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The various addressing modes in the intel 8085 microprocessor, including implied/implicit, immediate, direct, register, and register indirect addressing modes. Each mode is defined by how the source and destination operands are specified in the instruction.
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To perform any operation, we have to give the corresponding instructions to the microprocessor.
In each instruction, programmer has to specify 3 things:
(^) In implied/implicit addressing mode the operand is hidden and the data to be operated is available in the instruction itself. (^) If address of source of data as well as address of destination of result is fixed, then there is no need to give any operand along with the instruction. (^) Examples: (^) CMA (complement accumultor ) (^) RRC (rotate accumulator A right by one bit) (^) RLC (rotate accumulator A left by one bit)
(^) The data to be operated is available inside a memory location and that memory location is directly specified as an operand. (^) The operand is directly available in the instruction itself. (^) Examples: (^) LDA 2050 (load the contents of memory location into accumulator A) (^) LHLD address (load contents of 16-bit memory location into H-L register pair)
(^) In this mode, the operand is in general purpose register. (^) The data to be operated is available inside the register(s) and register(s) is(are) operands. Therefore the operation is performed within various registers of the microprocessor. (^) Examples: (^) MOV A, B (move the contents of register B to register A) (^) ADD B (add contents of registers A and B and store the result in register A) (^) INR A (increment the contents of register A by one)
(^) First of all the 16-bit address is placed on the address bus from the program counter. (^) Let say the address is 2005H where the data is placed. (^) The higher order address i.e. 20H is placed on the address bus A8-A while the lower order address i.e. 05h is placed on the multiplexed address and data bus ADO-AD7. (^) The lower order address continues to remain on this address bus so long as ALE (Address Latch Enable) remains positive. Once ALE goes low it carries data. (^) The control unit sends the signal to indicate what type of operation is to be performed. Since the data is to be read from the memory therefore it sends to enable the memory chip.
(^) The byte from the memory location is then placed on the data bus i.e. 4F saved in location 2005H is placed on the data bus and sent to the instruction decoder. (^) The instruction is decoded and accordingly the task is performed by the ALU i.e. Arithmetic and logic unit.