What is Addressing Mode in Computer Architecture
This lecture is dedicated to students and learners those are eager to know the concept of Addressing mode in computer architecture.
by seeing this lecture you will be able to understand the concept of addressing mode. what is it and its significance and different types.
Addressing modes are an aspect of the instruction set architecture(ISA) in CPU designs.
The various addressing modes are defined in a ISA that define “ how machine language identify the operand of each instruction”.
Purpose : To reduce the number of bits in addressing field of instruction.
In computer architecture, Addressing modes are the manner or way to access or specify the data.
“ The way in which “operand of an instruction is specified”.
“The technique for specifying the address of an operands are known as Addressing Modes”.
It works like a pointer that indicate the data/ operand location in memory / register.
The addressing mode specifies a rule for interpreting or modifying the address field of an instruction.
The operation field of an instruction specifies the operation to be performed.
The way , the operands are chosen during program execution is dependent on the addressing mode of the instruction.
Types :
Depending upon the type of instruction applied, addressing modes are of various types such as :-
Direct addressing mode: The instruction contains the address of data(operand) in memory (in which the address field contains the effective address of the -operand).
“ To access the data directly from memory(storage location)”.
Ex: LDA A Load the operand stored @ location A.
Indirect Addressing(Register Indirect Addressing): The operand is present @ specified location of memory but its address is stored into a Register.
“ To access the data indirectly from memory(storage location)”.
Register Addressing: The operands are stored in Registers, the instruction then specify the address of Register(Address field refers to a register rather than a main memory address) .
Ex: MOV R1,R2 transferred data are stored from R1 to R2.
Immediate addressing: The operand itself is given in the instruction Or The operand is an immediate value is stored explicitly in the instruction (the operand is actually present in the instruction).
The simplest form of addressing is immediate addressing, in which the operand is actually present in the instruction : OPERAND = A
Ex: LDA 10101 (Load operand 10101 into AC)
Implicit addressing(Implied Addressing): The location of operand is understood from the instruction itself (Either the source or destination address (or sometimes both) is implied by the opcode)
“ Access the data implicitly by AC(Accumulator)”.
Ex: CMA (compliment Accumulator) implicitly specifies that the operand is in AC.
For example, the operation a := b + c; can be done using the sequence load b; add c; store a; the destination (the accumulator) is implied in every "load" and "add" instruction; the source (the accumulator) is implied in every "store" instruction.
Indexed Addressing(indexed registers): This register contains the address of operand with respect to a fixed memory address.
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