Clocking Methodology in Computer Architecture: Quiz and Flashcards

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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 branch instruction

What happens to the offset field in the branch instruction?

It is shifted left 1 bit

What is the purpose of shifting the offset field?

To increase the range of the offset field

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

The branch target address becomes the new PC Signup and view all the answers

What are the two operations performed by the branch datapath?

Computing the branch target address and testing the register contents Signup and view all the answers

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

To generate the branch target address Signup and view all the answers

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

4 Signup and view all the answers

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

It computes the setting of the control lines Signup and view all the answers

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

2 Signup and view all the answers

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

The 7-bit opcode field Signup and view all the answers

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

It is incremented Signup and view all the answers

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

1 Signup and view all the answers

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

To denote the operation and format of an instruction Signup and view all the answers

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

To specify the base register for load and store instructions Signup and view all the answers

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

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

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

To specify the destination register Signup and view all the answers

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

It simplifies the control of the datapath Signup and view all the answers

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

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

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

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

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

The longest possible path in the processor Signup and view all the answers

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

1 Signup and view all the answers

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

Because it was simple and suitable for simple instruction sets Signup and view all the answers

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

It results in a significant penalty in performance Signup and view all the answers

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

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

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

To make the hardware more predictable Signup and view all the answers

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

To update state elements only on a clock edge Signup and view all the answers

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

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

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

Because they need stable input values Signup and view all the answers

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

The value of the read could be unpredictable Signup and view all the answers

What determines the output values of combinational logic elements?

The inputs from state elements Signup and view all the answers

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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.