8086 Instruction Format-II

Introduction:

Depending on the addressing mode used, an instruction of 8086 microprocessor can be one to seven byte long. These are:

• One Byte Instruction
• Two Byte Instruction
• Three Byte Instruction
• Four Byte Instruction
• Five Byte Instruction
• Six Byte Instruction
• Seven Byte Instruction

The table shows the general format of 8086 instructions. The following paragraph gives the different instruction format with examples
1. One Byte Instruction: These instructions in the category uses the implied addressing mode and register mode instruction. The implied mode instruction does not use the operands, and the register mode uses the operand as one register.
a. Format-1: implied Type

Byte	Addressing Mode	Format/ Examples
1 Byte Format	Implied Addressing Mode Examples	OP CODE
		CLC, CLD, STC, CMC, CBW, DAA, DAS, AAA, AAS, RET, CLI, HLT etc..
	Examples	REP, REPZ / REPE,  REPNE / REPNZ, LOCK, LODS / LODSB / LODSW

1. Format-2:Register Addressing Mode

1 Byte Format	Register Addressing Mode INC reg(16 bit reg) DEC reg(16 bit reg) Examples (However INC/DEC for 8 bit register is 2byte instruction)	OP CODE	REG
		11111	Reg (3 bit)
		INC DEC	BX CX

2.  Two Byte Instruction: These instructions use two operands and there are two formats of this type of instruction.
   a. Format-1: Register to Register
   Both the source and the destination are the registers. The table below give the format and example of this type.

2 Byte	Description	FORMAT/ Examples
2 byte Format	( Reg to Reg)	OP CODE		Operands
		6-bit	2-bit	MOD			REG	R/M
	Size	****** dw			11	3 bits		3 bits
	Examples	100010dw 11  000  001 10001000 11 000 001 MOV  AL, CL w=0 indicate it is 1 byte operation d=0 indicate REG field­ is source

       b. Format-2 : Register to / from memory with no displacement
One of the operand is REG and the second  may be REG or MEMORY and instruction does not use any displacement. The table below give the format and example of this type.

2 Byte Format	Reg to/from memory with no displacement	OPCODE
		OP CODE		MOD	REG	R/M
		6-bit	2-bit	2-bit	3-bit	3-bit
		******dw		00	***	***
	Examples In this example the data is present in a memory location in DS whose offset address is in BX. The effective address of the data is given as DSx10H +[BX]	100010dw  00  000  111 10001010   00  000 111 MOV AL,  [BX] MOV AX, [BX] d=1, indicate AL  is destination w=0 indicate it is 1 byte operation

3. Three Byte Instructions: There are two formats for this type of instructions
   a. Register to /from memory with 8 bit displacement :This type of instruction uses one of the operand as register, the other operand may be register/memory and uses 8 bit displacement value.

3 Byte	Description	FORMAT/ Examples
3 Byte Format	Reg to/from memory with 8 bit disp.	OP CODE	MOD	REG	R/M	Displacement
		1 Byte	2 Bits	3 bits	3 bits	1 hex Byte
		******dw	01	***	***	********
	Examples	10001010 01  000 111  00100000 MOV  8-bit DISP  AL, [BX] 20h MOV AL, 20h[BX] 10001000  01  110 100 01000011 MOV 43h[SI],DH d=1 indicate REG field is destination; w=0 indicate it is 1 byte operation In this example the data is present in a memory location in DS whose offset address is [BX +20h]. The effective address of the data is given as DSx10H +[BX+20h]

   b. Immediate Operand (8 Bits data) to Register:This is also 3 byte instruction and use an 8 bit  immediate data and other operand is an 8 bit register(high or low order) when operand is 16-bit then it is 4-byte instruction. In this format, the first byte as well as the 3 bites from the second byte which are used for REG  field in case of register to register format are used for opcode.

3 Byte	Description	FORMAT/ Examples
3 Byte Format	Reg to/from memory with 8 bit disp.	OP CODE	Reg	Data low Byte	Data high Byte
		5bit	3 Bits	1  bits	1 Byte
		****w	***	********	********
	Examples	10111 000  00010000  00000000 MOV  AX, 0010h; it is 3-byte instruction w=1, indicate 16-bit operand MOV  AL, 8-bit-operand MOV  AL, 35h; it is 2-byte instruction 10110  000 00111010 here high byte is not there AND AL, 59h is as below: 00100000 01011001h w=0, indicate 8-bit  immediate data

4. Four Byte Instructions:
   a. Format-1: register to /from memory with 16 bit displacement

4 Byte	Description	FORMAT/ Examples
4 Byte Format	Reg to/from memory with 8 bit disp.	OP CODE	MOD	REG	R/M	16 bit Displacement
		1 Byte	2 Bits	3 bits	3 bits	Low Byte	High Byte
	Examples	10001001 01  110 100  0100001100100011 MOV 2343h[SI],DH

b. Format-2: Segment Register to Memory/reg

4 Byte	Description		FORMAT/ Examples
5 byte format	Immediate Operand (8 Bits data) to Memory with 16 bit displacement	OP CODE	MOD	OP CODE	SR	R/M	Displacement (16 bits)
		1 Byte	2 Bits	1 bits		3 bits	Low byte	High byte
	MOV  1234h[BX+SI], SS	10001100	10	0	10	000	00110100	00010010
	Examples	MOV  1234h[BX+SI], SS = 8C 90 34 12h This instruction moves the content of the segment register SS to a memory location, whose EA is the sum of the 16-bit displacement 1234h and the content of BX and SI register.

5. Five Byte Instruction:Immediate Operand (8 Bits data) to Memory with 16 bit displacement

4 Byte	Description	FORMAT/ Examples
5 byte format	Immediate Operand (8 Bits data) to Memory with 16 bit displacement	OP CODE	MOD	OP CODE	R/M	Displacement (16 bits)	Immediate data
		1 Byte	2 Bit	3 bits	3 bits	Low byte	High byte	1 Byte
	MOV Byte Ptr 0F246h[BP], 34h	11000110	10	000	110	01000110	11110010	00110100
	Examples	MOV Byte Ptr 0F246h[BP], 34h = C6 86 46 F2 34 This instruction moves the byte value 34h to a memory location whose EA is given as the sum of the 16-bit offset value F246h and the content of the BP register.

6. Six Byte Instruction: Immediate Operand (16 Bits data) to Memory with 16 bit displacement

4 Byte	Description	FORMAT/ Examples
6 byte format	Immediate Operand (16 Bits data) to Memory with 16 bit displacement	OP CODE	MOD	OP CODE	R/M	Displacement (16 bits)		Immediate data (16 bit)
		1 Byte	2 Bits	3 bits	3 bits	Low Byte	High Byte	Low Byte	High Byte
	MOV WORD PTR 0F2h[BP],34h	11000111	10	000	110	F2	00	34	00
	Examples	MOV WORD PTR 0F2h[BP],34h = C7 86 F2 00 34 00h This instruction moves 0034 to a memory location whose EA is given as the sum of the 16-bit offset value F2h and content of BP register.

7. Seven Byte Instruction:
   FORMAT: Immediate data to a memory location

7 Byte	Description		FORMAT/ Examples
7 byte format	Immediate data to a memory location (intersegment)	SOP	OP CODE	MOD	OP CODE	R/M	Displacement (16 bits)		Immediate data (16 bit)
		8-bit	1 Byte	2 Bits	3 bits	3 bits	Low Byte	High Byte	Low Byte	High Byte
	MOV DS:0F246h[BP], 1234h	3E	11000111	10	000	110	F2h	46h	34h	12h
	Examples		MOV DS:0F246h[BP], 1234h = 3E C7 86 F2 46 12 34h; This instruction moves the value 1234h to a memory location whose EA is given as the sum of the 16-bit offset value and the content of BP register. The physical address is generated using DS as the segment register. Segment override (SOP) is necessary, because by default BP refers in the stack segment(SS) register.