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General Instruction Formats in 8086 Microprocessor

8086 Microprocessor General Instruction Formats: Here, we are going to learn about the various general instruction formats in 8086 Microprocessor.
Submitted by Uma Dasgupta, on February 10, 2020

Introduction:

In this article, we are going to discuss about the 6 general formats of instructions.

1. One byte instruction:
   This is only one byte long and may have implied data and register.
2. Register to register:
   This instruction is 2 byte long. First byte specified opcode and width(w) of opcode and second byte specified register operand and R/M field.
   
   Here, MSB - Most significant bit and LSB - Least significant bit
3. Register to/from memory with no displacement:
   This format is also 2-byte form and similar to the register except the MOD field.
4. Register to/from memory with displacement:
   This is same as the third one having one or two additional byte for displacement.
   
   
5. Immediate operand to register:
   In it first byte as well as three bits from the second byte which are used for the register field. In case of register to register format is reserved for the opcode. It also contains 1 or 2 bytes of immediate data.
   
   
6. Immediate Operand to memory with 16-bit displacement:
   
   

Opcode have single bit indicators their significance is as follows,

• W bit: It tells us whether the instruction is to operate over an 8-bit or 16-bit

      W=0 /8bit
      W=1 /16bit
  

• D bit: It tells whether the register specified field is the source register or destination register.

      D=0    Source register
      D=1    Destination
  

• S bit: This is called sign extension bit; it is used along with the w bit.

      S   W
      0   0       8 bit operant with 8bit immediate
      0   1       operant. 16 bit operant with 6 bit
      0   1       immediate operant. 16 bit operation
      1   1       with sign extended immediate data.
  

• V bit: Used in case of shift and rotate instruction if v=0, then shift count is 1. If v=1 then count is stored into the CL register and count should be count=2.
• Z bit: This is used along with the string primitives such as REP for comparison with ZF flag.

Conclusion:

In the above article, I have discussed the different instruction formats of the microprocessors briefly, with relative diagrams also. As I said earlier at the beginning of this article also that this topic of computer system organization is very important from the examination point of view and also the base of your knowledge about computer architecture. For further queries shoot your questions in the comment section below!