CAP 2 - Processor Operation and RISC/CISC Processors

Questions and Answers

What are the five basic parts of a processor as illustrated in the von Neumann machine?

Memory, Arithmetic Logic Unit, Program Control Unit, Input Equipment, Output Equipment

Which phase involves write back to memory and is usually slower than the other phases in instruction execution?

Memory Access Phase

What is the main differentiation in terms of instruction set architectures classes?

Internal Storage Type

In which storage type are both instructions and operands stored in memory following a specific abstract data type?

Stack Storage Type Signup and view all the answers

Which special register is involved in instructions when using the accumulator storage type?

ACC register Signup and view all the answers

What types of instructions and operands are implicitly on the top of stack in the stack storage type?

ALU operands Signup and view all the answers

Which of the following statements about the accumulator architecture code and the GPR (register-memory) architecture code is correct?

The GPR architecture code has 31.5% fewer memory accesses compared to the accumulator architecture code. Signup and view all the answers

Why are instructions with very different execution times or very different numbers of phases not suitable for a production line (pipeline)?

They can cause pipeline stalls, reducing overall performance. Signup and view all the answers

What is the main advantage of using a Reduced Instruction Set Computer (RISC) architecture over a Complex Instruction Set Computer (CISC) architecture?

Simplified control logic and faster execution times. Signup and view all the answers

In the context of a GPR (register-memory) architecture, what is the purpose of the control unit?

To generate control signals that coordinate the execution of instructions. Signup and view all the answers

What is the main drawback of a single-cycle datapath processor design?

Lower performance compared to pipelined processor designs. Signup and view all the answers

Which component of a processor is responsible for fetching instructions from memory and providing them to the control unit?

Instruction Fetch Unit Signup and view all the answers

Which of the following is NOT one of the five basic parts of a processor with accumulator architecture?

Program status word (PSW) Signup and view all the answers

What is the primary function of the control unit in a processor with accumulator architecture?

To fetch and decode instructions from memory Signup and view all the answers

Which of the following is NOT a class of instruction set architectures (ISAs) mentioned in the text?

Very long instruction word (VLIW) Signup and view all the answers

In the microprogram representation for the processor with accumulator architecture, what is the purpose of the instruction labeled '#15 Jump'?

To load the program counter with the address stored in the data register Signup and view all the answers

What is the purpose of the instruction labeled '#16 Jump if positive' in the microprogram representation?

To jump to the address stored in the data register if the accumulator value is positive Signup and view all the answers

Which of the following instructions is NOT present in the microprogram representation for the processor with accumulator architecture?

Shift Signup and view all the answers

What are the five phases involved in the execution of an instruction in a processor?

Operation code fetch, operation code decode, operands fetch, effective instruction execution, results store Signup and view all the answers

Explain the main difference between the stack and accumulator storage types in instruction set architectures.

In stack storage type, both instructions and operands are stored in memory following the stack data type, while in accumulator storage type, instructions involve a special register (ACC register) and sometimes memory. Signup and view all the answers

Why does the phase involving memory access, such as writing back to memory, impact program performance significantly?

It can be 10 times slower than other phases, affecting the overall execution speed of the program. Signup and view all the answers

What is the primary differentiation factor among instruction set architectures classes?

The internal storage type in a processor. Signup and view all the answers

What is the main advantage of using a Reduced Instruction Set Computer (RISC) architecture over a Complex Instruction Set Computer (CISC) architecture?

RISC architectures are typically more efficient, faster, and easier to optimize due to their simpler instruction set. Signup and view all the answers

Why is a single-cycle datapath processor design considered a drawback in modern processors?

It limits the processor's performance scalability and efficiency due to executing all instructions in a single clock cycle. Signup and view all the answers

Explain why instructions with very different execution times or very different numbers of phases are not suitable for a production line (pipeline) in processor design.

Instructions with very different execution times or very different numbers of phases are not suitable for a production line (pipeline) because they can cause imbalances and bottlenecks in the pipeline. Pipelines work best when all instructions have similar execution times and number of phases, allowing for efficient parallel processing. Differences in these factors can lead to some instructions taking much longer to complete, stalling the pipeline and reducing overall performance. Signup and view all the answers

Describe the main purpose of the control unit in a processor with a GPR (register-memory) architecture.

In a processor with a GPR (register-memory) architecture, the primary function of the control unit is to manage the flow of instructions and data between the various components of the processor. This includes fetching instructions from memory, decoding them, and then orchestrating the appropriate data transfers and arithmetic/logical operations to execute those instructions. The control unit ensures the correct registers are accessed, memory locations are read/written, and that the overall execution of instructions proceeds in the proper sequence. Signup and view all the answers

What is the main differentiation in terms of instruction set architectures (ISAs) classes, and how does this impact processor design?

The main differentiation in ISA classes is between RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer) architectures. RISC ISAs typically have a smaller, simpler set of instructions that can be executed in a single clock cycle, while CISC ISAs have a larger, more complex set of instructions that may take multiple clock cycles to execute. This difference in instruction complexity impacts processor design, as RISC processors can be designed with simpler, more efficient pipelines, while CISC processors require more complex control logic and microarchitectures to handle the varied instruction set. Signup and view all the answers

Explain the purpose of the microprogram and the role of the '#15 Jump' and '#16 Jump if positive' instructions in the microprogram representation for a processor with an accumulator architecture.

In a processor with an accumulator architecture, the microprogram represents the low-level control sequence that executes each instruction. The '#15 Jump' instruction is used to transfer control to a different part of the microprogram, allowing the processor to execute a different sequence of microoperations for a given instruction. The '#16 Jump if positive' instruction is a conditional jump that checks the sign of a value (e.g., the accumulator) and transfers control to a different part of the microprogram based on the result. These jump instructions provide the necessary control flow within the microprogram to execute the various instructions in the instruction set. Signup and view all the answers

Contrast the accumulator architecture code and the GPR (register-memory) architecture code in terms of the number of instructions, memory accesses, and the overall efficiency of the two approaches.

The accumulator architecture code in Listing 2.8 has 19 instructions and performs 38 memory accesses (19 fetches and 19 operand fetches), while the GPR (register-memory) architecture code in Listing 2.9 has 15 instructions and performs 26 memory accesses (15 fetches and 11 operand fetches). This represents a 31.5% reduction in memory accesses for the GPR architecture compared to the accumulator architecture. The GPR architecture is more efficient because it can perform more operations using the registers, reducing the need for memory accesses. The fewer instructions and memory accesses in the GPR code also suggest a more compact and optimized implementation compared to the accumulator architecture. Signup and view all the answers

Explain the main drawback of a single-cycle datapath processor design and how it compares to a more advanced pipelined design.

The main drawback of a single-cycle datapath processor design is that it requires all instructions to complete in a single clock cycle, regardless of their complexity. This means that the processor must be designed to accommodate the longest-running instruction, which can lead to inefficient use of the processor's resources and slower overall performance. In contrast, a pipelined design divides the execution of instructions into multiple stages, allowing different instructions to be processed concurrently. This enables the processor to achieve a higher throughput and better utilize its resources, as each stage can be optimized for the specific tasks it needs to perform. However, pipelining also introduces additional complexity in terms of hazard detection and resolution, which must be carefully managed to ensure correct program execution. Signup and view all the answers

Explain the key differences between the stack and load-store classes of instruction set architectures (ISAs) in terms of memory access instructions.

The stack class has push and pop instructions for memory access, while the load-store class has separate load and store instructions for memory access. Signup and view all the answers

Describe the purpose and connections of the key components in the processor with accumulator architecture illustrated in Figure 2.2.

The accumulator (ACC) register is connected to the ALU as its implicit operand. There is also a data register (DR), address register (AR), program counter (PC), and instruction register (IR). The control unit coordinates the operation of these components. Signup and view all the answers

Explain the purpose of the instructions labeled '#15 Jump' and '#16 Jump if positive' in the microprogram representation for the processor with accumulator architecture shown in Figure 2.3.

The '#15 Jump' instruction causes the program counter (PC) to be loaded with the address value stored in the data register (DR), allowing for an unconditional jump. The '#16 Jump if positive' instruction checks if the value in the accumulator (ACC) is positive, and if so, loads the PC with the address in DR to perform a conditional jump. Signup and view all the answers

What is the main advantage of using a Reduced Instruction Set Computer (RISC) architecture over a Complex Instruction Set Computer (CISC) architecture? Provide a specific example to illustrate your answer.

The main advantage of RISC over CISC is simpler instructions that can be executed more quickly. For example, in a RISC architecture, memory access is done via separate load and store instructions, rather than complex instructions like 'load effective address' in CISC that require multiple cycles. Signup and view all the answers

In the context of a general-purpose register (GPR) architecture like MIPS, what is the role of the control unit? Describe two key responsibilities it has during instruction execution.

The control unit in a GPR architecture is responsible for coordinating the datapath components to implement each instruction. Two key responsibilities are: 1) Generating control signals to enable reading/writing of registers and memories, and 2) Determining the next instruction address (e.g. for branches). Signup and view all the answers

What is the main drawback of a single-cycle datapath processor design? Explain how pipelining can help overcome this limitation.

The main drawback is that every instruction, regardless of complexity, must be completed in one cycle, limiting performance. Pipelining overcomes this by breaking instruction execution into stages that can be overlapped, allowing a new instruction to begin each cycle. Signup and view all the answers