TAMS / Java / Hades / **applets** (print version): contents | previous | next#### Binary Adder/Subtractor (4 bit)

**Description**

A four-bit **adder/subtractor** demonstration.
While it is perfectly possible to design a custom circuit
for the subtraction operation,
it is much more common to re-use an existing adder and
to replace a subtraction by a two-complement's addition.

The applet shows how this is done.
Click the input switches or type the 's' bindkey to control the
*Sub/nAdd* switch, and '1' .. '8' to control the A and B
inputs.

When the *Sub/nAdd* input is low (0), the XOR-gates act as
non-inverting buffers, and the carry-input to the adder is 0.
Therefore, the adder calculates a four-bit sum plus carry-out:

(Cout,S3,S2,S1,S0) = (A3,A2,A1,A0) + (B3,B2,B1,B0)

If the *Sub/nAdd* input is high (1), the XOR-gates act as
inverting buffers, and the carry-input to the adder is 1.

(Cout,S3,S2,S1,S0) = (A3,A2,A1,A0) - (B3,B2,B1,B0)

Run the applet | Run the editor (via Webstart)

Impressum | 24.11.06

http://tams.informatik.uni-hamburg.de/applets/hades/webdemos/20-arithmetic/40-addsub/add-sub_print.html