计算机系统I-处理器基本原理

The Processor: Basic Principles

What is the architecture of computers

Von Neumann structure: data and programs are in memory.

$$Computer \begin{cases} CPU\begin{cases} Control Unit\\ Datapath\begin{cases} Path: multiplexors\\ ALU\\ Registers\\ \cdots \end{cases} \end{cases}\\ Memory\\ I/O Interface \end{cases}$$

What is the inside of Processor (CPU)

• Datapath: performs operations on data
• Control: sequences datapath, memory, …
• Cache memory
  • Small fast SRAM memory for immediate access to data

How to use CPU to solve problems

Design methodology 1: Common digital system

Finite state machine (FSM)

graph LR
	A[input]
	A -->B[Combinational circuit
			Status transmission conditions]
	A -->C[Combinational circuit
			Output processing]
	D[State storage] -->B
	C-->E[Output]
	B-->D
	B-->C

Design methodology 2: Control of Register Transfers

Control of Register Transfers

Register transfers performed on registers control that supervises the sequencing of the register transfers.

Design methodology 3: Control of Register Transfers

General system: Program state machine (PSM)

Turing Machine

Basic idea

• Use machine to simulate mathematical computation process of humans by paper and pen.
• Two simple operations
  • Write or delete a symbol from the paper
  • Move the attention from one position to another of the paper
• The next operation depends on
  • the symbol in one position of the paper that the person pays attention to
  • the thinking state of the person currently
• Turing constructs an ideal machine

Components of Turing Machine

1. An infinitely long paper tape TAPE
2. A read/write head HEAD
3. A set of control rules TABLE
4. A state register

Instruction structure of Turing Machine

$M=\{Q,\Sigma,\Gamma,\delta,q0,qaccept,qreject\}$, where $Q,\Sigma,\Gamma$​are all finite sets

1. Q is the state set
2. $\Sigma$ is the input alphabet, which does not include the special empty " ";
3. $\Gamma$ is the alphabet, where $b\in\Gamma$ is the empty, and $\Sigma\in\Gamma$
4. $\delta$ is the transfer function: $Q\times\Sigma\to Q\times\Sigma\{L,R\}$, where L and R represent the read/write head moves left and right, respectively
5. q0$\in$Q is the beginning state
6. qaccept is the accept state
7. qreject is the reject state, and qreject$\ne$​qaccept

Program of Turing Machine

$\delta=\{q_i,S_j,S_k,R(L\ or\ N),q_l\}$

• $q_i$ is the current state of the machine
• $S_j$ is the symbol read by the machine
• $S_k$ is the symbol waiting to be written to replace $S_j$
• R, L, N represent moving right one step, moving left one step, or remain static
• $q_l$ is the next state of the machine

Practical Turing Machine – A Real Computer

A large enough storage to replace the tape

• Magnetic storage for replacement
• Semiconductor storage for replacement

Storing the inner state

• Inner state does not store immediate, downgrading the efficiency
• Register to store the immediate
• Can significantly simplify computational structure and improve computing efficiency

Enhance the processing capability of HEAD–CPU

• Require a stronger HEAD
• Can be implement by Control of Register Transfer Technique: CPU
• Satisfactory Universal Turing Machine

CPU

Logics in CPU

Unlocked vs. Clocked

Clocks used in synchronous logic

Performance

Being able to gauge the relative performance of a computer is an important but tricky task. There are a lot factors that can affect performance

• Architecture
• Hardware implementation of the architecture
• Compiler for the architecture
• Operating system

Furthermore, we need to be able to define a measure of performance

• Single users on a PC -> a minimization of response time
• Large data -> a maximization of throughput

Response Time and Throughput

• Latency (Response time)
  • is the time between the start and completion of an event
  • How long it takes to do a task
• Throughput (bandwidth)
  • is the total amount of work done in a given period of time
  • Total work done per unit time

How are response time and throughput affected by

• Replacing the processor in a computer with a faster processor
• Adding more processors?

Measuring Execution Time

Elapsed time

• Total response time, including all aspects
• Determines system performance

CPU time

• Time spent processing a given job
• Comprises user CPU time and system CPU time
• Different programs are affected differently by CPU and system performance

How can computers run fast

The main goal of architecture improvement is to improve the performance of the system.

• Describe a thing with the least instructions – Algorithms, compiling
• Do more things in a Clock cycle – Architecture
• Increase the speed of “core” – Main frequency

CPU Clocking

Operation of digital hardware governed by a constant-rate clock

• Clock period: duration of a clock cycle
  e.g., 250ps = 0.25ns = $250\times10^{-12}s$
• Clock frequency (rate): cycles per second
  e.g., 4.0GHz = 4000MHz = $4.0\times10^{9}$Hz