4.3 RISC-V Datapath Design

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Questions and Answers

What does it indicate when the Zero signal from the ALU is asserted?

It indicates that the register values are equal.

How is the branch instruction offset modified for execution?

The branch instruction offset is modified by adding the PC to the instruction's 12 bits shifted left by 1 bit.

What is a significant challenge when creating a single datapath for instruction execution?

A significant challenge is that no datapath resource can be used more than once per instruction.

Why is separate memory needed for instructions and data in a single datapath implementation?

<p>Separate memory is needed because the simplest datapath executes all instructions in one clock cycle, requiring distinct resources for instruction and data.</p> Signup and view all the answers

What is the role of control signals in the datapath for executing multiple instruction classes?

<p>Control signals determine how the various datapath components are connected and operate for different instruction classes.</p> Signup and view all the answers

What is the purpose of the program counter (PC) in the RISC-V datapath?

<p>The program counter (PC) holds the address of the current instruction being executed.</p> Signup and view all the answers

Why is it necessary to have an adder within the RISC-V datapath?

<p>The adder is used to increment the program counter (PC) to point to the next instruction's address.</p> Signup and view all the answers

Describe the role of control signals in the datapath components.

<p>Control signals determine the operation of datapath elements, such as directing an ALU to perform an addition.</p> Signup and view all the answers

What distinguishes a control signal from a data signal in the context of a datapath?

<p>A control signal instructs components on how to operate, while a data signal carries the actual data being processed.</p> Signup and view all the answers

What is the significance of the register file being read and written in the same clock cycle?

<p>It allows for fast and efficient access to register data, which is essential for executing instructions in RISC-V architecture.</p> Signup and view all the answers

What is the role of the program counter (PC) in the instruction fetching process?

<p>The program counter (PC) holds the address of the current instruction and increments to point to the next instruction, typically 4 bytes later.</p> Signup and view all the answers

Describe the function of R-format instructions within the context of the processor's datapath.

<p>R-format instructions read two registers, perform an ALU operation on their contents, and write the result back to a register.</p> Signup and view all the answers

Explain why instruction memory is treated as combinational logic in the datapath.

<p>Instruction memory is treated as combinational logic because it provides output based solely on the address input at any time, without requiring write access during instruction execution.</p> Signup and view all the answers

How does the adder function in relation to the program counter and instruction fetching?

<p>The adder takes the current value of the program counter and computes the address of the next instruction by adding 4 to it.</p> Signup and view all the answers

What characteristics of the program counter enable it to function effectively at the end of each clock cycle?

<p>The program counter is a 32-bit register that is automatically written at the end of every clock cycle, eliminating the need for a separate write control signal.</p> Signup and view all the answers

Flashcards

Program Counter (PC)

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

Instruction Memory

A component that stores instructions in a program.

R-format Instructions

Instructions that read two registers, perform an ALU operation, and write the result to a register.

ALU Operation

Arithmetic or logical operation performed by the Arithmetic Logic Unit (ALU).

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Instruction Fetch

The process of retrieving an instruction from memory.

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Incrementing the PC

Adding a fixed value (usually 4 bytes) to the program counter to get the address of the next instruction.

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RISC-V Datapath

A pathway for data movement and operations in a RISC-V processor, involving components like instruction memory, registers, ALU, and adders.

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Register File

A collection of registers, used to store data temporarily in a processor, used for both reading and writing.

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Instruction Memory

Memory storing program instructions.

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Program Counter (PC)

A register holding the memory address of the current instruction.

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Adder (datapath element)

A combinational circuit that adds two numbers used to increment the program counter for the next instruction.

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ALU

Arithmetic Logic Unit, a component that performs arithmetic and logical operations in a processor.

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Datapath elements

The components used for operating, holding, or transferring data, including the instruction and data memories, the register file, the ALU, and adders.

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64-bit RISC-V architecture

A version of the RISC-V architecture that processes 64 bits of data at a time.

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ALU Zero Signal

Signals if the result of an ALU operation is zero, used for equality checks in conditional branches.

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Branch Instruction

An instruction that modifies the program counter (PC) by adding an offset value, allowing conditional jumps.

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Branch Offset

The 12-bit value in a branch instruction, adding this offset to PC to compute a new PC address.

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Single Datapath

A complete datapath combining all components needed to execute different instruction types, aiming for one-cycle execution.

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Instruction Memory (separate from Data Memory)

Separate memory for storing instructions, distinct from the memory used for storing data.

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Duplicated Functional Units

Functional units that might need to be duplicated in a single datapath to avoid conflicts when different instructions need to use them during the same clock cycle.

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One-Clock Cycle Execution

The goal of the single datapath design is to execute each instruction in a single clock cycle.

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Study Notes

Building a Datapath

  • A reasonable datapath design starts by examining major components needed to execute RISC-V instructions.
  • The design progresses from the top down, observing which datapath elements each instruction needs.
  • Abstraction is used in the explanation, starting with the bottom-up approach. Control signals are shown alongside datapath elements.

Instruction Memory and Program Counter

  • Figure 4.5a shows a memory unit storing program instructions accessible by address.
  • Figure 4.5b shows the Program Counter (PC) as a register holding the current instruction's address.
  • An adder is needed to increment the PC to the next instruction address. This adder can be built from an Arithmetic Logic Unit (ALU) configured for addition operation.

Datapath for Instruction Fetching

  • Figure 4.6 illustrates combining elements (memory, PC, adder) to fetch instructions and increment the PC.

R-Format Instructions

  • R-format Instructions (Figure 2.19, page 127):
    • Read two registers.
    • Perform an ALU operation on register contents.
    • Write the result to a register.

Register File

  • The processor's 32 general-purpose registers are stored in a register file.
  • Any register can be read or written by specifying its number within the file.
  • The register file holds the register state of the computer.
  • The register file needs two read ports and one write port.

Arithmetic Logic Unit (ALU)

  • An ALU is needed to operate on register values.
  • The ALU takes two 32-bit inputs and produces a 32-bit result.
  • A 4-bit control signal dictates the ALU operation.

Data Memory

  • Needed to fetch and write data; has read and write control signals, address input and input for data to be written.
  • Figure 4.8 illustrates the data memory unit and its input/outputs.

Immediate Generation Unit (ImmGen)

  • Converts 12-bit instruction offset into 32-bit signed value for load, store.
  • ImmGen uses instruction opcode to select the appropriate 12-bit field (for load, store, and conditional branch)
  • Figure 4.8 illustrates the Immediate Generation Unit.

Branch Instructions (e.g., beq)

  • Branch instructions (e.g., beq x1, x2, offset) compute the branch target address by adding the sign-extended offset field and the PC.
  • They calculate whether the target address becomes the new PC or if the program proceeds sequentially (branch not taken).
  • Figure 4.9 illustrates the datapath structure for branch instructions; it involves immediate generation and comparison using the ALU.

Datapath for memory instructions and R-type Instructions

  • Figure 4.10 diagrams a single datapath for both memory and R-type instructions, showing the need for multiplexors (for ALU and Register file data inputs).
  • The figure emphasizes how a single datapath can be implemented using multiplexors to select appropriate inputs based on the instruction type.

Combined Datapath (Figure 4.11)

  • The final combined datapath integrates components for instruction fetch, R-type/memory instructions and branches on a single design.
  • Multiplexors are used for different data and control inputs.

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