A set of logical extents, each having compressed logical tracks of data, is mapped to a head physical extent and, if the head physical extent is determined to have been filled, to at least one overflow extent having spatial proximity to the head physical extent. The central body is defined by a base wall, and by an annular wall from whose outer face the blades extend. The readhead has at least one sensor which produces an output which is dependent on the angular alignment of the readhead relative to the scale. In one embodiment, the second controlled atmosphere is a vacuum atmosphere, whereas the first controlled atmosphere includes at least an inert gas. The support body is adapted to receive assembly modules for both thermal ink printers and ribbon ink printers such that the modular printer can be easily converted from one to the other. The MCU is configured for reading the digital output of the ADC module.