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.. |
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| Examples | REP, REPZ / REPE, REPNE / REPNZ, LOCK, LODS / LODSB / LODSW |
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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 |
- 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 |
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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 Â is destination w=0 indicate it is 1 byte operation |
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- 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] |
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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 |
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- 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 |
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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) |
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| 1 Byte | 2 Bits | 1 bits | 3 bits | Low byte | High byte | |||
| MOVÂ 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. |
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- 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. |
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- 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. |
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- 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. |
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