This circuit consists of two registers A & B and a half adder. The half adder will add together the values loaded in to the registers. The values are output on the 7 segment displays.
At the top left is register A. The load line is on and the pulse input provides a 0 or 1 value. This value is stored in the D flip flop at the next rising pulse of the clock. The yellow LED represents a 1 or 0 value being loaded.
At the bottom left is register B. Set up in the same way as A. The blue LED represents a 1 or 0 value being loaded.
The Q output of the D flip flops will load a 0 or 1 to the inputs of the half adder.
Input A from Register A
Input B from Register B
The half adder will add together the values of register A and B
0 + 0
0 + 1
1 + 0
1 + 1
On the half adder the green LED is the value of the SUM output and the red LED the value of the CARRY output.
The green and red LED's represent the values 0, 1 and 2 in binary.
Yellow 7-seg = A Register Value
Blue 7-seg = B Register Value
Red 7-seg = A Reg plus B Reg
7-Segment LED's output the values in decimal.
For more info check out Ben Eater on YouTube - 8 bit computer build - https://www.youtube.com/user/eaterbc
