8051 Instruction set

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Microprocessors 1:

1 Microprocessors 1 The 8051 Instruction Set

Instruction Groups:

2 Instruction Groups The 8051 has 255 instructions Every 8-bit opcode from 00 to FF is used except for A5. The instructions are grouped into 5 groups Arithmetic Logic Data Transfer Boolean Branching

Arithmetic Instructions:

3 Arithmetic Instructions ADD 8-bit addition between the accumulator (A) and a second operand. The result is always in the accumulator. The CY flag is set/reset appropriately. ADDC 8-bit addition between the accumulator, a second operand and the previous value of the CY flag. Useful for 16-bit addition in two steps. The CY flag is set/reset appropriately.

Example – 16-bit Addition:

4 Example – 16-bit Addition Add 1E44H to 56CAH CLR C ; Clear the CY flag MOV A, 44H ; The lower 8-bits of the 1 st number ADD A, CAH ; The lower 8-bits of the 2 nd number MOV R1, A ; The result 0EH will be in R1. CY = 1. MOV A, 1EH ; The upper 8-bits of the 1 st number ADDC A, 56H ; The upper 8-bits of the 2 nd number MOV R2, A ; The result of the addition is 75H The overall result: 750EH will be in R2:R1. CY = 0.

Arithmetic Instructions:

5 Arithmetic Instructions DA Decimal adjust the accumulator. Format the accumulator into a proper 2 digit packed BCD number. Operates only on the accumulator. Works only after the ADD instruction. SUBB Subtract with Borrow. Subtract an operand and the previous value of the borrow (carry) flag from the accumulator. A  A - <operand> - CY. The result is always saved in the accumulator. The CY flag is set/reset appropriately.

Example – BCD addition:

6 Example – BCD addition Add 34 to 49 BCD CLR C ; Clear the CY flag MOV A, #34H ; Place 1 st number in A ADD A, #49H ; Add the 2 nd number. ; A = 7DH DA A ; A = 83H

Arithmetic Instructions:

7 Arithmetic Instructions INC Increment the operand by one. The operand can be a register, a direct address, an indirect address, the data pointer. DEC Decrement the operand by one. The operand can be a register, a direct address, an indirect address. MUL AB / DIV AB Multiply A by B and place result in A:B. Divide A by B and place result in A:B.

Logical Operations:

8 Logical Operations ANL / ORL Work on byte sized operands or the CY flag. ANL A, Rn ANL A, direct ANL A, @Ri ANL A, #data ANL direct, A ANL direct, #data ANL C, bit ANL C, /bit

Logical Operations:

9 Logical Operations XRL Works on bytes only. CPL / CLR Complement / Clear. Work on the accumulator or a bit. CLR P1.2

Logical Operations:

10 Logical Operations RL / RLC / RR / RRC Rotate the accumulator. RL and RR without the carry RLC and RRC rotate through the carry. SWAP A Swap the upper and lower nibbles of the accumulator. No compare instruction. Built into conditional branching instructions.

Data Transfer Instructions:

11 Data Transfer Instructions MOV 8-bit data transfer for internal RAM and the SFR. MOV A, Rn MOV A, direct MOV A, @Ri MOV A, #data MOV Rn, A MOV Rn, direct MOV Rn, #data MOV direct, A MOV direct, Rn MOV direct, direct MOV direct, @Ri MOV direct, #data MOV @Ri, A MOV @Ri, direct MOV @Ri, #data

Data Transfer Operations:

12 Data Transfer Operations MOV 1-bit data transfer involving the CY flag MOV C, bit MOV bit, C MOV 16-bit data transfer involving the DPTR MOV DPTR, #data

Data Transfer Instructions:

13 Data Transfer Instructions MOVC Move Code Byte Load the accumulator with a byte from program memory. Must use indexed addressing MOVC A, @A+DPTR MOVC A, @A+PC

Data Transfer Instructions:

14 Data Transfer Instructions MOVX Data transfer between the accumulator and a byte from external data memory. MOVX A, @Ri MOVX A, @DPTR MOVX @Ri, A MOVX @DPTR, A

Data Transfer Instructions:

15 Data Transfer Instructions PUSH / POP Push and Pop a data byte onto the stack. The data byte is identified by a direct address from the internal RAM locations. PUSH DPL POP 40H

Data Transfer Instructions:

16 Data Transfer Instructions XCH Exchange accumulator and a byte variable XCH A, Rn XCH A, direct XCH A, @Ri XCHD Exchange lower digit of accumulator with the lower digit of the memory location specified. XCHD A, @Ri The lower 4-bits of the accumulator are exchanged with the lower 4-bits of the internal memory location identified indirectly by the index register. The upper 4-bits of each are not modified.

Boolean Operations:

17 Boolean Operations This group of instructions is associated with the single-bit operations of the 8051. This group allows manipulating the individual bits of bit addressable registers and memory locations as well as the CY flag. The P, OV, and AC flags cannot be directly altered. This group includes: Set, clear, and, or complement, move. Conditional jumps.

Boolean Operations:

18 Boolean Operations CLR Clear a bit or the CY flag. CLR P1.1 CLR C SETB Set a bit or the CY flag. SETB A.2 SETB C CPL Complement a bit or the CY flag. CPL 40H ; Complement bit 40 of the bit addressable memory

Boolean Operations:

19 Boolean Operations ORL / ANL OR / AND a bit with the CY flag. ORL C, 20H ; OR bit 20 of bit addressable memory with the CY flag ANL C, /34H ; AND complement of bit 34 of bit addressable memory with the CY flag. MOV Data transfer between a bit and the CY flag. MOV C, 3FH ; Copy the CY flag to bit 3F of the bit addressable memory. MOV P1.2, C ; Copy the CY flag to bit 2 of P1.

Boolean Operations:

20 Boolean Operations JC / JNC Jump to a relative address if CY is set / cleared. JB / JNB Jump to a relative address if a bit is set / cleared. JB ACC.2, <label> JBC Jump to a relative address if a bit is set and clear the bit.

Branching Instructions:

21 Branching Instructions The 8051 provides four different types of unconditional jump instructions: Short Jump – SJMP Uses an 8-bit signed offset relative to the 1 st byte of the next instruction . Long Jump – LJMP Uses a 16-bit address. 3 byte instruction capable of referencing any location in the entire 64K of program memory.

Branching Instructions:

22 Branching Instructions Absolute Jump – AJMP Uses an 11-bit address . 2 byte instruction The upper 3-bits of the address combine with the 5-bit opcode to form the 1 st byte and the lower 8-bits of the address form the 2 nd byte. The 11-bit address is substituted for the lower 11-bits of the PC to calculate the 16-bit address of the target. The location referenced must be within the 2K Byte memory page containing the AJMP instruction. Indirect Jump – JMP JMP @A + DPTR

Branching Instructions:

23 Branching Instructions The 8051 provides 2 forms for the CALL instruction: Absolute Call – ACALL Uses an 11-bit address similar to AJMP The subroutine must be within the same 2K page. Long Call – LCALL Uses a 16-bit address similar to LJMP The subroutine can be anywhere. Both forms push the 16-bit address of the next instruction on the stack and update the stack pointer.

Branching Instructions:

24 Branching Instructions The 8051 provides 2 forms for the return instruction: Return from subroutine – RET Pop the return address from the stack and continue execution there. Return from ISV – RETI Pop the return address from the stack. Restore the interrupt logic to accept additional interrupts at the same priority level as the one just processed . Continue execution at the address retrieved from the stack. The PSW is not automatically restored.

Branching Instructions:

25 Branching Instructions The 8051 supports 5 different conditional jump instructions. ALL conditional jump instructions use an 8-bit signed offset. Jump on Zero – JZ / JNZ Jump if the A == 0 / A != 0 The check is done at the time of the instruction execution. Jump on Carry – JC / JNC Jump if the C flag is set / cleared.

Branching Instructions:

26 Branching Instructions Jump on Bit – JB / JNB Jump if the specified bit is set / cleared. Any addressable bit can be specified. Jump if the Bit is set then Clear the bit – JBC Jump if the specified bit is set. Then clear the bit.

Branching Instructions:

27 Branching Instructions Compare and Jump if Not Equal – CJNE Compare the magnitude of the two operands and jump if they are not equal. The values are considered to be unsigned. The Carry flag is set / cleared appropriately. CJNE A, direct, rel CJNE A, #data, rel CJNE Rn, #data, rel CJNE @Ri, #data, rel

Branching Instructions:

28 Branching Instructions Decrement and Jump if Not Zero – DJNZ Decrement the first operand by 1 and jump to the location identified by the second operand if the resulting value is not zero. DJNZ Rn, rel DJNZ direct, rel No Operation NOP