Logical or boolean operations

Introduction

The name “boolean” comes from Geroge Boole, an English mathematician, philosopher and logician, who was the first to introduce the algebraic logical system in the 19^th century.

To start with, let’s introduce the so called “truth tables” and with the variables “True” or “False”, often used in mathematics, in equations and inequalities, and for valid expression ranges. We are not going to go deeper into this issues we study on mathmatics and let’s go directly to their applicability in computer science. However, let’s just take a look in the most common operations – “AND”, “OR”, “eXclusive-OR” and “NOT” – using the letters T and F to represent True and False, respectively.

This means we can express these tables as:

• True AND True = True
• True AND False = False
• False AND True = False
• False AND False = False

• True OR True = True
• True OR False = True
• False OR True = True
• False OR False = False

• True “eXclusive OR” (XOR) True = False
• True XOR False = True
• False XOR True = True
• False XOR False = False

• NOT True = False
• NOT False = True

Let’s now take a computer science approach and represent False as 0 and True as 1. The truth tables will look like as follows:

On Maths, we can also use some of these operations on the set theory:

• AND – Intersection ( symbol ˄ )
• OR – Union ( symbol ˅ )
• NOT – Negation ( symbol ¬ )

We can also find an interesting equivalence in binary for the AND and XOR operations and the arithmetic operations of Multiplication and Addition:

AND and Multiplication:

XOR and Addition:

Operations with more than one bit

These logical operations can also be performed with a greater number of bits. In this case, the operation is done bit by bit. In the examples below, column by column:

Practical example

To end with, let’s see some practical examples with these operators.

Let’s imagine we have a controller register of 8 bits which controls the “switch on” or “switch off” of some devices. Let’s also assume that the 3^rd bit (remember that we can number the bits from right to left from 0 to 7) controls the PC speaker and the 2^nd bit controls a second screen.

And let’s also assume we have this register with the following binary values:

Bit	7	6	5	4	3	2	1	0
Val.	1	0	1	1	0	1	0	1

We have the 3^rd bit (speaker – in green) set to 0, which means that the speaker is switched off, and the 2^nd bit (2^nd screen – in yellow) set to 1, which means that the second screen is switched on.

We want to perform an operation to switch on both devices, independently if they are already switched on or not. To do so, we use the OR operator where 0 is the identity element and 1 is the absorbing element:

	1	0	1	1	0	1	0	1
OR	0	0	0	0	1	1	0	0
	1	0	1	1	1	1	0	1

Notice that in the first row we have the register bits and in the second row the bits used to perform the operation. Also notice that all the bits on this second row are set to 0 with the exception of the 2^nd and 3^rd bits which are set to 1. By doing so, we assure that with the remaining bits set to 0 we won’t change the state of the other bits in the register! We only want to activate the 2^nd and 3^rd bits and that’s the reason why the same bits on the second row are set to 1. In fact, by observing the third row, which represents the new state of the register, only the 2^nd and 3^rd bits were forced to be set to 1 and there are no changes on the remaining bits.

Let’s go back to the initial state of the register. Now, we want to switch off those devices, independently if they are already switched off or not. In this case we will use the AND operator where 1 is the identity element and 0 is the absorbing element:

	1	0	1	1	0	1	1	0
AND	1	1	1	1	0	0	1	1
	1	0	1	1	0	0	1	0

Now, notice that in the second row, all the bits used for the operation are set to 1 with the exception of the 2^nd and 3^rd bits, that we want to switch off, which are set to 0. In this case, we assure that we are only changing the 2^nd and 3^rd bits to 0! The remaining bits won’t be changed!

Let’s finish with a last example and let’s consider again the initial values of the register. Now we want to “invert” the state of the devices from switched on to switched off and vice-versa! To do so, we will use the XOR operator where we can say that the 0 is the identity element and 1 is the “inverter” element:

	1	1	0	1	0	1	1	0
XOR	0	0	0	0	1	1	0	0
	1	1	0	1	1	0	1	0

Notice that we have some similarities to the OR operator where the bits used in the operation that are set to 0 won’t change the result of the same bits in the register. However, having the 2^nd and 3^rd bits set to 1 we are “inverting” the corresponding bits in the register. So, we had the speaker switched off and now it is switched on and the second screen, which was switched on, is now switched off.

The case of the second screen is something more specific on display adapters and it was just used here as a mere example. However, there is a controller on the main board that use one of its bits to switch on or off the PC speaker (PIT – Programmable Interval Timer). In the past, where sound cards were just a mirage, some “melodies” were reproduced by using another register to set the sound frequency and the bit 1 of the PIT was used to switch on or off the PC speaker. (In fact, it was an input/output port of that controller, but we will speak about this later).

In all these examples, the binary number used in the operations is commonly called a bit mask. We will talk about this with more details in a future article about logical circuits…

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