4.1 Computer Architecture RISC-V Chapter

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Questions and Answers

What role does the program counter (PC) play in the RISC-V instruction execution process?

The program counter provides the instruction address to the instruction memory, starting the process of fetching the instruction.

How are register operands specified in a RISC-V instruction?

Register operands are specified by fields within the instruction itself.

What happens to the ALU result when an arithmetic-logical instruction is executed?

The result from the ALU is written back to a register.

In a load or store operation, what does the ALU result represent?

<p>The ALU result serves as the address for either loading a value from memory into the registers or storing a value from the registers into memory.</p> Signup and view all the answers

How does the RISC-V architecture determine the next instruction address when executing a branch?

<p>The next instruction address is determined by the ALU output, which can either sum the PC and branch offset or increment the PC by four.</p> Signup and view all the answers

What are the two essential steps that occur for every RISC-V instruction implementation?

<p>Fetch the instruction from memory using the program counter (PC) and read one or two registers based on instruction fields.</p> Signup and view all the answers

How does the subset of RISC-V instructions affect the design of the datapath?

<p>The subset influences the complexity and organization of the datapath, focusing on essential operations while omitting advanced features like shifting and multiplication.</p> Signup and view all the answers

Explain the significance of the instruction set architecture in the implementation of RISC-V.

<p>The instruction set architecture determines many implementation aspects, including the structure of the datapath and control logic.</p> Signup and view all the answers

What is the primary difference in actions required after reading registers for different instruction classes in RISC-V?

<p>The actions vary based on the instruction class, as they dictate how to manipulate data or control the flow of execution.</p> Signup and view all the answers

In what ways does the RISC-V implementation illustrate the principle of 'Simplicity favors regularity'?

<p>By maintaining a consistent set of operations for different instruction types, the design simplifies implementations and enhances predictability.</p> Signup and view all the answers

What role does the ALU play in the execution of RISC-V instructions?

<p>The ALU is used for address calculation in memory-reference instructions and for performing operations in arithmetic-logical instructions.</p> Signup and view all the answers

How does a conditional branch instruction determine the next instruction address?

<p>A conditional branch instruction compares values and can change the next instruction address accordingly; if no change is needed, the program counter (PC) increments by four.</p> Signup and view all the answers

Why can't data lines simply be wired together in a RISC-V implementation?

<p>Data lines cannot be simply wired together because the system requires a multiplexor to select one source from multiple inputs based on control signals.</p> Signup and view all the answers

What is the function of a multiplexor in a RISC-V processor?

<p>A multiplexor serves as a data selector that chooses among several inputs to steer one to its destination based on control lines derived from the instruction being executed.</p> Signup and view all the answers

How do the actions required for memory-reference instructions differ from those for arithmetic-logical instructions?

<p>Memory-reference instructions require access to memory for reading or writing data, while arithmetic-logical instructions involve writing results from the ALU or memory back into a register.</p> Signup and view all the answers

Flashcards

RISC-V Instruction Subset

A selection of core RISC-V instructions, including memory-reference (lw, sw), arithmetic-logical (add, sub, and, or), and conditional branch (beq) instructions, excluding shifts, multiplies, divides, and floating-point instructions.

Instruction Fetch

The process of retrieving an instruction from memory using the program counter (PC).

Register Read

Reading data from one or two registers, identified by fields within the instruction.

Instruction Class

Categorization of instructions based on their functionalities (memory-reference, arithmetic-logical, or branch).

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Program Counter (PC)

A register that holds the memory address of the next instruction to be executed.

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Data Path

The physical components and connections within a computer that perform the actions specified by instructions.

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Control Unit

The component that determines the sequence of operations needed to carry out instructions, coordinating the actions of the data path.

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Clock Rate

Speed of the processor's clock, influencing the instruction execution time.

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CPI

Cycles per instruction; a measurement of how many clock cycles are required to execute one instruction.

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Register File Write

The register file is updated with a new value only during a load operation or after an ALU computation.

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Instruction Fetch

The process of obtaining an instruction from the memory using the program counter (PC) address.

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Register Operands

Values stored in designated registers that an instruction uses.

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ALU Operation

Arithmetic and logical operations performed by the Arithmetic Logic Unit (ALU).

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Memory Address Calculation

Calculating memory addresses, typically for load/store instructions with ALU.

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Branch Instruction

An instruction that changes the program counter's value based on an ALU's result or condition.

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Program Counter (PC)

A special register that holds the memory address of the next instruction to be fetched.

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RISC-V instruction set simplicity

The RISC-V instruction set is designed to make the implementation of many instructions similar, using a common arithmetic-logical unit (ALU).

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ALU in Instruction Execution

All instruction classes in RISC-V use the ALU after register reading, whether for address calculation (memory), operation execution (arithmetic), comparison (branches).

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Memory-reference instructions

These instructions access memory for either reading data (load) or writing data (store) after the ALU operation.

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Arithmetic/logic instructions

These instructions execute operations using the ALU and write the results to a register.

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Conditional branch instructions

These instructions modify the program counter (PC) based on the comparison result, or increment the PC by four if no branching occurs.

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Multiplexor (Data Selector)

A device that chooses between multiple data sources to send data to a single destination, as needed in processor circuits.

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Instruction-dependent Control

Different instruction types require control signals to different parts of the processor, like enabling memory read/write on specific instructions.

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Program Counter (PC)

A register that holds the address of the next instruction to be executed.

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Study Notes

4.1 Introduction

  • Computer performance depends on instruction count, clock cycle time, and clock cycles per instruction (CPI).
  • Instruction count is determined by the compiler and instruction set architecture.
  • Processor implementation determines clock cycle time and CPI.
  • This chapter details datapaths and control units for RISC-V instruction set implementations.
  • A simple overview of processor implementation principles is presented initially.
  • Pipelined RISC-V implementation is followed by more complex instruction sets like x86.

Basic RISC-V Implementation

  • Implementation includes a subset of core RISC-V instructions.
    • Memory-reference instructions (lw, sw)
    • Arithmetic-logical instructions (add, sub, and, or)
    • Conditional branch instruction (beq)
  • This subset excludes shift, multiply, divide, and floating-point instructions.

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