Microcontrollers SEM 2 UNIT 2

 Unit 2: 8051 Instruction Set

Q.IMP Write assembly language program on subroutine using loop: Multiply 15 by 10 using the technique of a repeated addition (7 Marks)

Code snippet

; Main program

MOV A, #15 ; Load value 15 into accumulator

CALL MULTIPLY ; Call the subroutine to multiply by 10

; After subroutine returns, the result will be in accumulator A

; (Accumulator will contain 150)

 

; Rest of your program code...

 

; Subroutine definition

MULTIPLY:

MOV R0, #10 ; Set counter (R0) to 10 for 10 repetitions

LOOP:

ADD A, #15 ; Add 15 to accumulator (repeated addition)

DJNZ R0, LOOP ; Decrement counter and loop if not zero

 

; After the loop, A will have the result (15*10)

RET ; Return from the subroutine

Question 1 (6 Marks): Explain the concept of addressing modes in the 8051 instruction set. Provide examples for different addressing modes.

Answer:

Addressing modes specify how the 8051 microcontroller locates the data operand for an instruction. Here are some common addressing modes:

• Register Direct: The operand is directly specified by a register name (A, B, C, D, E, H, or L).
• Example: MOV A, B (Move the content of register B to the accumulator)
• Immediate: The operand is the data value itself, included within the instruction.
• Example: ADD A, #10 (Add the immediate value 10 to the accumulator)
• Direct: The operand is located at a memory address specified by an 8-bit value following the instruction.
• Example: MOV A, 35h (Move the value from memory location 35h to the accumulator)
• Indirect: The operand address is stored in a register, and the microcontroller retrieves the data from that memory location.
• Example: MOV A, @R0 (Move the value from the memory location pointed to by register R0 to the accumulator)

Question 2 (6 Marks): Describe the different Data Transfer instructions in the 8051 instruction set. Give examples for each type.

Answer:

Data transfer instructions move data between various memory locations and registers within the 8051 microcontroller. Here are some common types:

• MOV (Move): The most basic data transfer instruction. It copies data from one location to another.
• Example: MOV A, B (Move the content of register B to the accumulator)
• MOVX (Move with Exchange): Transfers data and simultaneously swaps the contents of two locations.
• Example: MOVX A, @DPTR (Move the value from the memory location pointed to by DPTR to the accumulator and swap it with the original accumulator value)
• LDC (Load Constant): Loads a specific data value into a register directly from the instruction.
• Example: LDC A, #3Fh (Load the hexadecimal value 3Fh into the accumulator)
• XCH (Exchange): Swaps the contents of two registers.
• Example: XCH A, R1 (Swap the content of the accumulator with register R1)

Question 3 (6 Marks): Explain Arithmetic instructions in the 8051 and provide assembly language examples for addition and subtraction.

Answer:

Arithmetic instructions perform mathematical operations on data in the 8051. These instructions can affect the Program Status Word (PSW) flags that indicate the outcome of the operation.

• ADD (Add): Adds two operands and stores the result in the accumulator.
• Example: ADD A, #5 (Add the immediate value 5 to the accumulator)
• ADDC (Add with Carry): Adds two operands and the Carry flag (CY) to the accumulator.
• Example: ADDC A, B (Add the content of register B and the Carry flag to the accumulator)
• SUBB (Subtract with Borrow): Subtracts the second operand and the Borrow flag (borrowed from the previous operation) from the accumulator.
• Example: SUBB A, #10 (Subtract the immediate value 10 from the accumulator, potentially borrowing from the Carry flag)

Question 4 (6 Marks): Discuss Logical instructions in the 8051 instruction set and provide examples for AND and OR operations.

Answer:

Logical instructions perform bitwise operations on data in the 8051. These operations manipulate individual bits (0s and 1s) within the data.

• ANL (AND): Performs a bitwise AND operation on two operands. The result in each bit position is 1 only if both corresponding bits in the operands are 1.

• ORL (OR): Performs a bitwise OR operation on two operands. The result in each bit position is 1 if at least one corresponding bit in the operands is 1.

• XRL (eXclusive OR): Performs a bitwise XOR operation on two operands. The result in each bit position is 1 if the corresponding bits in the operands are different.

Question 5 : Briefly describe Branch instructions in the 8051 and give an example of a conditional jump.

Answer:

Branch instructions alter the normal program flow of the 8051 based on certain conditions. They allow the program to jump to a different code section depending on the state of the Program Status Word (PSW) flags or the result of a comparison.

• JMP (Jump): Unconditionally jumps to a specific memory address.
• JC (Jump if Carry): Jumps to a specified address if the Carry flag (CY) is set.
• JZ (Jump if Zero): Jumps to a specified address if the Zero flag (Z) is set (indicating the result of the previous operation was zero).

Question 6: What are Bit manipulation instructions in the 8051?

Answer:

Bit manipulation instructions allow you to control individual bits within a byte of data in the 8051. These instructions are useful for setting, clearing, toggling, or isolating specific bits for various operations.

• SETB (Set Bit): Sets a specific bit position in a register to 1.
• CLR (Clear Bit): Clears a specific bit position in a register to 0.
• CPL (Complement): Inverts all the bits in a register (0s become 1s and vice versa).