The 4008 is a 4-bit full adder that takes two 4-bit binary numbers, A[3:0] and B[3:0] and carry-in signal, Cin and adds them to produce a 5-bit result, made up of a sum, S[3:0] and a carry-out.
Thr 4008 uses full carry lookahead to generate the carry-out bit. This means that when several 4008s are used in ripple configuration, the inter-chip carries are generated that they would be if one had to wait for the previous chips's summation to complete. While not as fast as full lookahead across all bits, this is significantly faster than a plain ripple counter would be.
Truth table of a single adder (1 of 4)
Cin
A
B
Cout
S
0
0
0
0
0
0
0
1
0
1
0
1
0
0
1
0
1
1
1
0
1
0
0
0
1
1
0
1
1
0
1
1
0
1
0
1
1
1
1
1
A gate-level schematic of the Philips HEF4008 device.
