Unit 2
Logic gates.
AND, OR, NOT and XOR, truth tables, and how a few gates add two numbers, up to a processor's arithmetic unit.
Before this unit
The unit, lesson by lesson
Lessons
4 lessons · 21 practice questions · about 2 hours
AND, OR and NOT
The three basic gates: AND needs both inputs, OR settles for one, NOT flips; and how to choose a gate for what a circuit has to do.
Truth tables
A table that describes a gate completely: a row for every combination of inputs, the columns of AND, OR and XOR, gates built from gates, and how many different gates there are at all.
A half adder
Gates that compute: adding two bits, a carry that is AND and a sum that is XOR, how the NOT blocks the sum at 1 + 1, and why it is only half an adder.
A full adder and the processor
Three bits in a column: the full adder, eight of them in a row in a processor's arithmetic unit, binary addition with carries, and what happens when the sum does not fit in a byte.