ALU design

Imagine, that you would try to build an
ALU with the basic functionality
AND, OR, XOR, ADD.

You may need some additional circuitry
outside the ALU:

to invert B for calculating A-B,
to set B to zero for passing through A
(or for incrementing A by forcing the ALU carry input to 1),
to set B to -1 (all Bits high) for decrementing A,
and maybe some circuitry for shifting A left and right
(some 74245, for instance).

All those things already went explained in my
previous articles about ALU design, so we now
skip this part.

Here a schematic for one Bitslice of the ALU:

The blue boxes connect to the control signals.

The carry chain is built with a majority gate
based on a CMOS AND.
To change the functionality from ADD to XOR,
the carry chain is simply turned off, reducing
the three_input XOR gate (built from two 2_input
XOR gates) of the full adder to a simple 2_input
XOR gate fed from the ALU inputs A and B.

Our ALU has a three state output, so the
output of said XOR is buffered with a 74HC125
(...or 74HC245). For the AND/OR functionality, there is another
majority gate which can be configurated to work
as an AND/OR gate.
To simplify things, we just took another buffer.

Warning:
be aware of the current which the inputs of your
majority gates will sink/source.
The red marked resistor would be going to be a
problem if you go for speed and low resistor
values.
It may be a good idea to place a buffer in
front of the resistor for every Bitslice of
the ALU.

Now for another idea:

A 74HC/AC245 will give a very compact 8 Bit
carry chain.

The rest of the functionality is hidden
inside the 74251 8:1 multiplexer...
which, by the way, has a LOW active output enable.

The select inputs A,B,C of the multiplexer
are tied to the ALU inputs: A,B and carry input.

The functionality of the multiplexer is set by the
data inputs I7..0, which now work as control
lines (MSB first):


AND: 10001000
OR : 11101110
XOR: 01100110
ADD: 10010110

Downside is, that generating the control signals
may require a few additional logic gates.

By the way: when setting the control word to
10101010, the ALU will pass through the input A,
11001100 will pass through the ALU input B.

For more details, please consult/dissect my
previous articles about using multiplexers
for ALU design.

Now for some more boring theory...

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