Operand And Instructions Representation

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Operand And Instructions Representation

• By
• Dave Maung

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Different types of Operand/Instruction

• Zero Operands per instruction
• One Operands per instruction
• Two Operands per instruction
• Three operands per instruction

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Zero Operand Per Instruction

• Also known as Stack Architecture because Operand
  are kept in stack.
• To add an instruction using zero operand, take
  two values and add them together and put the
  result back to then stack.
• Operand must be implicit

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An example of Instruction used on a stack
computer

• Push X
• Push 7
• Add
• Pop X

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One Operand Per Instruction

• Architecture that limit each instruction to
  single operand is known as 1-address design.
• Relies on implicit operand for each instruction
  to store in special register known as an
  accumulator.

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Step to process one operand per Instruction

• Processor extracts the current value of the
  accumulator
• Performs the specified operation using the
  extracted value and the operand
• place the result back in the accumulator

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Example of One operand per instruction

• Add X
• Accumulator ? accumulator X

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Disadvantage of One Operand Per Instruction

• It does not allow instructions to specify two
  values.
• It requires two instructions that load the value
  into the accumulator and then need to store the
  values back in the new location

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Two Operands Per Instruction

• Overcomes the limitation 1-address systems
• Using 2 address process, an operation can be
  applied to a specified value.
• It also offers data movement instructions that
  treat the operand as source and destination.

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Example two operand per instruction

• Add X Y
• Specifies that the value of X is to be added to
  the current value of Y
• Y ? Y X

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Three Operand Per Instruction

• Similar as Two Operand Per Instruction except
  that it can operate three input value per
  instruction

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Example of three operand per instruction

• Add X Y Z
• Specifies an assignment of
• Z ? X Y

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Operand that specifies a source must be

• A signed constant
• An Unsigned constant
• The content of a register
• The value in a memory location

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Operand the specifies a destination must be

• A signed register
• A pair of contiguous registers
• A memory location

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Von Nuemann Boottleneck

• Operand addressing is the central weakness of a
  von Neumann architecture.
• memory access may become a bottleneck because
  instructions are stored in memory and a processor
  must make at least one memory per instruction.
• To Optimize the performance, operand must be
  taken from a register instead of memory.

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Types of Encoding

• 1. Explicit Operand Encoding
• 2. Implicit Operand Encoding

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Implicit Operand Encoding

• The Opcode specifies the types of operands.
• A processor that uses a implicit encoding
  contains multiple operand code for a given
  operation

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Example of Implicit Encoding
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Disadvantage of Implicit encoding

• Multiple opcodes are needed for a given
  operation.
• A list of opcodes can become large, when a
  processor allows many types of operands.

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Explicit Encoding

• Using Explicit can overcome the disadvantage of
  implicit encoding.
• Each operand is represented by two fields
• Type of Operand
• Type of Value

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Example of Explicit Operand Encoding
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Tradeoffs in the choice of operands

• Ease of Programming
• Fewer Instructions
• Smaller Instructions
• Larger Range of Immediate Values
• Faster Operand Fetch and Decode
• Decreased Hardware size

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Ease of Programming

• We know that complex forms of operands make
  programming easier
• 3-address approach mean we do not need to code
  separate instructions.

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Fewer Instructions

• Reduces the number of instruction in a program by
  increasing the expressive power of operands
• Lowers the count of instructions by increasing
  the number of addresses
• Disadvantage
• Each instruction will be larger

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Disadvantage of keeping smaller instruction

• Decreases the expressive power.
• Increases the number of instructions

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Smaller Instructions

• How to keep smaller Instruction?
• 1. Limiting the number of operands
• 2. Limiting the set of operands types
• 3. Limiting maximum size of an operand keep
  instruction small

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Larger Range of Immediate values

• Increasing the size allows larger values because
  the size of a field in the operand determines the
  numeric range of immediate values.
• Disadvantage
• Larger instruction results

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Faster Operand Fetch and Decode

• How to operate hardware faster?
• First, limit the number of operands
• Second, limit the possible types of each operand

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Decreased Hardware size

• How to decrease the hardware size?
• limits the types and complexity of operands
• Reduces the size of circuitry

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Values in Memory

• Every processors include at least one instruction
  that allows an operand to specify value in memory
• Values in the operand interpreted as an memory
  address.
• This memory address is for the professor to
  perform memory lookup.

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Advantage of memory look up

• Helps ease programming
• Helps allowing arbitrary instruction to
  reference memory.
• improves performance.

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Operand Addressing Modes

• Instruction register is used to hold an
  instruction that is being decoded.
• A immediate value can be found in instruction
  register

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Summary

• Covered different type of Operand instruction,
  Von Neumann Bottleneck, different choice of
  operand