Napier008

自我介绍

Sanshui, Foshan, Guangdong Province, China

我的测试区

Advantages and Disadvantages

Pipelining does not help in all cases. There are several possible disadvantages. An instruction pipeline is said to be fully pipelined if it can accept a new instruction every clock cycle. A pipeline that is not fully pipelined has wait cycles that delay the progress of the pipeline.

並非在所有情況下管線技術都起作用。可能有一些缺點。如果一條指令管線能夠在每一個時脈週期(clock cycle)接納一條新的指令，被稱為完整管線化(fully pipelined)。因管線中的指令需要延遲處理而要等待數個時脈週期，被稱為非完整管線化。

Advantages of Pipelining:

1. The cycle time of the processor is reduced, thus increasing instruction issue-rate in most cases.

1. 减少了处理器执行指令所需要的时钟週期，在通常情況下增加了指令的輸入頻率(issue-rate)。

2. Some combinational circuits such as adders or multipliers can be made faster by adding more circuitry. If pipelining is used instead, it can save circuitry vs. a more complex combinational circuit.

2. 一些集成電路(combinational circuits)，例如加法器(adders)或者乘法器(multipliers)，通過添加更多的環路(circuitry)使其工作得更快。如果以管線化替代，能相對地減少環路。

Disadvantages of Pipelining:

1. A non-pipelined processor executes only a single instruction at a time. This prevents branch delays (in effect, every branch is delayed) and problems with serial instructions being executed concurrently. Consequently the design is simpler and cheaper to manufacture.

1. 非管線化的處理器每次(at a time)只執行一個指令。防止分支延時（事實上，每個分支都會產生延時）和串行指令被并行執行產生的問題。 同時，設計比較簡單和較低生產成本。

2. The instruction latency in a non-pipelined processor is slightly lower than in a pipelined equivalent. This is because extra flip flops must be added to the data path of a pipelined processor.

2. 在執行相同的指令時，非管線化處理器的指令傳輸延遲時間(The instruction latency)比管線化處理器明顯較短。這是因為管線化的處理器必須在數據路徑(data path)中添加額外正反器(flip-flops)。

3. A non-pipelined processor will have a stable instruction bandwidth. The performance of a pipelined processor is much harder to predict and may vary more widely between different programs.

3. 非管線化處理器有固定指令位寬(a stable instruction bandwidth)。管線化處理器的性能更難以預測，並且不同的程序之間的變化(vary)可能更大。

Execution is as follows: 執行順序如以下列表所示

Generic pipeline

To the right is a generic pipeline with four stages:

  1. 讀取指令(Fetch)
  2. 指令解碼（Decode）
  3. 執行指令(Execute)
  4. 寫回執行结果(Write-back)

(for lw and sw memory is accessed after execute stage)

The top gray box is the list of instructions waiting to be executed; the bottom gray box is the list of instructions that have been completed; and the middle white box is the pipeline.

上方的大灰色方格是一連串未被執行的指令；下方的大灰色格是已被執行完成的指令；中間的大白色方格是管線。

Time Execution 時序 執行情況 0 Four instructions are awaiting to be executed 四條指令等待執行 1

   * the green instruction is fetched from memory

從主存儲器(memory)中讀取綠色指令

2

   * the green instruction is decoded

綠色指令被解碼

   * the purple instruction is fetched from memory

從主存儲器中讀取紫色指令 3

   * the green instruction is executed (actual operation is performed)

綠色指令被執行（事實上運算已經開始(performed)）

   * the purple instruction is decoded

紫色指令被解碼

   * the blue instruction is fetched

從主存儲器中讀取藍色指令 4

   * the green instruction's results are written back to the register file or memory

綠色指令的運算結果被寫回到寄存器(register)或者主存儲器

   * the purple instruction is executed

紫色指令被執行

   * the blue instruction is decoded

藍色指令被解碼

   * the red instruction is fetched

從主存儲器中讀取紅色指令

5

   * the green instruction is completed

綠色指令被執行完畢

   * the purple instruction is written back

紫色指令的運算結果被寫回到寄存器或者主存儲器

   * the blue instruction is executed

藍色指令被執行

   * the red instruction is decoded

紅色指令被解碼 6

   * The purple instruction is completed

紫色指令被執行完畢

   * the blue instruction is written back

藍色指令的運算結果被寫回到寄存器或者主存儲器

   * the red instruction is executed

紅色指令被執行 7

   * the blue instruction is completed

藍色指令被執行完畢

   * the red instruction is written back

紅色指令的運算結果被寫回到寄存器或者主存儲器 8

   * the red instruction is completed

紅色指令被執行完畢 9 All instructions are executed

Bubble 氣泡 A bubble in cycle 3 delays execution 一個氣泡在編號為3的時脈週期中，指令執行被延遲 Main article: Bubble (computing)

When a "hiccup" in execution occurs, a "bubble" is created in the pipeline in which nothing useful happens. In cycle 2, the fetching of the purple instruction is delayed and the decoding stage in cycle 3 now contains a bubble. Everything "behind" the purple instruction is delayed as well but everything "ahead" of the purple instruction continues with execution.

在指令執行中產生一個“打嗝”(hiccup),在管線中生成一個沒有實效的氣泡。在編號為2的時脈週期中，紫色指令的讀取被延遲，並且在編號為3的時脈週期中解碼層也產生了一個氣泡。所有在紫色指令之後的指令都被延遲執行，而在其之前已經執行了的指令則不受影響。

Clearly, when compared to the execution above, the bubble yields a total execution time of 8 clock ticks instead of 7.

由於氣泡使指令執行延遲了一個時脈週期，完成全部4指令執行共需要8個時脈週期。

Bubbles are like stalls, in which nothing useful will happen for the fetch, decode, execute and writeback. It can be completed with a nop code.