The way of specifying data to be operated by an instruction is known as addressing modes. This specifies that the given data is an immediate data or an address. It also specifies whether the given operand is register or register pair.

Here are the addressing modes discussed: Immediate: The operand is included in the instruction. Direct: The effective address of the operand in memory is part of the instruction. Indirect: The instruction contains a memory address, which contains the effective address of the operand in memory.

Applications of Addressing Modes-

These can also be said as the advantages of using addressing mode: To give programming versatility to the user by providing such facilities as pointers to memory, counter for loop control, indexing of data, and program relocation. To reduce the number of bits in the addressing field of the instruction.

An addressing mode specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere.

Immediate—Immediate addressing is not really an addressing mode into memory; rather, it is an instruction format that directly includes the data to be acted on as part of the instruction. This form of operand access simplifies the instruction execution cycle since no additional fetches are required.

[i′mēd·ē·ət in′strək·shən] (computer science) A computer program instruction, part of which contains the actual data to be operated upon, rather than the address of that data.

EA= PC + Address field value PC= PC + Relative value. Base register addressing mode:Base register addressing mode is used to implement inter segment transfer of control.In this mode effective address is obtained by adding base register value to address field value. EA= Base register + Address field value.

In direct addressing mode, the address field contains the address of the operand. Example: Add the contents of register 1001 to the accumulator….2. Immediate Addressing Mode:

In immediate addressing mode the source operand is always data. If the data is 8-bit, then the instruction will be of 2 bytes, if the data is of 16-bit then the instruction will be of 3 bytes. Examples: MVI B 45 (move the data 45H immediately to register B)

In direct addressing mode, address field in the instruction contains the effective address of the operand and no intermediate memory access is required. Now a days it is rarely used. Here 1001 is the address where operand is stored.

In the Motorola syntax, this addressing mode is indicated by enclosing the address register with round brackets. For example, the instruction MOVE. B (A0),D0 means move the contents of the memory location whose address is found in register A0 to the data register D0.

2) Index Mode Index Mode is used to access an array whose elements are in successive memory locations. The content of the instruction code, represents the starting address of the array and the value of the index register, and the index value of the current element.

An instruction format defines layout of bits of an instruction, in terms of its constituent parts. An instruction format must include an opcode and implicitly or explicitly, zero or more operands. Each explicit operand is referenced using one of addressing modes.

The definition of instruction is the act of educating, giving the steps that must be followed or an order. An example of instruction is someone giving another person detailed directions to the library.

Types of Instruction

• Direct instruction. The Direct instruction strategy is highly teacher-directed and is among the most commonly used.
• Indirect instruction.
• Independent study.
• Interactive instruction.
• Experiential Learning.

The instruction types include load and store with reservation, synchronization, and enforce in-order execution of I/O. They are especially useful for multiprocessing. Flow Control Instructions – These include branch, Condition-Register logical, trap, and other instructions that affect the instruction flow.

Three Categories of Instructions:

• Data manipulation. Arithmetic manipulation:
• Data transfer. transfer data between MM and CPU:
• Program control. branch to instruction other than the one following the current one conditionally or unconditionally (based on comparison between two operands or between one operand and zero):