Adressing Modes and Instruction Cycle

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:

1. To give the programming versatility to the user.
2. To reduce the number of bits in addressing field of instruction.

Types of Addressing Modes

Below we have discussed different types of addressing modes one by one:

Immediate Mode

In this mode, the operand is specified in the instruction itself. An immediate mode instruction has an operand field rather than the address field.

For example: ADD 7, which says Add 7 to contents of accumulator. 7 is the operand here.

Register Mode

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.

Advantages

• Shorter instructions and faster instruction fetch.
• Faster memory access to the operand(s)

Disadvantages

• Very limited address space
• Using multiple registers helps performance but it complicates the instructions.

Register Indirect Mode

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.

Auto Increment/Decrement Mode

In this the register is incremented or decremented after or before its value is used.

Direct Addressing Mode

In this mode, effective address of operand is present in instruction itself.

• Single memory reference to access data.
• No additional calculations to find the effective address of the operand.

For Example: ADD R1, 4000 - In this the 4000 is effective address of operand.

NOTE: Effective Address is the location where operand is present.

Indirect Addressing Mode

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.

Displacement Addressing Mode

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.

Relative Addressing Mode

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)

Base Register Addressing Mode

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.

Stack Addressing Mode

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.

Instruction Cycle

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:

1. Fetch the Instruction

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.

2. Decode the Instruction

The instruction in the IR is executed by the decoder.

3. Read the Effective Address

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.

4. Execute the 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.