introduction to 8086 micro-processor

The instructions of 8086 processor are classified into following 6 functional groups:-
Data transfer instructions
Arithmetic and logical instructions
Branch instructions
Processor control instructions
String operation instructions
Protection control instructions
Assembler Instruction Format:-
The general format of an assembly language instruction is label : MNEMONIC operand 1, operand 2 comment.
Where each part of the instructions is separated by space and every instructions, starts on a new line.
Data Transfer Instructions:-
This 8086 processor support a variety of instructions for transfer of data, immediate value. Data may reside in any register memory location or it may be immediate value or from ports. In the data movements, instruction depends on the addressing modes. Example can be in any of the following forms.
• Register to register
• Register to memory
• Memory to register
• Immediate value to register
• Immediate value to memory
Data movement instructions generally involve two operands. i.e,. the source and destination. The source can be a register memory location or an immediate value. The destination can be a register or a memory location. Both the source and destination con not be a refer to memory location in the same instruction. They must be of the same data type. i.e., either of the type byte or type word. Data transfer instructions do not affect the CPU flags. MOV[command] means move. General format is mov destination , source. Move the data from source to destination and find result will be stored in destination.
• Mov reg,reg = mov A1,b1
• Mov reg,memory = mov c1,(1050H)
• Mov memory,reg = mov (1100H),d1
• Mov reg,Immediate = mov bx,55H
• Mov memory,immediate = mov (1060H),66H.
The instruction move, transfers a byte or word or data from the source to destination
Example:- 1. Mov A1,B1 [a1<~b1]
Copy the content of B1 register to A1 register (8Bit).
Before move A1=SA, B1=F7.
After move A1=F7, B1=F7
2. mov dx,ax [dx<~ax]
Copy the content of ax register to dx register (16Bit). Suppose before move if it is dx=5AC5, ax=EF). Then after move dx=EF06, ax=EF06
XCHG:- Exchange
XCHG dest,src
Desti src
Exchange the contents of source and destination. The instruction XCHG swaps (exchange) the contents of source or memory with the contents of a destination register or memory. The source can be a register or a memory location. The destination can be a register or a memory location. Both source & destination cannot refer to memory locations in the same instructions. They must be of same type, either of type byte or type word.
Example:- 1. A1=55, B1=66
XCHG A1,B1
The contents of A1 & B1 gets exchange.
A1=66, B1=55.
2. AX=88FC, BC=EEEE
XCHG AX,BX
AX=EEEE, BX=88FC
IN:- Input from port
In accumulation port or DX. Input to all or ax from input port.
If it is byte: A1 [Port]
If it is word: A1 [Port], Ah [Port + 1]
This instruction in reads data from the port & copies into accumulator. An 8-bit access to port will place the data in ax registers. The instruction In has 2 formals
(1) Fixed port format:-
The address of a port will directly specified in instruction.
Eg:- In port
(2) Variable format:-
The address of a port is stored in the dx register before the instruction IN is executed.
Eg:- mov dx,port
In dx
The advantage of the variable format IN instruction is that the port address can be coded dynamically during the execution of a program.
OUT:- out to port
Out port (or dx),
Output from A1 or Ax to output port.
If it is byte: [port] A1.
If it is word : [port] Al, [port + 1] Ah.
The instruction out wrotes the contents of the accumulator [Al for 8-bit and Ax for 16-bit] to a specific port. The instruction out has 2 formats.
(1) Fixed port format:-
The address of the port is directly specified in the instruction.
Eg:- out port
(2) Variable port format:-
The address of the port is stored in the dx register before the instruction out is executed.
Eg:- mov dx,port
Out dx
XLAT / XLATB:- Translate byte
It translate al into a value in a translation from at B2
AL DS: [Bx+(AL)]
The instruction XLAT / XLATB replaces a byte in the AL register with a byte from the look up table before execution of the XLAT instruction BX should be loaded with the offset address of lookup table in the data segment (DS) and Al with the lode to be converted. When XLAT is executed the byte pointed to by (bx+al) is transferred to the Al register.
Eg:- mov AL,05
Mov Bx,1000H
XLAT
AL [Bx + AL]
AL [1000 + 05]
AL [1005]
LEA:- Load Effect Address
The instruction LEA determines offset of a variable or memory location indicated as a source (address) and places the offset in the specified 16-bit register. The source operand must be a memory variable and destination must be a 16-bit register.
Eg:- LEA Bx,N
Bx=offset address of N
LDS:- Load Data Segment
Load register and register from the memory locations. This instruction copies the content of first two memory locations to a specified 16-bit register & next consecutive two memory locations into the DS register. No flags are effected by this.
Syntax:- LES reg16, mem16
Eg:- LES Bx, [5300H]
This instruction copies the content of memory out 5300 to BL and 5301 to BH and next consecutive memory location 5302 & 5303 to register.
LAHF:- Load ah with flags
This instruction copies the lower byte of the flag reg to ah reg.
Syntax:- LAHF
Eg:- LAHF
SAHF:- Store ah in flag register
This instruction copies the content of ah into the lower byte of the flag register
Eg: SAHF
It is useful in modifying the flag register without the execution of arithmetic instructions. By this instruction SF,ZF,AF,DF,CF are affected.