The operation field of an instruction specifies the operation to be performed. This operation will be executed on some data which is stored in computer registers or the main memory. The way any operand is selected during the program execution is dependent on the addressing mode of the instruction. The purpose of using addressing modes is as follows:
Below we have discussed different types of addressing modes one by one:
In this mode, the operand is specified in the instruction itself. An immediate mode instruction has an operand field rather than the address field.
ADD 7, which says Add 7 to contents of accumulator. 7 is the operand here.
In this mode the operand is stored in the register and this register is present in CPU. The instruction has the address of the Register where the operand is stored.
In this mode, the instruction specifies the register whose contents give us the address of operand which is in memory. Thus, the register contains the address of operand rather than the operand itself.
In this the register is incremented or decremented after or before its value is used.
In this mode, effective address of operand is present in instruction itself.
ADD R1, 4000 -
In this the 4000 is effective address of operand.
NOTE: Effective Address is the location where operand is present.
In this, the address field of instruction gives the address where the effective address is stored in memory. This slows down the execution, as this includes multiple memory lookups to find the operand.
In this the contents of the indexed register is added to the Address part of the instruction, to obtain the effective address of operand.
EA = A + (R), In this the address field holds two values, A(which is the base value) and R(that holds the displacement), or vice versa.
It is a version of Displacement addressing mode.
In this the contents of PC(Program Counter) is added to address part of instruction to obtain the effective address.
EA = A + (PC), where EA is effective address and PC is program counter.
The operand is A cells away from the current cell(the one pointed to by PC)
It is again a version of Displacement addressing mode. This can be defined as
EA = A + (R), where A is displacement and R holds pointer to base address.
In this mode, operand is at the top of the stack. For example:
ADD, this instruction will POP top two items from the stack, add them, and will then PUSH the result to the top of the stack.
An instruction cycle, also known as fetch-decode-execute cycle is the basic operational process of a computer. This process is repeated continuously by CPU from boot up to shut down of computer.
Following are the steps that occur during an instruction cycle:
The instruction is fetched from memory address that is stored in PC(Program Counter) and stored in the instruction register IR. At the end of the fetch operation, PC is incremented by 1 and it then points to the next instruction to be executed.
The instruction in the IR is executed by the decoder.
If the instruction has an indirect address, the effective address is read from the memory. Otherwise operands are directly read in case of immediate operand instruction.
The Control Unit passes the information in the form of control signals to the functional unit of CPU. The result generated is stored in main memory or sent to an output device.
The cycle is then repeated by fetching the next instruction. Thus in this way the instruction cycle is repeated continuously.