Most people have learned to use the Decimal numbering system for counting and calculations. But micros use a different system. It's called Binary and that’s why our computers are called binary computers! All numbering systems follow the same rules. Decimal is Base 10, Binary is Base 2, and Hex(adecimal) is Base 16. The base of a system refers to how many possible numbers can be in each digit position. In decimal, a single digit number is 0 through 9. In binary a single digit number is 0 or 1. In hex a single digit number is 0 through 9, A,B,C,D,E, and F.
In decimal, as you count up from 0, when you reach 9, you add 1 more, then you have to add another digit position to the left and carry a 1 into it to get 10 (ten). Ten is a two digit decimal number.
In binary, as you count up from 0, when you reach 1, you add 1 more, then you have to add another digit position to the left and carry a 1 into it to get 10B (two decimal). While this is exactly what you do in decimal, the result looks like ten so we usually write the letter ‘B’ after to distinguish it from ten decimal. [ We should really denote ten decimal as 10D but we usually leave off the ‘D’]. So while decimal 10 (ten) looks like binary 10 (two decimal) they represent different decimal values. It is still useful to think in decimal, since that's what we're used to, but we have to get used to seeing numbers represented in binary.
In hexadecimal (hex), as you count up from 0, when you reach 9, you then go to A ten decimal), then B (eleven decimal), etc., until you reach F (15 decimal). When you reach F, you then add 1 more, then you have to add another digit position to the left and carry a 1 into it to get 10H (sixteen decimal). While this is exactly what you do in decimal, the result looks like ten or 10B, so we usually write the letter ‘H’ after to distinguish it from ten decimal and binary 10B. So while decimal 10 (ten) and binary 10B looks like hex 10H, they represent different values. It is still useful to think in decimal, since that's what we're used to, but we have to get used to seeing numbers represented in binary and hex.
Another small difference between decimal terminology and binary is that in binary a digit is called a bit. It gets even more confusing by the fact that 4 bits make a nibble. Two nibbles make a byte. Two bytes make a word. Most numbers used in a micro don't go beyond this, although there are others. Using what I've just said, if two nibbles make a byte, you could also say that a byte is eight bits.
To represent a binary number larger than 4 bits, or a nibble, a different numbering system is normally used. It is called hexadecimal, or Base 16. A shorter name for hexadecimal is simply hex, and that's what we'll use here after. In this system there are 16 possible numbers for each digit. For the first 10, 0 through 9, it looks like decimal. Unlike decimal, when you add 1 more to 9, you get A. I know that having a letter to represent a number is really confusing, but they had to call it something, and A is what they chose. So a hex A is a decimal 10 (ten). The numbers count up from A through F. Just to clarify this here is the sequence of counting in hex from 0 to where you have to add another digit position to the left... 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F (no G). This represents a decimal count of 0 through 15. At a count of F (15 decimal), if you add 1 more you get 10 (oh no! .. not another 10 that means something else!!). Sad but true.
Let's regroup here. A binary 10 (one zero) is decimal 2, a decimal 10 is ten, and a hex 10 is decimal 16. If you can get this concept, you will have conquered the most difficult part of learning micros.
I need to get past one more obstacle, the idea of significance. In a decimal number like 123, 3 is the least significant digit position (the right most digit) and 1 is the most significant digit position (the left most digit). Significance means the relative value of one digit to the next. In the number 123 (one hundred twenty three) , each number in the right hand most digit position (3) is worth 1. The value of each number in the next most significant digit position (2) is worth ten and in the most significant digit position (1) each is worth a hundred. I'm not trying to insult your intelligence here, but rather to point out the rule behind this. The rule is that no matter what base you're working in, as you start adding digits to the left, each one is worth the base times (multiplied) the digit to the right. In the decimal number 123 (base 10), the 2 digit position is worth 10 times the 3 digit position and the 1 digit position is worth 10 times the 2 digit position. Hence the familiar units, tens, hundreds, and so on. For some reason, for most people, this makes sense for decimal (base 10) but not for any other base numbering system.
The very same is true for binary. The only difference is the base. Binary is base 2. So in binary the least significant bit (remember bits?) is worth 1 ( this happens to be the same for all bases). The next most significant bit is worth 2, the next worth 4, the next worth 8, and so on. Each is 2 times (base 2) the previous one. So in an 8 bit binary number (or byte, remember bytes?), starting from the right and moving left, the values for each of the 8 bit positions are 1, 2, 4, 8, 16, 32, 64 , and 128. If you've got this, you have passed a major milestone, you should go celebrate your passage. If you haven't, I would re-read the above until you do, and then go celebrate!! ( By the way, if you didn't get this the first time through, join the crowd. I didn't either!!)
In hex (base 16) the same rule applies. In a 4 digit hex number, starting at the right and working left, the first digit is worth 1 ( hey that's just like decimal and binary!!), the next is worth 16 (base times the previous digit), the next is worth 256 (16 X 16), and the most significant is worth 4096 (256 X 16). One last note, hex is just binary described another way. A hex digit is a binary nibble ( remember nibbles?). Both are 4 bit binary values.
Trying to wrap all this confusion up in review, 4 bits is a nibble or a hex digit. Two hex digits is a byte or 8 bit binary. A 4 digit hex number is a word, or 16 bit binary or 4 nibbles, or 2 bytes. You may have to review the previous paragraphs a few times (I did!!) to get all the relationships down pat, but it is crucial that you are comfortable with all I've said, so that what follows will make more sense.
Let's take a few example numbers and find the decimal equivalent of each. Let's start with the binary number 1011, a nibble. Starting at the right and moving left, the first digit is worth one, because there is a 1 there. The next is worth two, because there is a 1 in it. The next would be worth 4, but since there is a 0, it's worth zero. The last or most significant is worth eight, since there is a 1 in it. Add all these up and you get eleven. So a binary 1011 is decimal 11. Also, since this could be a hex digit, the hex value would be B.
Let's take a longer binary number 10100101. Starting at the right moving left, the first is worth 1, the next is worth 0, the next is worth 4, the next is 0, the next is 0, the next is worth 32, the next is 0, and the last is 128. Adding all these up you get decimal 165. Dividing this number up into two hex digits, you get A5. So binary 10100101, decimal 165, and hex A5 are all the same value. Using hex, the rightmost digit is worth 5, and the most significant digit is worth 160 (10 X 16), resulting in decimal 165. If you understand this, you’re ready to move on, leaving the different systems behind. If you don't, keep reviewing and studying until you do. If you don't understand and still continue, you will be confused by what follows. I promise that once you get this, the rest is easier.
This ends the second lesson. I hope this wasn't too daunting or difficult, but there was a lot to get through. The rest of the course should be a little easier. Learning a new numbering system is like learning a new language. It's a little cumbersome at first, but it gets easier.