Epitaxial circuitry offers a revolution in silicon technology, with components that can be fabricated on atomic scales. We perform the first ab initio calculation of atomically thin epitaxial nanowires in silicon, investigating the fundamental electronic properties of wires two P atoms thick, similar to those produced this year by Weber et al. For the first time, we catch a glimpse of disorder-related effects in the wires--a prerequisite for understanding real fabricated systems. Interwire interactions are made negligible by including 40 ML of silicon in the vertical direction (and the equivalent horizontally). Accurate pictures of band splittings and the electronic density are presented, and for the first time the effective masses of electrons in such device components are calculated.