Microcomputer Architecture
Digital or Binary Logic

Computers are electronic devices that function by employing digital logic. All of the data, programs and circuitry are digital or binary in nature, meaning that they are comprised of the elements 0 and 1. This is somewhat different than radio, television, and vinyl or LP records, which operate on an analog basis. Analog electronics means that devices use a signal has amplitude or magnitude instead of a stream of 0's and 1's.

Why binary? Because the storage of information on magnetic media is accomplished by arranging bits of metalic oxide in one of two alignments, corresponding to 0 or 1. This allows for massive numbers of 0s and 1s to be stored in a very small space. This smallest unit of information (a 0 or a 1) is called a bit.

Collections of bits can be organized into larger chunks.

  • 4 bits = 1 nibble
  • 8 bits = 2 nibbles = 1 byte
A byte is also often referred to as a word. It is the customary building block of most digital systems. (There are exceptions - eg.g. Control Data Computers). In order to really begin to understand microcomputers, even in everyday life, a cursory familiarity with binary arithmetic is essential. Hence, learn to count in binary, and to perform binary/decimal conversions.

A Binary to Decimal Conversion table
Decimal Binary Decimal Binary
0 0 8 1000
1 1 9 1001
2 10 10 1010
3 11 11 1011
4 100 12 1100
5 101 13 1101
6 110 14 1110
7 111 15 1111

The use of binary numbering becomes more apparant when we look at larger numbers and when we look at the relationship between the number of bits and the range of numbers that can be formed from that many bits. This begins to reveal a great deal about microcomputers and their construction. Note that the largest number that can be made with n bits is 2n-1. So 1 byte, comprised of eight bits can make 256 different numbers (0..255). This is the basis of the ASCII character set, which is comprised of 256 charcters.

Binary Numbers and the Powers of 2:
Why the same numbers keep cropping up all the time.
# of bits 2n Related to:
1 2 a bit
2 4  
3 8  
4 16 4 bit color = a 16 color palette
5 32  
6 64  
7 128  
8 256 8 bits = 1 byte
the ASCII character set
8 bit color = a 256 color palette
Largest value addressed by a 16 bit address bus
9 512  
10 1024 1K
11 2048 2K
12 4096  
13 8192  
14 16384  
15 32768  
16 65536 64K
16 bit color = a 65636 color palette
# of bytes in 64K of memory - often called 1 page of memory
Largest value addressed by a 16 bit address bus
17 131072 128K
18 262144 256K
19 524288 512K
20 1048576 1024K, 1M
21 2097152  
22 4194304  
23 8388608  
24 16777216 24 bit color = a 16.7 Million color palette (True-color)