Instruction set of 8086 Microprocessors

The instruction set of 8086 Microprocessors

The instruction format contains two fields:

➡operation code / opcode 

➡Operand field

OPERATION CODE / OPCODE: 

 ➡ It indicates the type of operation to be performed by CPU 

✔Example: MOV, ADD … 

OPERAND: 

➡The CPU executes the instruction using the information that resides in these fields.

➡There are six general formats of instructions in the 8086 instruction set.

➡The instruction of 8086 vary from 1to 6 bytes length

ONE BYTE INSTRUCTION: 

➡It is only one byte long and may have implied data or register operands.

➡The least three significant 3 bits of the opcode are used for specifying register 

operand if any otherwise all the 8 bits form an opcode and the operands are implied.

REGISTER TO REGISTER

➡The format is 2 bytes long 

➡The first byte of the code specifies the opcode and width 

➡The second byte of the code shows the register operand and R/M field 

REGISTER TO/FROM MEMORY WITH NO DISPLACEMENT

➡The format is 2 bytes long 

➡This is similar to the register to register format except for the MOD field is shown. 

➡The MOD field shows the mode of addressing 

REGISTER TO/FROM MEMORY WITH DISPLACEMENT

➡The format contains one or two additional bytes for displacement along with 2 bytes Register to/from memory with no displacement.