The PC register is different from the others in that it has two additional input control wires. One is labeled PC-INCR. When both PC-INCR and PC-LD are 1, the binary number in the PC register goes up by one. Overflow is ignored and the numbers wrap around. Put another way, if PC contains 1111111111111111 (16 1's), and PC-INCR=1 and PC-LD=1, the new value will be 0000000000000000 (16 0's). There is also a PC-CLR input which puts 0 in the PC register, when PC-LD is high.