipate heat. The company already keeps the weight down somewhat by using a tethered battery, and the patent document says it could also incorporate the processing power into this puck, which Apple refers to as a cartridge. A cartridge can include a housing, a processor positioned within the housing to provide video output to a display unit of a head-mounted display (HMD), and an attachment interface to removably attach the cartridge to the display unit. One option Apple describes is to have the processing unit in a removable unit that could be inserted into the headset or into the tethered casing. A head-mountable assembly can include a processor and a display unit defining a slot to receive the processor. In a first mode, the processor can be positioned within the slot to provide image data to the display unit. In a second mode, the processor can be removed from the slot and provide image data to the display unit.