The advantages of 8 bit microprocessor

1. The 8 bit word length is twice 4 bits.

2. The 8 bit word length allows two BCD numbers for each CPU data word

3. The 8 bit word length can hold all the data needed for one character in

American Standard Code for Information Interchange (ASC II), ASCII characters are used widely in data processing to represent numbers, letters, and many special symbols.

Each time the microprocessor’s word length doubles, the processor becomes more powerful. Greater word lengths have required improved LSI technology. For example, the LSI used to develop some of the new 64 bit microprocessors uses a similar sized chip, but it comes over 23 million transistors.

Another common measure of microprocessor power is the number of memory bytes that the microprocessor can address. For example, a 4 bit microprocessor stores 4 bit word in memory. The length of the data word is the same as the length of the data word used by the microprocessor. Each word in memory is assigned a location number or address.

Binary Address		Memory contents (4 bits long)
1111		Data word 15
1110		Data word 14
1101		Data word 13
1100		Data word 12
1011		Data word 11
1010		Data word 10
1001		Data word 9
1000		Data word 8
0111		Data word 7
0110		Data word 6
0101		Data word 5
0100		Data word 4
0011		Data word 3
0010		Data word 2
0001		Data word 1
0000		Data word 0

Figure: A 16bit word memory addressed by a 4bit

Figure shows the memory-addressing power of single 4 bit word. 4 bits can address 16 words in memory. We number these 16 words from 0 to 15. A single 8 bit word has an address range of 256 memory words. A 16 bit word has an address range of 65,536 memory words. Most microprocessors can use more than a single word to address memory. Therefore the memory address range is not limited by the length of the microprocessor’s data word.

A shorthand notation is used in specifying the number of bytes. The symbol “K” is used to say “times 1000”. The symbol “M” means “times 1 million”. The symbol “G” means “times 1 billion”.

Data word length	4bit	8bit	16bit	32bit
Memory address range	4096k
	8192k
		65,536(64k)
			32,768(32k)
			65,536(64k)
			1,048,576(1M)
			2,097,152(2M)
			4,194,304(4M)
				4,294,967,296(4G)
				34,359,738,367(32G)

A third common measure of microprocessor power is the speed with which microprocessor executes an instruction. Speed is determined by the time it takes the microprocessor to complete the fetch / execute cycle for one program step. Some microprocessors are 20 to 100 times faster than others. Each one has oscillator circuit is called the microprocessor’s clock. Slow microprocessors may use a clock that at a few hundred kilohertz (KHz). It takes such a microprocessor 10 to 20 microseconds (ms) to execute one instruction.