Chapter 5.3, Problem 16CYU

Program Plan Intro

Addressing instructions:

• Instruction consists of two parts as operands and opcode.
• Opcode defines a mnemonic to represent the operation which is to be performed.
• Operands are the variables on which the operations are performed.
• Depending on the technique employed to store the operands, instruction formats are categorized into two types. They are as follows.
  • Two address instruction
  • Three address instruction.

Two address instruction:

• This format uses two registers for computation purposes.
• In this case, there is often a situation that an address is overwritten.
• Example: The following example show the two address method. When the registers used R1 and R2.

        $\begin{array}{l}\text{R1}\leftarrow \text{X; stores X to R1}\\ \text{R2}\leftarrow \text{R1+Y; adds Y to R1}\\ \text{R1}\leftarrow \text{C};\text{stores}\text{Z}\text{to}\text{R1}\\ \text{R1}\leftarrow \text{R2*R1;}\text{stores}\text{the}\text{result}\text{of}\text{R2*R1}\text{into}\text{R1}\end{array}$

Three address instruction:

• This format uses three register to perform the computation. Two registers for storing the operands and the third register for storing the result.
• Example: If one have to perform the following computation using three address: A+B*C.
• If 3 registers used for the computation R1, R2, and R3.
• Then the sequence of operations will be as follows.

                    $\begin{array}{l}\text{R1}\leftarrow \text{A; stores A to R1}\\ \text{R2}\leftarrow \text{B; }\text{stores}\text{B}\text{in}\text{register R2}\\ \text{R3}\leftarrow \text{R1+R3;}\text{stores}\text{the}\text{result}\text{of}\text{R1+R2}\text{into}\text{R3}\\ \text{R1}\to \text{C;}\text{ stores C in register R3}\\ \text{R2}\leftarrow \text{R1*R3; performs the computation of R1*R2 and stores the result in R2}\text{.}\end{array}$