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题型分析：Hardware and Virtual Machines

题型 1：RISC vs CISC 比较​

题目特征​

  • 完成比较表格
  • 勾选特征归属（RISC / CISC / Both）
  • 描述差异并给出推荐
标准解题方法
  1. 根据指令集大小、CPI、寻址模式、流水线效率、代码密度等维度比较
  2. RISC 关键词：simple, fixed-length, one cycle, load/store, many registers
  3. CISC 关键词：complex, variable-length, many cycles, micro-programmed
  4. 应用场景：RISC 适合低功耗设备，CISC 适合复杂计算
评分标准（MS 模式）
  • M1：每组正确比较得 1 分
  • A1：使用表格或成对陈述
  • B1：给出具体的特征描述而非模糊回答

典型例题 1：9618/s22/qp/31 Q4​

Tick the boxes to show whether each characteristic applies to RISC, CISC or both.

CharacteristicRISCCISC
Each instruction takes approximately one clock cycle
Variable instruction length
Large number of addressing modes
Micro-programmed control unit
Many general purpose registers
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CharacteristicRISCCISC
Each instruction takes approximately one clock cycleM1 ✅
Variable instruction lengthA1 ✅
Large number of addressing modesA1 ✅
Micro-programmed control unitB1 ✅
Many general purpose registersB1 ✅

典型例题 2：9618/w22/qp/31 Q10​

(a) Describe two differences between RISC and CISC processors. (b) A computer designer needs to choose between a RISC and a CISC processor for a new device. State which processor you would recommend for each application and justify your answer. (i) A battery-powered mobile phone (ii) A desktop computer used for video editing

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(a) Differences (any 2):

  • M1：RISC has fewer / simpler instructions; CISC has more / complex instructions
  • A1：RISC has fixed instruction length; CISC has variable instruction length
  • A1：RISC typically executes one instruction per clock cycle; CISC may need multiple cycles
  • B1：RISC uses load/store architecture; CISC allows memory-to-memory operations

(b)(i) Mobile phone:

  • M1：RISC recommended — lower power consumption
  • A1：RISC generates less heat / more energy efficient

(b)(ii) Video editing:

  • M1：CISC recommended — complex instructions handle multimedia tasks efficiently
  • A1：CISC offers higher code density / backward compatibility

题型 2：流水线（Pipelining）​

题目特征​

  • 描述流水线各阶段
  • 绘制 timing diagram
  • 分析 hazard 及其解决方法
标准解题方法
  1. 五阶段顺序：IF → ID → OF → IE → WB
  2. 每个阶段通常需要一个时钟周期
  3. Timing diagram：横轴为时钟周期，纵轴为指令编号
  4. Hazard 会使流水线停顿（stall / bubble）
  5. 数据相关（data hazard）可用 forwarding 解决
评分标准（MS 模式）
  • M1：正确列出流水线阶段名称
  • A1：正确绘制 timing diagram
  • B1：识别 hazard 类型并解释

典型例题 1：9618/s23/qp/32 Q8​

A processor uses a five-stage pipeline: IF (Instruction Fetch), ID (Instruction Decode), OF (Operand Fetch), IE (Instruction Execute), WB (Write Back). (a) Explain what is meant by pipelining. (b) The following three instructions are to be executed: I1: LDR R1, 100 I2: ADD R2, R1, #5 I3: SUB R3, R4, #1 Complete a timing diagram showing the execution of these three instructions through the pipeline. Assume no hazards.

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(a) Pipelining:

  • M1：将指令执行分成多个阶段，各阶段重叠执行不同的指令（overlapping execution of multiple instructions）
  • A1：提高 CPU 吞吐量（throughput），但不减少单条指令的执行时间

(b) Timing diagram:

  • M1：Cycle 1 → I1: IF
  • A1：Cycle 2 → I1: ID, I2: IF
  • A1：Cycle 3 → I1: OF, I2: ID, I3: IF
  • B1：Cycle 4 → I1: IE, I2: OF, I3: ID
  • B1：Cycle 5 → I1: WB, I2: IE, I3: OF
  • B1：Cycle 6 → I2: WB, I3: IE
  • B1：Cycle 7 → I3: WB

典型例题 2：Data Hazard​

A processor has a five-stage pipeline. Explain what happens when a data hazard occurs. Suggest how this hazard can be resolved.

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  • M1：Data hazard occurs when an instruction depends on the result of a previous instruction that has not yet been written back
  • A1：Pipeline stalls / inserts a bubble (NOP) until the data is available
  • B1：Solution: forwarding / bypassing (forward the result directly from the execute stage to the next instruction without waiting for write back)

题型 3：计算机体系结构（Computer Architectures / Flynn's Taxonomy）​

题目特征​

  • 解释 SISD / SIMD / MISD / MIMD
  • 给出例子
  • 匹配应用场景
标准解题方法
  1. 明确"指令流"和"数据流"的概念
  2. SISD：单核顺序执行（一条指令处理一组数据）
  3. SIMD：一条指令对多组数据并行操作
  4. MISD：多条指令处理同一组数据（容错系统）
  5. MIMD：多条指令处理多组数据（多核处理器）
评分标准（MS 模式）
  • M1：正确解释每种分类的指令流和数据流
  • A1：为每种分类给出合适例子
  • B1：在比较中明确区分相似类别（如 SIMD vs MIMD）

典型例题 1：9618/w23/qp/31 Q5​

(a) Complete the table using SISD, SIMD, MISD or MIMD.

DescriptionClassification
A single processor executes one instruction on one item of data
Each processor executes different instructions on different data
One instruction is executed on multiple items of data simultaneously

(b) Describe one advantage of a massively parallel computer.

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(a) Table:

  • M1：A single processor executes one instruction on one item of data → SISD
  • A1：Each processor executes different instructions on different data → MIMD
  • A1：One instruction is executed on multiple items of data simultaneously → SIMD

(b) Massively parallel computer advantage:

  • B1：可以同时处理大量数据 / 解决复杂计算问题
  • B1：高性能计算（HPC）适用 scientific simulations / weather forecasting

典型例题 2：SIMD vs MIMD​

(a) State what is meant by SIMD. (b) State what is meant by MIMD. (c) Give one application of SIMD processors.

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(a) SIMD (Single Instruction, Multiple Data):

  • M1：一条指令同时对多组数据执行相同的操作
  • A1：用于 vector processing / parallel processing

(b) MIMD (Multiple Instruction, Multiple Data):

  • M1：多条指令同时对多组数据执行不同的操作
  • A1：每个处理器可以独立执行不同的任务

(c) Application of SIMD:

  • B1：图形处理 / GPU / image processing / multimedia processing

题型 4：虚拟机（Virtual Machines）​

题目特征​

  • 解释虚拟机的概念
  • 列出优点和局限性
  • 比较 host OS 和 guest OS
标准解题方法
  1. 虚拟机 = 软件模拟完整计算机，可以运行独立 OS
  2. Hypervisor (VMM) 在物理硬件和 guest OS 之间
  3. Host OS 运行在物理机上，Guest OS 运行在 VM 内
  4. 优点：隔离、资源共享、测试、迁移
  5. 缺点：性能开销、硬件访问限制
评分标准（MS 模式）
  • M1：正确定义虚拟机
  • A1：描述 hypervisor / VMM 的作用
  • A1：列出优点（each valid point = 1）
  • B1：列出局限性

典型例题 1：9618/s24/qp/32 Q9​

(a) Describe the role of a hypervisor in a virtual machine environment. (b) Explain one limitation of using virtual machines compared to running the operating system directly on the hardware.

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(a) Hypervisor / VMM:

  • M1：管理物理硬件资源（CPU, memory, I/O）
  • A1：在多个 guest OS 之间分配资源
  • A1：隔离不同 VM 之间的资源访问

(b) Limitation:

  • B1：性能开销（performance overhead）— hypervisor 层增加了额外处理步骤
  • B1：Guest OS 无法直接访问物理硬件，I/O 操作速度较慢

典型例题 2：虚拟机应用​

A company uses virtual machines on its server. (a) Explain what is meant by a virtual machine. (b) Describe two benefits of using virtual machines on the server.

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(a) Virtual machine:

  • M1：软件模拟的计算机环境（software simulation of a computer system）
  • A1：可以运行自己的 operating system (guest OS)，与物理机隔离

(b) Benefits (any 2):

  • A1：隔离性 — 一个 VM 崩溃不影响其他 VM
  • A1：资源利用 — 多个 VM 共享同一物理硬件
  • B1：便于测试 — 可以快速创建销毁测试环境
  • B1：便于迁移 — VM 可以移动到其他物理服务器
常见陷阱
  • 混淆 RISC（精简指令）和 CISC（复杂指令）的核心特征
  • 流水线阶段顺序错误，特别是 ID 和 OF 的顺序
  • 认为 pipelining 减少单条指令延迟（实际提高 throughput）
  • 混淆 SIMD 和 MIMD 的区别（指令流 vs 数据流）
  • 认为虚拟机没有性能开销
  • 混淆 host OS 和 guest OS 的关系
  • 遗漏 hypervisor 在虚拟化中的核心作用