Logic functions are often faster and less expensive than the equivalent microprogram memory. The main advantage of the microprogram control unit is the simplicity of its structure. A microprogram is much like a player-piano roll. A "specialized computer" is usually a conventional computer with special-purpose control logic or microprogram. A microsequencer, based around the Am2910, directed the control flow through the microprogram. A diode matrix is used as the control store or microprogram in many early computers. The microprogram could be rewritten and reinstalled, which was very useful for a research computer. In a typical implementation a horizontal microprogram word comprises fairly tightly defined groups of bits.