Introduction to Microprocessor by Arun S

Prelude 

BIT=Binary Digit: 0's and 1's
1 nibble=4 Bits

1 Byte= 8Bits    for example 1010 0101

1KiloByte(KB)= 1024 Bytes

1 Mega Byte(MB)= 1024KB 

Digital Abstraction

The digital circuit is a device that works in binary.There are only two symbols in binary(0's and 1's) . It means, input and output of digital circuits are 0’s and 1’s. Digital circuits are used to implement electronics device and it is used in real world. But in real world, there are no 0’s and 1’s. Instead of 0’s and 1’s, continuously varying voltages will be used as inputs and outputs. Because the world is analog, whatever available in reality will vary continuously. But in digital circuits 0’s and 1’s will be considered as inputs and outputs. In digital circuits range of voltages are interpreted as binary 0 and 1.Digital circuit model enables one to focus on the digital behavior of a circuit(0's and 1's), to ignore analog and transient phenomena, and to easily build larger, more complex circuits out of small circuits.

 What is Logic '0' and Logic '1'?

Range of voltage close to Vcc is represented as logic ‘1’(High) and range of voltage close to gnd is represented as logic ‘0’(Low). Logic '1' and '0' are just symbols, it is not available in the real world. 

For Example consider CMOS logic levels 

Logic levels of CMOS Logic family

How this logic levels related to Microprocessor?

Communication with Microprocessor or any other digital circuit can be done using logic '0' and logic '1'. But logic '0' and '1' are not available in the real world. so, we are considering digital abstraction by assuming certain range of voltage close to Vcc as logic '1' and voltage close to ground as logic '0'.
Microprocessor is also Digital Integrated circuit. Digital Integrated circuits can be designed using logic gates.Those logic gates are designed using transistors. Voltage should be applied to either switch on or switch off the transistor.In order to switch on the transistor certain range of voltage should be applied and in order to switch off the transistor certain range of voltage should be applied.

Microprocessor Age

1.  4004 is the world first Central processing unit(CPU) designed by Intel  in 1971.It is 4-bit CPU, and fabricated in P-channel MOSFET technology. It can address 4096 4-bit wide memory locations. A 4-bit-wide memory location is often called a nibble. 4004 has totally 45 instructions and it will execute with the speed of 50 Kilo-instruction per second (KIPs). The weight of 4004 is less than an ounce(28.3grams). 

 2.  8008 microprocessor is designed and manufactured by Intel in April 1972. It is extended 8 bit version of 4004. It is an 8-bit CPU with an external 14-bit address bus that can address 16 KB of memory.

3.  8080 microprocessor is designed and manufactured by Intel in 1974. It is the first modern 8 bit processor from Intel. About 6 months after 8080 microprocessor release , Motorola corporation introduced MC6800 microprocessor.

4.  Intel introduced 8085 in 1977 which is the updated version of Intel 8080 microprocessor. The 8085 is the last 8-bit, general-purpose microprocessor developed by Intel. Although only slightly more advanced than an 8080 microprocessor, the 8085 executed software at an even higher speed. An addition that took 2.0 μs (500,000 instructions per second on the 8080) required only 1.3 μs (769,230 instructions per second) on the 8085. The main advantages of the 8085 were its internal clock generator, internal system controller, and higher clock frequency. This higher level of component integration reduced the 8085’s cost and increased its usefulness.
5.  Intel released 8086 microprocessors in 1978 and a year later, it released 8088. Both 8086 and 8088 are 16-bit microprocessors, which execute instruction in 400ns which is 2.5 million of Instruction per Second (MIPs). While considering execution speed 8086 is significantly faster than 8085. 8086 and 8088 can address 1MB(megabyte) of memory, Which is 16 times more than 8085.

Introduction to Processor 
Microprocessor recognize and operate in binary number. Each microprocessor has its own set of binary words, meanings and language. The word is defined as the number of bits the microprocessor recognize and process at a time.The word length ranges from 4 bits for small microprocessor based systems to 64 bits for high speed large computer.

Each machine has its own set of instructions based on the design of its CPU. One must give instruction in binary language (machine language) to communicate with CPU.It is difficult for most of the people to write program in set of 0’s and 1’s.So, computer manufactures have decided English like words to represent the binary instruction of a machine which is nothing but Assemble language. Assembly language is specific to a given machine, programs written in assembly language are not transferable from one machine to another.

Machine Language 
The number of bits is a word for a given machine is fixed, and words are formed through various combination of these bits.For example, a machine with a word length of eight bits can have 256(2^8) combination of eight bits. However not all these words need to be used in the machine as instruction.

The microprocessor design engineer selects the combination of bit patterns and gives a specific meaning to each combination by using electronic logic circuit. That combination of bits is called as instruction.
The set of instructions designed in to the machine makes up its machine language a binary language, composed of 0’s and 1’s that is specific for each computer. Machine language depends on the design of the machine. For example 8-bit processor designed by Intel design engineer will be different when compared to the 8-bit processor designed by Motorola engineer. It is because the designer is different, so,the assumption of combination of bits considering for the instruction also will be different.We know that, Considering different combination of bits will end up with different digital circuit for the assumed combination of bits.

8085 Machine Language  

• The 8085 is a microprocessor with 8 bit word length. Its instruction set is designed by using various combination of eight bits.
•  An instruction is a binary pattern entered through an input device in memory to command the microprocessor to perform specific function. 
• For Example,0011 1100:is an instruction that increments the number in the register called accumulator by one.1000 0000:is an instruction that adds the number in the register called B to the number in the accumulator, and keep the sum in accumulator. 
• The 8085 microprocessor has 256 such bit patterns, amounting to 74 different instructions are called an instruction set the binary language with predefined instruction set is called the 8085-machine language.
•  It is tedious for people to recognize and write instruction in binary language, so, the instruction is for convenience, written in hexadecimal code and entered in a single board microcomputer by using hexadecimal keys.
• For example, the binary instruction 0011 1100 is equivalent to 3C in hexadecimal. This   instruction can be entered in a single board microcomputer with a hex key board by pressing 3 and C.

8085 Assembly Language 

• Even though the instruction can be written in hexadecimal code, it is still difficult to understand a program written in hexadecimal numbers.
•  Each manufacturers of a microprocessor have deviced a symbolized code for each instruction, called a mnemonic, the mnemonics for particular instruction consist of letter that suggests the operation to be performed by the instruction. 
• For example: the binary code 0011 1100(3CH) of the 8085 microprocessor is represented by the mnemonic INR A. (H - hexadecimal number- refer Appendix 1)
• INR A-INR stands for increment, and A represent the accumulator. This symbol suggests the operation of increments the accumulator content by one.
• Similarly, the binary code 1000 0000(80H) is represented as ADD B, ADD stands for addition and B represents the content is register B. This symbol suggests the addition of the contents in register B and the contents in the accumulator.
• The program written in assembly language is called assembly language program.
• Again, the assembly language or mnemonics, is specific to each microprocessor. For example, 6800 Motorola microprocessor has an entirely different set of binary codes and mnemonics than the 8085 therefore, the assembly language of 6800 is far different from that of 8085.
• An assembly language program written for one microprocessor is not transferable to a computer with another microprocessor unless the microprocessor is compatible in their machine codes.
• Machine language and assembly language are microprocessor specific and both are considered as low-level language.
• Machine language is in binary, and the assembly language is in English like words. Microprocessor understand only binary language.
  How the assembly language mnemonics written and translated in to machine language or binary code?
  The mnemonics can be written by hand on paper and translated manually is hexadecimal codes, called hand assembly.
  The mnemonic can be written electronically on a computer using a program called editor in the ASCII Code and translate in to binary code by using the program called an assembler.

ASCII Code  

• A computer is binary machine, to communicate with computer in alphabetical letters and decimal numbers, translation codes are necessary.
• The commonly used code is ASCII-American standard code for information interchange.It is 7-bit code with 128(2^7) combination, and each combination from 00H to 7FH is assigned to either a letter, a decimal number a symbol or a machine command.
• For example, hexadecimal 30H to 39H represent 0 to 9 decimal digits,41H to 5AH represents capital letters A through Z.
• In microcomputers systems, keyboards, video screens and printers are typical examples of device that use ASCII codes. 
• When the key 9 is pressed on the ASCII keyboard, the computer receives 39H in binary called an ASCII character and system program translates ASCII character in to appropriate number.

 Appendix 1

Hexadecimal number system

Base or radix of hexadecimal number is 16. It means 16 symbols available in hexadecimal number system. Combination of those symbols can be used to write numbers in hexadecimal. The 16 hexadecimal symbols are 0,1,2,3,4,5,6,7,8,9, A, B, C, D, E, F.

For example, base or radix of decimal number is 10. It means 10 symbols available in decimal number system. Combination of those symbols can be used to write numbers in decimal. The 10 decimal symbols are 0,1,2,3,4,5,6,7,8,9.

Base or radix for binary number is 2. The 2 binary symbols are 0,1.

Following table represent the relationship between decimal binary and hexadecimal numbers.

Decimal Numbers (Base10)	Binary Numbers (Base 2)	Hexadecimal Numbers (Base 16 or H)
0	0000	0
1	0001	1
2	0010	2
3	0011	3
4	0100	4
5	0101	5
6	0110	6
7	0111	7
8	1000	8
9	1001	9
10	1010	A
11	1011	B
12	1100	C
13	1101	D
14	1110	E
15	1111	F

(4 base 10)= (0100) base 2= (4 base 16 or 4H)

4 in decimal = 0100 in binary = 4 in hexadecimal

10 in decimal =1010 in binary = A in hexadecimal(AH)

45 in decimal = 0010 1101 in binary = 2D in hexadecimal (I can represent as 2DH)

How to convert binary number in to hexadecimal number?

 Let as consider a random binary number i) 101011001

Step 1: From right hand side group 4 bits (Last group may have less than 4 bits in the group. If it is less pad with zeros).

Step 2: Refer table and write the equivalent hexadecimal number for each group.

1	0101	1001	Group 4, 4 bits starting from right
1	5	9	Refer table and write equivalent hexadecimal number

Padding zeros to the last group. Padding zeros to the left will not change that number. For example, 45 in decimal is equivalent to 0045 in decimal.

0001	0101	1001	Group 4, 4 bits starting from right
1	5	9	Refer table and write equivalent hexadecimal number

 101011001 in binary = 159H

ii) binary number =11110111101011

Step 1: From right hand side group 4 bits (Last group may have less than 4 bits in the group. If it is less pad with zeros).

Step 2: Refer table and write the equivalent hexadecimal number for each group.

0011	1101	1110	1011	Group 4, 4 bits starting from right
3	D	E	B	Refer table and write equivalent hexadecimal number

 11110111101011 in binary = 3DEBH

Why numbers are represented in hexadecimal instead of binary?

Hexadecimal numbers are used for representing binary numbers with a smaller number of digits. Hexadecimal number are only used for representation purpose, but processor or other digital circuits will not understand hexadecimal.

Whether microprocessor understand hexadecimal numbers?

No, microprocessor understand only binary numbers (Machine language).  To represent binary numbers with less number of digits hexadecimal number system is used.