Counting clock

// A program counter, built from NAND gates, counting on a step clock.
//
// Press Run. RESET starts at 1: click the clock once and the counter loads 0.
// Click RESET to 0, then click the clock: every click is one tick, and every
// tick adds one. Set the clock's mode to run and it counts by itself.
//
// LOAD is tied to 0 by a constant bit, so the counter never jumps. Wire a
// switch to LOAD instead and a tick with it on loads IN, which is how a CPU
// jumps to another instruction.
//
// The PC chip is a REG8, an INC8 and two MUX8s. Unpack it to look inside.