3.1 Types of instructions

MikroElektronika

Depending on operation they perform, all instructions are divided in several groups:

  • Arithmetic Instructions
  • Branch Instructions
  • Data Transfer Instructions
  • Logic Instructions
  • Bit-oriented Instructions

The first part of each instruction, called MNEMONIC refers to the operation an instruction performs (copy, addition, logic operation etc.). Mnemonics are abbreviations of the name of operation being executed. For example:

  • INC R1 – Means: Increment register R1 (increment register R1);
  • LJMP LAB5 – Means: Long Jump LAB5 (long jump to the address marked as LAB5);
  • JNZ LOOP – Means: Jump if Not Zero LOOP (if the number in the accumulator is not 0, jump to the address marked as LOOP);

The other part of instruction, called OPERAND is separated from mnemonic by at least one whitespace and defines data being processed by instructions. Some of the instructions have no operand, while some of them have one, two or three. If there is more than one operand in an instruction, they are separated by a comma. For example:

  • RET – return from a subroutine;
  • JZ TEMP – if the number in the accumulator is not 0, jump to the address marked as TEMP;
  • ADD A,R3 – add R3 and accumulator;
  • CJNE A,#20,LOOP – compare accumulator with 20. If they are not equal, jump to the address marked as LOOP;

Arithmetic instructions

Arithmetic instructions perform several basic operations such as addition, subtraction, division, multiplication etc. After execution, the result is stored in the first operand. For example:

ADD A,R1 – The result of addition (A+R1) will be stored in the accumulator.

ARITHMETIC INSTRUCTIONS
Mnemonic Description Byte Cycle
ADD A,Rn Adds the register to the accumulator 1 1
ADD A,direct Adds the direct byte to the accumulator 2 2
ADD A,@Ri Adds the indirect RAM to the accumulator 1 2
ADD A,#data Adds the immediate data to the accumulator 2 2
ADDC A,Rn Adds the register to the accumulator with a carry flag 1 1
ADDC A,direct Adds the direct byte to the accumulator with a carry flag 2 2
ADDC A,@Ri Adds the indirect RAM to the accumulator with a carry flag 1 2
ADDC A,#data Adds the immediate data to the accumulator with a carry flag 2 2
SUBB A,Rn Subtracts the register from the accumulator with a borrow 1 1
SUBB A,direct Subtracts the direct byte from the accumulator with a borrow 2 2
SUBB A,@Ri Subtracts the indirect RAM from the accumulator with a borrow 1 2
SUBB A,#data Subtracts the immediate data from the accumulator with a borrow 2 2
INC A Increments the accumulator by 1 1 1
INC Rn Increments the register by 1 1 2
INC Rx Increments the direct byte by 1 2 3
INC @Ri Increments the indirect RAM by 1 1 3
DEC A Decrements the accumulator by 1 1 1
DEC Rn Decrements the register by 1 1 1
DEC Rx Decrements the direct byte by 1 1 2
DEC @Ri Decrements the indirect RAM by 1 2 3
INC DPTR Increments the Data Pointer by 1 1 3
MUL AB Multiplies A and B 1 5
DIV AB Divides A by B 1 5
DA A Decimal adjustment of the accumulator according to BCD code 1 1

Branch Instructions

There are two kinds of branch instructions:

Unconditional jump instructions: upon their execution a jump to a new location from where the program continues execution is executed.

Conditional jump instructions: a jump to a new program location is executed only if a specified condition is met. Otherwise, the program normally proceeds with the next instruction.

BRANCH INSTRUCTIONS
Mnemonic Description Byte Cycle
ACALL addr11 Absolute subroutine call 2 6
LCALL addr16 Long subroutine call 3 6
RET Returns from subroutine 1 4
RETI Returns from interrupt subroutine 1 4
AJMP addr11 Absolute jump 2 3
LJMP addr16 Long jump 3 4
SJMP rel Short jump (from –128 to +127 locations relative to the following instruction) 2 3
JC rel Jump if carry flag is set. Short jump. 2 3
JNC rel Jump if carry flag is not set. Short jump. 2 3
JB bit,rel Jump if direct bit is set. Short jump. 3 4
JBC bit,rel Jump if direct bit is set and clears bit. Short jump. 3 4
JMP @A+DPTR Jump indirect relative to the DPTR 1 2
JZ rel Jump if the accumulator is zero. Short jump. 2 3
JNZ rel Jump if the accumulator is not zero. Short jump. 2 3
CJNE A,direct,rel Compares direct byte to the accumulator and jumps if not equal. Short jump. 3 4
CJNE A,#data,rel Compares immediate data to the accumulator and jumps if not equal. Short jump. 3 4
CJNE Rn,#data,rel Compares immediate data to the register and jumps if not equal. Short jump. 3 4
CJNE @Ri,#data,rel Compares immediate data to indirect register and jumps if not equal. Short jump. 3 4
DJNZ Rn,rel Decrements register and jumps if not 0. Short jump. 2 3
DJNZ Rx,rel Decrements direct byte and jump if not 0. Short jump. 3 4
NOP No operation 1 1

Data Transfer Instructions

Data transfer instructions move the content of one register to another. The register the content of which is moved remains unchanged. If they have the suffix “X” (MOVX), the data is exchanged with external memory.

DATA TRANSFER INSTRUCTIONS
Mnemonic Description Byte Cycle
MOV A,Rn Moves the register to the accumulator 1 1
MOV A,direct Moves the direct byte to the accumulator 2 2
MOV A,@Ri Moves the indirect RAM to the accumulator 1 2
MOV A,#data Moves the immediate data to the accumulator 2 2
MOV Rn,A Moves the accumulator to the register 1 2
MOV Rn,direct Moves the direct byte to the register 2 4
MOV Rn,#data Moves the immediate data to the register 2 2
MOV direct,A Moves the accumulator to the direct byte 2 3
MOV direct,Rn Moves the register to the direct byte 2 3
MOV direct,direct Moves the direct byte to the direct byte 3 4
MOV direct,@Ri Moves the indirect RAM to the direct byte 2 4
MOV direct,#data Moves the immediate data to the direct byte 3 3
MOV @Ri,A Moves the accumulator to the indirect RAM 1 3
MOV @Ri,direct Moves the direct byte to the indirect RAM 2 5
MOV @Ri,#data Moves the immediate data to the indirect RAM 2 3
MOV DPTR,#data Moves a 16-bit data to the data pointer 3 3
MOVC A,@A+DPTR Moves the code byte relative to the DPTR to the accumulator (address=A+DPTR) 1 3
MOVC A,@A+PC Moves the code byte relative to the PC to the accumulator (address=A+PC) 1 3
MOVX A,@Ri Moves the external RAM (8-bit address) to the accumulator 1 3-10
MOVX A,@DPTR Moves the external RAM (16-bit address) to the accumulator 1 3-10
MOVX @Ri,A Moves the accumulator to the external RAM (8-bit address) 1 4-11
MOVX @DPTR,A Moves the accumulator to the external RAM (16-bit address) 1 4-11
PUSH direct Pushes the direct byte onto the stack 2 4
POP direct Pops the direct byte from the stack/td> 2 3
XCH A,Rn Exchanges the register with the accumulator 1 2
XCH A,direct Exchanges the direct byte with the accumulator 2 3
XCH A,@Ri Exchanges the indirect RAM with the accumulator 1 3
XCHD A,@Ri Exchanges the low-order nibble indirect RAM with the accumulator 1 3

Logic Instructions

Logic instructions perform logic operations upon corresponding bits of two registers. After execution, the result is stored in the first operand.

LOGIC INSTRUCTIONS
Mnemonic Description Byte Cycle
ANL A,Rn AND register to accumulator 1 1
ANL A,direct AND direct byte to accumulator 2 2
ANL A,@Ri AND indirect RAM to accumulator 1 2
ANL A,#data AND immediate data to accumulator 2 2
ANL direct,A AND accumulator to direct byte 2 3
ANL direct,#data AND immediae data to direct register 3 4
ORL A,Rn OR register to accumulator 1 1
ORL A,direct OR direct byte to accumulator 2 2
ORL A,@Ri OR indirect RAM to accumulator 1 2
ORL direct,A OR accumulator to direct byte 2 3
ORL direct,#data OR immediate data to direct byte 3 4
XRL A,Rn Exclusive OR register to accumulator 1 1
XRL A,direct Exclusive OR direct byte to accumulator 2 2
XRL A,@Ri Exclusive OR indirect RAM to accumulator 1 2
XRL A,#data Exclusive OR immediate data to accumulator 2 2
XRL direct,A Exclusive OR accumulator to direct byte 2 3
XORL direct,#data Exclusive OR immediate data to direct byte 3 4
CLR A Clears the accumulator 1 1
CPL A Complements the accumulator (1=0, 0=1) 1 1
SWAP A Swaps nibbles within the accumulator 1 1
RL A Rotates bits in the accumulator left 1 1
RLC A Rotates bits in the accumulator left through carry 1 1
RR A Rotates bits in the accumulator right 1 1
RRC A Rotates bits in the accumulator right through carry 1 1

Bit-oriented Instructions

Similar to logic instructions, bit-oriented instructions perform logic operations. The difference is that these are performed upon single bits.

BIT-ORIENTED INSTRUCTIONS
Mnemonic Description Byte Cycle
CLR C Clears the carry flag 1 1
CLR bit Clears the direct bit 2 3
SETB C Sets the carry flag 1 1
SETB bit Sets the direct bit 2 3
CPL C Complements the carry flag 1 1
CPL bit Complements the direct bit 2 3
ANL C,bit AND direct bit to the carry flag 2 2
ANL C,/bit AND complements of direct bit to the carry flag 2 2
ORL C,bit OR direct bit to the carry flag 2 2
ORL C,/bit OR complements of direct bit to the carry flag 2 2
MOV C,bit Moves the direct bit to the carry flag 2 2
MOV bit,C Moves the carry flag to the direct bit 2 3