Computer Architecture: Registers and Clocking
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Questions and Answers

What is the base for the branch address calculation according to the instruction set architecture?

  • The address of the previous instruction
  • The address of the next instruction
  • The address of the memory location
  • The address of the branch instruction (correct)

    • What happens to the offset field in the branch instruction?

  • It is shifted left 1 bit (correct)
  • It is shifted right 1 bit
  • It is divided by 2
  • It is multiplied by 2

    • What is the purpose of shifting the offset field?

  • To reduce the number of bits required
  • To make the calculation more complex
  • To decrease the range of the offset field
  • To increase the range of the offset field (correct)

    • What happens when the condition is true in a branch instruction?

    <p>The branch target address becomes the new PC</p> Signup and view all the answers

    What are the two operations performed by the branch datapath?

    <p>Computing the branch target address and testing the register contents</p> Signup and view all the answers

    What is the function of the immediate generation unit in the branch datapath?

    <p>To generate the branch target address</p> Signup and view all the answers

    What is the total number of steps involved in the execution of an R-type instruction?

    <p>4</p> Signup and view all the answers

    What is the function of the main control unit during the execution of an R-type instruction?

    <p>It computes the setting of the control lines</p> Signup and view all the answers

    How many registers are read from the register file during the execution of an R-type instruction?

    <p>2</p> Signup and view all the answers

    What is the input to the control unit in the simple datapath?

    <p>The 7-bit opcode field</p> Signup and view all the answers

    What happens to the PC during the execution of an R-type instruction?

    <p>It is incremented</p> Signup and view all the answers

    How many clock cycles are required to execute an R-type instruction?

    <p>1</p> Signup and view all the answers

    What is the purpose of the opcode field in an instruction?

    <p>To denote the operation and format of an instruction</p> Signup and view all the answers

    What is the purpose of the rs1 field in an instruction?

    <p>To specify the base register for load and store instructions</p> Signup and view all the answers

    What is the purpose of the rs2 field in an instruction?

    <p>To specify the register operand that gets copied to memory for store instructions</p> Signup and view all the answers

    What is the purpose of the rd field in an instruction?

    <p>To specify the destination register</p> Signup and view all the answers

    What is the benefit of the first design principle from Chapter 2?

    <p>It simplifies the control of the datapath</p> Signup and view all the answers

    How does RISC-V compare to MIPS in terms of instruction formats?

    <p>RISC-V has instruction formats that look more complicated but actually simplify the hardware</p> Signup and view all the answers

    Why is the single-cycle design not used in modern processors?

    <p>Because it is too inefficient due to the longest possible path in the processor</p> Signup and view all the answers

    What determines the clock cycle in a single-cycle design?

    <p>The longest possible path in the processor</p> Signup and view all the answers

    What is the CPI (Cycles Per Instruction) for a single-cycle implementation?

    <p>1</p> Signup and view all the answers

    Why was the single-cycle design used in early computers?

    <p>Because it was simple and suitable for simple instruction sets</p> Signup and view all the answers

    What is the problem with using a single-cycle design with a fixed clock cycle?

    <p>It results in a significant penalty in performance</p> Signup and view all the answers

    Why wouldn't the single-cycle design work well with a floating-point unit or complex instructions?

    <p>Because it would not be able to handle the complexity of the instructions</p> Signup and view all the answers

    What is the primary reason for specifying a clocking methodology in computer design?

    <p>To make the hardware more predictable</p> Signup and view all the answers

    What is the purpose of an edge-triggered clocking methodology?

    <p>To update state elements only on a clock edge</p> Signup and view all the answers

    What is the significance of a clock edge in an edge-triggered clocking methodology?

    <p>It is a quick transition from low to high or vice versa</p> Signup and view all the answers

    Why do combinational logic elements need to have their inputs come from a set of state elements?

    <p>Because they need stable input values</p> Signup and view all the answers

    What is the consequence of writing and reading a signal at the same time in a computer design?

    <p>The value of the read could be unpredictable</p> Signup and view all the answers

    What determines the output values of combinational logic elements?

    <p>The inputs from state elements</p> Signup and view all the answers

    Study Notes

    Branch Instructions

    • Branch instructions have two important details:
      • The base for branch address calculation is the address of the branch instruction itself.
      • The offset field is shifted left 1 bit, making it a half-word offset, and increasing the effective range by a factor of 2.

    Branch Datapath

    • The branch datapath must perform two operations:
      • Compute the branch target address.
      • Test the register contents to determine if the branch is taken or not.
    • The datapath includes an immediate generation unit and an adder.

    Instruction Execution

    • A simple implementation scheme involves four steps:
      • Instruction fetch and PC increment.
      • Reading two registers from the register file.
      • Executing the instruction.
      • Writing the result to the register file.
    • These steps occur in one clock cycle.

    Instruction Format

    • The opcode field is always in bits 6:0.
    • The funct3 field (bits 14:12) and funct7 field (bits 31:25) serve as an extended opcode field.
    • The first register operand is always in bits 19:15 (rs1) for R-type and branch instructions.
    • The second register operand is always in bits 24:20 (rs2) for R-type and branch instructions.

    Single-Cycle Implementation

    • A single-cycle implementation has a fixed clock cycle, which is determined by the longest possible path in the processor.
    • This path is likely a load instruction, which uses five functional units in series.
    • Although the CPI is 1, the overall performance of a single-cycle implementation is likely to be poor due to the long clock cycle.

    Clocking Methodology

    • A clocking methodology defines when signals can be read and written.
    • An edge-triggered clocking methodology means that values stored in a sequential logic element are updated only on a clock edge.
    • This methodology ensures that hardware is predictable and that values are not read and written simultaneously.

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    Description

    This quiz covers the operation of registers and clocking methodology in computer architecture, including the timing of reads and writes.

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