Instruction Word Size in Microprocessor - GeeksforGeeks

21 Aug, 2019
The 8085 instruction set is classified into 3 categories by considering the length of the instructions. In 8085, the length is measured in terms of “byte” rather then “word” because 8085 microprocessor has 8-bit data bus. Three types of instruction are: 1-byte instruction, 2-byte instruction, and 3-byte instruction.
1. One-byte instructions –In 1-byte instruction, the opcode and the operand of an instruction are represented in one byte.
Example-1:Task- Copy the contents of accumulator in register B.Mnemonic- MOV B, A
Opcode- MOV
Operand- B, A
Hex Code- 47H
Binary code- 0100 0111 
Mnemonic- MOV B, A
Opcode- MOV
Operand- B, A
Hex Code- 47H
Binary code- 0100 0111 
Example-2:Task- Add the contents of accumulator to the contents of register B. Mnemonic- ADD B
Opcode- ADD
Operand- B
Hex Code- 80H
Binary code- 1000 0000 
 Mnemonic- ADD B
Opcode- ADD
Operand- B
Hex Code- 80H
Binary code- 1000 0000 
Example-3:Task- Invert (complement) each bit in the accumulator.Mnemonic- CMA
Opcode- CMA
Operand-  NA
Hex Code- 2FH
Binary code- 0010 1111 
Mnemonic- CMA
Opcode- CMA
Operand-  NA
Hex Code- 2FH
Binary code- 0010 1111 
Note – The length of these instructions is 8-bit; each requires one memory location. The mnemonic is always followed by a letter (or two letters) representing the registers (such as A, B, C, D, E, H, L and SP).
2. Two-byte instructions –Two-byte instruction is the type of instruction in which the first 8 bits indicates the opcode and the next 8 bits indicates the operand.
Example-1:Task- Load the hexadecimal data 32H in the accumulator.Mnemonic- MVI A, 32H
Opcode- MVI
Operand- A, 32H
Hex Code- 3E
32
Binary code- 0011 1110
0011 0010 
Mnemonic- MVI A, 32H
Opcode- MVI
Operand- A, 32H
Hex Code- 3E
32
Binary code- 0011 1110
0011 0010 
Example-2:Task- Load the hexadecimal data F2H in the register B.Mnemonic- MVI B, F2H
Opcode- MVI
Operand- B, F2H
Hex Code- 06
F2
Binary code- 0000 0110
1111 0010 
Mnemonic- MVI B, F2H
Opcode- MVI
Operand- B, F2H
Hex Code- 06
F2
Binary code- 0000 0110
1111 0010 
Note – This type of instructions need two bytes to store the binary codes. The mnemonic is always followed by 8-bit (byte) data.
3. Three-byte instructions –Three-byte instruction is the type of instruction in which the first 8 bits indicates the opcode and the next two bytes specify the 16-bit address. The low-order address is represented in second byte and the high-order address is represented in the third byte.
Example-1:Task- Load contents of memory 2050H in the accumulator.Mnemonic- LDA 2050H
Opcode- LDA
Operand- 2050H
Hex Code- 3A
50
20
Binary code- 0011 1010
0101 0000
0010 0000 
Mnemonic- LDA 2050H
Opcode- LDA
Operand- 2050H
Hex Code- 3A
50
20
Binary code- 0011 1010
0101 0000
0010 0000 
Example-2:Task- Transfer the program sequence to the memory location 2050H.Mnemonic- JMP 2085H
Opcode- JMP
Operand- 2085H
Hex Code- C3
85
20
Binary code- 1100 0011
1000 0101
0010 0000 
Mnemonic- JMP 2085H
Opcode- JMP
Operand- 2085H
Hex Code- C3
85
20
Binary code- 1100 0011
1000 0101
0010 0000 
Note – These instructions would require three memory locations to store the binary codes. The mnemonic is always followed by 16-bit (or adr).
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