INSTRUCTION SET
.Itis a command given to computer to perform
specified operation/task
.The collection of instructions that to execute-
instruction set
.The instn set of one MP cannot be used by other MP
.The sequence of instruction-program
Structure of an instruction
.Instruction-command given to computer to perform specified task/operation
.Instruction contains two field
OPCODE OPERAND
OPCODE:-Operational code-Specifies the given task to be performed by the
computer
OPERAND- It is data to be operated
it is in the form of 8-bit or 16bit data/ address, internal register,
register and memory location
some cases,the operand or address/data may be absent in the
instruction.
INSTRUCTION FORMAT
ACC.to word size of instruction, the
instruction set classified as
❖ .1-byte instruction
❖ .2-byte instruction
❖ 3-byte instruction
1-byte instruction
.Includes the opcode and operand in the same byte
for operation to be performed
.The information regarding the operand is contained
in opcode itself.
Example:
1.MOV B,C (the contents o f B-register to
accumulator)
MOV-opcode
operand specified in the register B ,C
2) ADD B (To add the contents of B register to
accumulator content)
Operand B is specified, Accumulator assumed
3)CMA (Compliment the contents of Accumulator)
Operand A is absent ,the information regarding this
is specified in opcode itself
2 byte instruction
.Instruction requires 2 memory locations to store
binary codes
.first byte-opcode
Second byte-operand (data/address)
Example:
MVI A,14H-(Load 14H data in accumulator)
MVI-opcode-1ST BYTE
14H and A-in the 2nd byte
3 byte instruction
1st byte-OPCODE
2nd byte-low order byte of data or address
3rd –Higher orderbyte of data,/address
Example;
LDA address-LDA 2050H(load contents of memory 2050h into A)
OPCODE 3A- 1st
50-2nd (low order address)
20-3rd (high order address)
JMP 2085H,LDA address, STA address
Addressing modes
• The technique of specifying the data for instructions.
• The way of representing the operand in the instruction is
called addressing modes
• 5 addressing mode
1.Direct addressing
2.register addressing
3.Register indirect addressing
4.Immediate addressing
5.Implied/implicit addressing mode
DIRECT ADDRESSING MODE
.Inthis mode, the address of the operand is specified
within the instruction itself
Eg: STA 2010H (Store the content of accumulator in the
memory location 2010H)
2010H is the memory address, where data is stored.
.3-byte instruction format
REGISTER ADDRESSING MODE
.When the operands are in general purpose registers,
only those registers are used to be specified as the
address of operands
.1-byte instruction
Eg: MOV C,D.
ADD B
REGISTER INDIRECT ADDRESSING MODE
The address of the operand is specified by
.
a register pair.
.The content of the register pair are
assumed to be the address of operand
.The register pair is specified to contain 16-
bit address of operand
.1-byte instruction
.eg: MOV A,M,ADD M
IMMEDIATE ADDRESSING
.The operand is specified as data within the instruction itself.
.In this mode,the address of operand is not specified as in all
other modes
. 1-byte or 2-byte instructions within the instructions are used
to specify the data itself
Eg:for instruction using immediate addressing mode
LXI H,2100H (HL pair is loaded with the cntents of 2100H)
MVI A,04H (Get 04h to the register A)
IMPLIED OR IMPLICITE ADDRESSING
In the instruction,there is no operand,the
instructionis implied/impolite addressss
eg: , NOP, RIM etc…
Classification of instructions
8085 instructions have classified in to five
groups as per convenience of program
1-data transfer group
2 arithmetic group
3.Logical group
4.Branch group
5.stack,I/O and machine control group
