Addressing a Card

In a PC, the CPU is the active component, controlling all other parts.
For that, it is required, that each component can be identified WITHOUT interference of other components. This is done by assigning a UNIQUE address (Think about it like a discussion in a group of people: when you ask the question: “John, how are you doing ?” and there is more than one “John” in the group, you would get most probably simultaneously an answer from multiple persons, which you then would not understand. It is the same with computers)
Addresses in a PC are numbers, but in PC’s it is custom to express such addresses in a special method: “Hexadecimal“.
To explain that, lets first see, how humans are counting, which is based on having 10 fingers on 2 hands:

1		6
2	7
3	8
4	9
5	10

As soon we have reach our maximum number of fingers, we need a second pair of hands to continue to count:

		10
		11
		12
		13

and it continues. Staring at 99/100, a third pair of hands is required.
But computers do NOT have hands with fingers, they work with electricity, which is either on or off.
Lets continue with the pictures of a hand, but lest use only 1 thumb of 1 hand, which is:

	UP: Power ON
	Down: Power OFF

Such an item with status ON/OFF is called: a BIT.
Numbers expressed in “Bits” are “Binary” number.
Lets count now again, and you see, we need a lot of additional thumbs:

Counting “Binary”	Counting Decimal
	1	1
		10	2
		11	3
			100	4
			101	5
			110	6
			111	7
				1000	8
				1001	9
				1010	10
				1011	11
				1100	12
				1101	13
				1110	14
				1111	15
					10000	16
					10001	17
					10010	18
					10011	19
					10100	20
					10101	21

These binary number are getting quickly very long and are very difficult to remember, so it is now custom to express such binary numbers as “hexadecimal” number, i.e. we combine 4 binary Bits to a single character:

Binary:		Decimal:	Hexadecimal:
0 1 10 11 100 101 110 111 1000 1001	0 1 2 3 4 5 6 7 8 9	0 1 2 3 4 5 6 7 8 9
1010 1011 1100 1101 1110 1111	10 11 12 13 14 15	A B C D E F	Up to the number 9, the counting is the same for Decimal and Hexadecimal. But the numbers 10-15 are expressed using the characters A-F
1 1 1	0000 0001 0010	16 17 18	10 11 12

Hexadecimal numbers are often marked with an ‘h’ at the end, to avoid to have them mistaken as decimal numbers ( example: 100h = 256).
Why do we need to take it up with these “hexadecimal” number ?
Because it has become custom on PC-compatible hardware components to express the addresses as Hexadecimal numbers !
These hardware addresses are also called:
– Input/Output Address
– Input/Output Port
– I/O Port
or just:
– Port
You can view the addresses already assigned in your Windows95 system:

There are some defaults: