Parul Polytechnic Institute Subject Code : Name Of Subject : Microprocessor and assembly language programming Name of Unit : Instruction cycle and Timing diagram Topic : 8085 instruction and timing Name of Faculty : H.M.Avaiya & N.D.Dhameliya Name of Students: (i) Muskan Sharma(181) (ii) Khanal Vonodbahadur(169)

ADDRESSING MODES An instruction specifies opcode and operands. The operands are data values on which operation is to be performed. There are various source in which operands can be stored like CPU registers or memory location. It is also possible that sometimes operands are specified within instruction itself. An instruction specifies from which of above source the operands for the operation to be performed are available. The various ways of specifying operands for an instruction are called addressing modes.

The 8085 uses mainly five different addressing modes as follows:  Implied addressing  Register addressing  Immediate addressing  Direct addressing  Indirect addressing

Implied addressing : In implied addressing,the addressing mode of an instruction is implied by the function of instruction itself. Implied instruction generally does not use any register or memory location. For example, STC (set carry flag) sets the CY flag to 1. CMC (complement carry) instruction is another example of implied addressing mode instruction.

Register addressing:  The addressing mode of an instruction is register.  If it specifies its operands using register only.  For example, MOV A,B uses register B as source register and the accumulator as destination register.  The register instructions are always single byte instruction which occupies in 1-byte in storage.

 They are also faster in execution as they do not need to refer any memory location for operands.  The memory operation is relatively slow operation.  Fig. shows the execution of MOV A,B instruction.  After execution, the accumulator gets the contents of register B.  The contains of register B remains unchanged.

Immediate addressing:  Immediate instruction store the data immediately after the opcode in the memory.  For example, MVI A,32h stores 8-bit data 32h immediately after the opcode in the memory as Instruction Memory location Machine code MVI A, 32h 2000h 2001h 3Eh (opcode) 32h (data)

Direct addressing:  The instruction that use direct addressing mode are 3-byte instruction.  The first byte store the opcode,while second and third byte specifies the 16-bit direct address as OPCODE first byte Lower byte of address second byte Higher byte of address third byte

Example, LDA 3050h loads the content of memory location by 16-bit address into the accumulator. Fig shows the execution of LDA 3050h instruction.

Indirect addressing:  The instruction that use the indirect addressing specifies the address of an operand indirectly into the register pair.  For example, MOV A, M instruction copies the contents of memory location specified by HL register pair into the accumulator.  The execution of MOV A, M instruction is shown in fig.

Combined addressing:  Sometimes an instruction follows more than one than one addressing mode.  If an instruction follows more than ane addressing mode, it is known as combined addressing.  For example, MVI M, 35h follows immediate and indirect addressing both.  As it loads immediate value 35h into memory location.  It is immediate addressing.  The address of memory location is indirectly given in HL registerpair.  So it is also indirect.

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