How to write ALU control lines?

Question

I know logical ADD in binary is 0010, and logical AND in binary is 0000. How do I go from this to a full operation?

The diagram has an error with the a-> green arrow extending too far, going right past the inverter; it should look like the b-> green arrow instead, which stops at the inverter and becomes black post inverter. — Erik Eidt
– Erik Eidt, Commented Jun 16, 2022 at 16:30

Erik Eidt · Accepted Answer · 2022-06-20 23:29:43Z

2

There's two control lines: Ainvert, and Bnegate, which can be used to invert values before combining them.

Use them as needed!

The Bnegate control line does these two things:

inverts the bits of the b input to ~b, which is then fed to the other circuits as the b-side/lower input.
and also feeds a 1 into the carry input of the adder, +.

The combined effect of Bnegate with the + Operation, will accomplish a + ~b + 1, which is the same as a-b and also a + -b, because in two's complement, -x = ~x+1.

Though we can, we don't have to use the + with Bnegate, as others operations are still available.

Given this logic diagram, what are some things that cannot be done?

a + ~b (though ~a + b can be done)
a | -b
a & -b
-a + b

edited Jun 20, 2022 at 23:29

answered Jun 16, 2022 at 16:17

Erik Eidt

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