What is a microprocessor?

• What is a microcomputer?

The words “Microprocessor” and “Microcomputer” are used to mean the same thing, but in fact these words have different meanings. The microprocessor is an IC (data processing and control). The microcomputer is a complete computing system built around a microprocessor.

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.

Microprocessor-Based System with Bus Architecture

ALU (Arithmetic/Logic Unit) – It performs such arithmetic operations as addition and subtraction, and such logic operations as AND, OR, and XOR. Results are stored either in registers or in memory.

Register Array – It consists of various registers identified by letter such as B, C, D, E, H, L, IX, and IY. These registers are used to store data and addresses temporarily during the execution of a program.

Control Unit – The control unit provides the necessary timing and control signals to all the operations in the microcomputer. It controls the flow of data between the microprocessor and memory and peripherals.

Input – The input section transfers data and instructions in binary from the outside world to the microprocessor. It includes such devices as a keyboard, switches, a scanner, and an analog-to-digital converter.

Output – The output section transfers data from the microprocessor to such output devices as LED, CRT, printer, magnetic tape, or another computer.

Memory – It stores such binary information as instructions and data, and provides that information to the microprocessor. To execute programs, the microprocessor reads instructions and data from memory and performs the computing operations in its ALU section. Results are either transferred to the output section for display or stored in memory for later use.

System bus – It is a communication path between the microprocessor and peripherals. The microprocessor communicates with only one peripheral at a time. The timing is provided by the control unit of the microprocessor.

Microprocessor Instructions

Even the incredibly simple microprocessor shown in the previous example will have a fairly large set of instructions that it can perform. The collection of instructions is implemented as bit patterns, each one of which has a different meaning when loaded into the instruction register. Humans are not particularly good at remembering bit patterns, so a set of short words are defined to represent the different bit patterns. This collection of words is called the assembly language of the processor. An assembler can translate the words into their bit patterns very easily, and then the output of the assembler is placed in memory for the microprocessor to execute.

Here's the set of assembly language instructions that the designer might create for the simple microprocessor in our example:

· LOADA mem - Load register A from memory address

· LOADB mem - Load register B from memory address

· CONB con - Load a constant value into register B

· SAVEB mem - Save register B to memory address

· SAVEC mem - Save register C to memory address

· ADD - Add A and B and store the result in C

· SUB - Subtract A and B and store the result in C

· MUL - Multiply A and B and store the result in C

· DIV - Divide A and B and store the result in C

· COM - Compare A and B and store the result in test

· JUMP addr - Jump to an address

· JEQ addr - Jump, if equal, to address

· JNEQ addr - Jump, if not equal, to address

· JG addr - Jump, if greater than, to address

· JGE addr - Jump, if greater than or equal, to address

· JL addr - Jump, if less than, to address

· JLE addr - Jump, if less than or equal, to address

· STOP - Stop execution

If you have read How C Programming Works, then you know that this simple piece of C code will calculate the factorial of 5 (where the factorial of 5 = 5! = 5 * 4 * 3 * 2 * 1 = 120):

a=1;

f=1;

while (a <= 5)

{

    f = f * a;

    a = a + 1;

}

At the end of the program's execution, the variable f contains the factorial of 5.

Assembly Language
A C compiler translates this C code into assembly language. Assuming that RAM starts at address 128 in this processor, and ROM (which contains the assembly language program) starts at address 0, then for our simple microprocessor the assembly language might look like this:

// Assume a is at address 128

// Assume F is at address 129

0   CONB 1      // a=1;

1   SAVEB 128

2   CONB 1      // f=1;

3   SAVEB 129

4   LOADA 128   // if a > 5 the jump to 17

5   CONB 5

6   COM

7   JG 17

8   LOADA 129   // f=f*a;

9   LOADB 128

10  MUL

11  SAVEC 129

12  LOADA 128   // a=a+1;

13  CONB 1

14  ADD

15  SAVEC 128

16  JUMP 4       // loop back to if

17  STOP

ROM
So now the question is, "How do all of these instructions look in ROM?" Each of these assembly language instructions must be represented by a binary number. For the sake of simplicity, let's assume each assembly language instruction is given a unique number, like this:

· LOADA - 1

· LOADB - 2

· CONB - 3

· SAVEB - 4

· SAVEC mem - 5

· ADD - 6

· SUB - 7

· MUL - 8

· DIV - 9

· COM - 10

· JUMP addr - 11

· JEQ addr - 12

· JNEQ addr - 13

· JG addr - 14

· JGE addr - 15

· JL addr - 16

· JLE addr - 17

· STOP - 18

The numbers are known as opcodes. In ROM, our little program would look like this:

// Assume a is at address 128

// Assume F is at address 129

Addr opcode/value

0    3             // CONB 1

1    1

2    4             // SAVEB 128

3    128

4    3             // CONB 1

5    1

6    4             // SAVEB 129

7    129

8    1             // LOADA 128

9    128

10   3             // CONB 5

11   5

12   10            // COM

13   14            // JG 17

14   31

15   1             // LOADA 129

16   129

17   2             // LOADB 128

18   128

19   8             // MUL

20   5             // SAVEC 129

21   129

22   1             // LOADA 128

23   128

24   3             // CONB 1

25   1

26   6             // ADD

27   5             // SAVEC 128

28   128

29   11            // JUMP 4

30   8

31   18            // STOP

You can see that seven lines of C code became 18 lines of assembly language, and that became 32 bytes in ROM.

Decoding
The instruction decoder needs to turn each of the opcodes into a set of signals that drive the different components inside the microprocessor. Let's take the ADD instruction as an example and look at what it needs to do:

1. During the first clock cycle, we need to actually load the instruction. Therefore the instruction decoder needs to:

· activate the tri-state buffer for the program counter

· activate the RD line

· activate the data-in tri-state buffer

· latch the instruction into the instruction register

2. During the second clock cycle, the ADD instruction is decoded. It needs to do very little:

· set the operation of the ALU to addition

· latch the output of the ALU into the C register

3. During the third clock cycle, the program counter is incremented (in theory this could be overlapped into the second clock cycle).

Every instruction can be broken down as a set of sequenced operations like these that manipulate the components of the microprocessor in the proper order. Some instructions, like this ADD instruction, might take two or three clock cycles. Others might take five or six clock cycles.