13.2.7 Worked Examples: A Better Beta | Summary and Q&A

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July 12, 2019
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13.2.7 Worked Examples: A Better Beta

TL;DR

This content discusses the addition of new instructions to the Beta computer architecture, including SWAPR, NEG, PC-relative Store, and BITCLR, and explores the requirements for implementing them.

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Questions & Answers

Q: What is the requirement for adding the SWAPR instruction to the Beta computer architecture?

Adding the SWAPR instruction requires the addition of new hardware because the existing Beta hardware cannot write to two different registers in the same clock cycle.

Q: How can the NEG instruction be implemented in the Beta architecture?

The NEG instruction can be implemented using a macro that subtracts the register's value from R31 and stores the result in another register, except for the largest representable negative number.

Q: Why can't the PC-relative store instruction be implemented as a macro?

The PC-relative store instruction's behavior differs from the existing store operation in Beta, making it impossible to implement as a macro.

Q: What changes are required in the Beta datapaths to implement the PC-relative store instruction?

Implementing the PC-relative store instruction requires additional hardware, such as an extra adder, to compute the memory address based on the program counter and a scaled constant value.

Summary & Key Takeaways

  • The content explains the process of adding new instructions to the Beta computer architecture, considering the minimum requirements for each instruction.

  • The SWAPR instruction, which swaps the contents of two registers in a single clock cycle, requires new hardware to be added.

  • The NEG instruction, which takes the two's complement negation of a register, can be implemented using a macro, except for the largest representable negative number.

  • The PC-relative store instruction cannot be implemented as a macro and requires changes to the Beta datapaths and control ROM.

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