cribed in Section 4.3.5.2. With direct progressive control a subscriber dialed a digit, and the first relay bank stepped to the dialed digit; the subscriber dialed a second digit, and the second stepping relay bank actuated, stepping to that digit level, and SO on, through the entire dialed number. With common control, on the other hand, the dialed number is first stored in a register. 13 These digits are then analyzed and acted upon by a marker, which is a hard-wired processor. Once the call setup is complete, the register and marker are free to handle other call setups. The marker was specifically developed for the crossbar switch. Such marker systems are most applicable to specialized crossbar switching matrices of crossbar switches. Stored program control (SPC) is a direct descendent of the crossbar common control system. SPC is described below. 4.3.7 Stored Program Control 4.3.7.1 Introduction. Stored program control (SPC) is a broad term designating switches where common control is carried out entirely by computer. In some exchanges, this involves a large, powerful computer. In others, two or more minicomputers may carry out the SPC function. Still with other switches, the basic switch functions are controlled by distributed microprocessors. Software may be hard-wired on one hand or programmable on the other. There is a natural marriage between a binary digital computer and the switch control functions. In most cases these also work in the binary digital domain. The crossbar markers and registers are typical examples. The conventional crossbar marker requires about half a second to service a call. Up to 40 expensive markers are required on a large exchange. Strapping points on the marker are available to laboriously reconfigure the exchange for subscriber change, new subscribers, changes in traffic patterns, reconfiguration of existing trunks or their interface, and SO on. Replacing register markers with programmable logic-a computer, if you vill-permits one device to carry out the work of 40. A simple inpu