Writing a Program for any Microcontroller consists of giving commands to the Microcontroller in a particular order in which they must be executed in order to perform a specific task. The commands to the Microcontroller are known as a Microcontroller’s Instructions Set.
Just as our sentences are made of words, a Microcontroller’s (for that matter, any computer) program is made of Instructions. Instructions written in a program tell the Microcontroller which operation to carry out.
An Instruction Set is unique to a family of computers. This tutorial introduces the 8051 Microcontroller Instruction Set also called as the MCS-51 Instruction Set.
As the 8051 family of Microcontrollers are 8-bit processors, the Instruction Set of 8051 Microcontroller is optimized for 8-bit control applications. As a typical 8-bit processor, the 8051 Microcontroller instructions have 8-bit Opcodes. As a result, the 8051 instruction set can have up to 2 8 = 256 Instructions.
Before going into the details of the 8051 Instructions Set, Types of Instructions and the Addressing Mode, let us take a brief look at the instructions and the instruction groups of the 8051 Instruction Set (the MCS-51 Instruction Set).
The following table shows the 8051 Instruction Groups and Instructions in each group. There are 49 Instruction Mnemonics in the Instructions Set of 8051 Microcontroller and these 49 Mnemonics are divided into five groups.
| DATA TRANSFER | ARITHMETIC | LOGICAL | BOOLEAN | PROGRAM BRANCHING |
| MOV | ADD | ANL | CLR | LJMP |
| MOVC | ADDC | ORL | SETB | AJMP |
| MOVX | SUBB | XRL | MOV | SJMP |
| PUSH | INC | CLR | JC | JZ |
| POP | DEC | CPL | JNC | JNZ |
| XCH | MUL | RL | JB | CJNE |
| XCHD | DIV | RLC | JNB | DJNZ |
| DA A | RR | JBC | NOP | |
| RRC | ANL | LCALL | ||
| SWAP | ORL | ACALL | ||
| CPL | RET | |||
| RETI | ||||
| JMP |
Before seeing the types of instructions, let us see the structure of the 8051 Microcontroller Instruction. An 8051 Instruction consists of an Opcode (short of Operation – Code) followed by Operand(s) of size Zero Byte, One Byte or Two Bytes.
The Op-Code part of the instruction contains the Mnemonic, which specifies the type of operation to be performed. All Mnemonics or the Opcode part of the instruction are of One Byte size.
Coming to the Operand part of the instruction, it defines the data being processed by the instructions. The operand can be any of the following:
There can multiple operands and the format of instruction is as follows:
MNEMONIC DESTINATION OPERAND, SOURCE OPERAND
A simple instruction consists of just the opcode. Other instructions may include one or more operands. Instruction can be one-byte instruction, which contains only opcode, or two-byte instructions, where the second byte is the operand or three byte instructions, where the operand makes up the second and third byte.
Based on the operation they perform, all the instructions of 8051 Microcontroller Instruction set are divided into five groups. They are:
We will now see about these instructions briefly.
The Data Transfer Instructions are associated with transfer of data between registers or external program memory or external data memory. The Mnemonics associated with Data Transfer are given below.
Mnemonic
Description