Digital Electronics: Principles, Devices and Applications

Microprocessors 535OperationAddressMemoryData(a)MOV A,30H09H07H22H31H30H29H(b)Figure 13.5Absolute addressing mode.OperationDataFigure 13.6Immediate addressing mode.13.5.2 Immediate Addressing ModeIn immediate addressing mode the value of the operand is held within the instruction itself (Fig. 13.6).This mode is useful for accessing constant values in a program. It is faster than the absolute addressingmode and requires less memory space. For example, the instruction MVI A, #30H moves the datavalue 30H into the accumulator. The sign # in the instruction tells the assembler that the addressingmode used is immediate.13.5.3 Register Direct Addressing ModeIn register direct addressing mode, data are accessed by specifying the register name in which theyare stored [Fig. 13.7(a)]. Operations on registers are very fast, and hence instructions in this moderequire less time than absolute addressing mode instructions. As an example, the instruction MOV A,R1 in the 8051 microprocessor moves the contents of register R1 into the accumulator [Fig. 13.7(b)].The contents of the accumulator after the instruction are 06H.13.5.4 Register Indirect Addressing ModeIn all the modes discussed so far, either the value of the data or their location is directly specified. Theindirect addressing mode uses a register to hold the actual address where the data are stored. That is,in this case the memory location of the data is stored in a register [Fig. 13.8(a)]. In other words, inindirect addressing mode, the address is specified indirectly and has to be looked up. This addressingmode is useful when implementing the pointer data type of high-level language.In the 8085 microprocessor, the R0 and R1 registers are used as an eight-bit index and the DPTR asa 16-bit index. The mnemonic symbol used for indirect addressing is @. As an example, the instruction