9.2.6 Constant Operands | Summary and Q&A

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July 12, 2019
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9.2.6 Constant Operands

TL;DR

Small constant operands in the Instruction Set Architecture (ISA) can optimize program size and performance, but may require additional hardware.

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Questions & Answers

Q: What are "features" in the context of ISA design?

In ISA design, "features" refer to additional instructions or modifications that can enhance the ISA's capabilities or improve its performance in certain scenarios.

Q: What is the benefit of allowing small constants as the second operand in ALU instructions?

Allowing small constants as the second operand in ALU instructions eliminates the need for load operations to fetch constant values from the memory, making programs shorter and potentially faster.

Q: What is the drawback of including small constant operands in the ISA?

The inclusion of small constant operands requires additional control and datapath logic to implement, increasing hardware costs and potentially impacting performance.

Q: How was the impact of small constant operands evaluated?

The impact was evaluated by modifying benchmark programs to use the ISA augmented with the feature and measuring the impact on simulated execution. The focus was not only on the code itself but on the instructions actually executed during program execution, considering that instructions within loops may be executed multiple times.

Summary & Key Takeaways

  • ISA designers often receive requests for additional instructions, called "features," that aim to improve the ISA in some way.

  • One such feature request is the inclusion of small constants as the second operand in ALU instructions, which can make programs shorter by eliminating the need for load operations to read constant values from memory.

  • Evaluating the impact of this feature using benchmark programs shows that small constants are frequently used as the second operands of arithmetic instructions and comparisons and are commonly found in address calculations. Adding support for small constant operands results in smaller and faster programs.

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