74181

The 74283, a well known adder:

The less known 8260 from Signetics.
Some kind of ALU chip, with a minimalistic functionality:
addition, XNOR, AND.

The 8260 had 2 control_lines, called C_INH and E_INH.
C_INH was used to select arithmetic or logic operation mode.
C_INH=0 enabled the carry chain, and the 8260 worked as an adder.
C_INH=1 disabled the carry chain by forcing all it's high_active
carry outputs to 1, what basically turned the XOR gate into an inverter.

When logic mode was selected, E_INH was used to force the
propagate_signals active, what made it possible to switch
from XNOR to AND.

The 74181 ALU:
A marvel of logic design.

It also has a control line, called M, to select
arithmetic or logic operation mode.

In logic mode, the carry chain is disabled, and the
high_active carry_outputs are forced to 1.
(In all examples mentioned here: high_active A,B,F and
low_active carry at the 74181 I/O_pins)
Like with the 8260, this turns the XOR at the output into an inverter.

Now to discuss how it works in detail.

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