Briefly explain the 8086 instruction format.

Subject Microprocessor and Assembly Language
NU Year Set: 4.(c) Marks: 5 Year: 2011

The 8086 Instruction 8086 Instruction Format varies from 1 to 6 bytes in length. Fig. 6.8 shows the instruction formats for 1 to 6 bytes instructions. As shown in Fig. 6.8, displacements and operands may be either 8-bits or 16-bits long depending on the instruction. The opcode and the addressing mode is specified using first two bytes of an instruction.

The opcode/addressing mode byte(s) may be followed by :

  • No additional byte
  • Two bytes EA (For direct addressing only).
  • One or two-byte displacement
  • One or two-byte immediate operand
  • One or two-byte displacement followed by a one or two-byte immediate operand
  • Two-byte displacement and a two-byte segment address (for direct intersegment addressing only).

Most of the opcodes in 8086 have a special 1-bit indicates. They are :

W-bit: Some instructions of 8086 can operate on byte or a word. The W-bit in the opcode of such instruction specifies whether instruction is a byte instruction (W = 0) or a word instruction (W = 1).

D-bit: The D-bit in the opcode of the instruction indicates that the register specified within the instruction is a source register (D = 0) or destination register (D =1).

S-bit: An 8-bit 2’s complement number can be extended to a 16-bit 2’s complement number by making all of the bits in the higher-order byte equal the most significant bit in the low order byte. This is known as sign extension. 

 

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