The Instruction Set

 The 8051 Instruction Set

• 1 Types of instructions
• 2 Description of the 8051 instructions

Introduction

The process of writing program for the microcontroller mainly consists of giving instructions (commands) in the specific order in which they should be executed in order to carry out a specific task. As electronics cannot “understand” what for example an instruction “if the push button is pressed- turn the light on” means, then a certain number of simpler and precisely defined orders that decoder can recognise must be used. All commands are known as INSTRUCTION SET. All microcontrollers compatibile with the 8051 have in total of 255 instructions, i.e. 255 different words available for program writing.

At first sight, it is imposing number of odd signs that must be known by heart. However, It is not so complicated as it looks like. Many instructions are considered to be “different”, even though they perform the same operation, so there are only 111 truly different commands. For example: ADD A,R0, ADD A,R1, ... ADD A,R7 are instructions that perform the same operation (additon of the accumulator and register). Since there are 8 such registers, each instruction is counted separately. Taking into account that all instructions perform only 53 operations (addition, subtraction, copy etc.) and most of them are rarely used in practice, there are actually 20-30 abbreviations to be learned, which is acceptable.

1- Types of instructions

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.