微处理器设计与VHDL数字逻辑

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"Digital Logic and Microprocessor Design With VHDL" 本书《Digital Logic and Microprocessor Design With VHDL》由Enoch Hwang编写，旨在介绍数字逻辑和微处理器设计的基础知识，并结合VHDL（Very High-Speed Integrated Circuit Hardware Description Language）进行深入探讨。书中内容包括了从微处理器的概述到VHDL的使用，以及数字电路的基础知识。作者首先介绍了微处理器的基本概念，阐述了微处理器在现代电子系统中的核心作用。微处理器是一个复杂的系统，它能够执行指令集，处理各种计算任务。书中通过设计抽象层次的概念，帮助读者理解从行为级、门级到晶体管级的设计过程，这些都是构建微处理器的基础。书中详细讲解了2-to-1多路选择器作为例子，展示了如何从高层次的描述逐步细化到低层次的实现。在行为级，多路选择器被描述为根据控制信号选择两个输入之一的逻辑行为；在门级，用基本的逻辑门（如与门、或门、非门）来实现；在晶体管级，进一步细化到mosfet等实际物理元件的布局。接着，作者引入了VHDL，这是一种用于硬件描述的语言，广泛用于数字系统的建模和综合。VHDL允许设计师以结构化的方式表达电路的行为和结构，方便设计的验证和自动化综合到实际芯片布局。书中还讨论了数字电路的基础，包括二进制数系统，二进制开关，基本逻辑运算符和逻辑表达式，以及真值表等相关概念。这些是理解和设计任何数字电路的基础，无论是在微处理器设计还是其他领域。在每一章的末尾，作者都提供了总结清单，帮助读者回顾和巩固所学知识，同时还设置了问题部分，鼓励读者通过解决问题来加深对理论的理解和应用。《Digital Logic and Microprocessor Design With VHDL》是一本面向初学者和专业人员的实用教程，涵盖了从基础数字逻辑到高级微处理器设计的关键概念，并结合VHDL，为读者提供了一个全面学习和实践的平台。

Chapter 1

Designing Microprocessors

Control

Signals

Status

Signals

MUX

'0'

Data

Inputs

Data

Outputs

Datapath

ALU

Output

Logic

Next-

state

Logic

Control

Inputs

Control

Outputs

State

Memory

Control Unit

Microprocessor

Being a computer science or electrical engineering student, you probably have assembled a PC before. You may

have gone out to purchase the motherboard, CPU (central processing unit), memory, disk drive, video card, sound

card, and other necessary parts, assembled them together, and have made yourself a state-of-the-art working

computer. But have you ever wondered how the circuits inside those IC (integrated circuit) chips are designed? You

know how the PC works at the system level by installing the operating system and seeing your machine come to life.

But have you thought about how your PC works at the circuit level, how the memory is designed, or how the CPU

circuit is designed?

In this book, I will show you from the ground up, how to design the digital circuits for microprocessors, also

known as CPUs. When we hear the word “microprocessor,” the first thing that probably comes to many of our

minds is the Intel Pentium

CPU, which is found in most PCs. However, there are many more microprocessors that

are not Pentiums, and many more microprocessors that are used in areas other than the PCs.

Microprocessors are the heart of all “smart” devices, whether they be electronic devices or otherwise. Their

smartness comes as a direct result of the decisions and controls that microprocessors make. For example, we usually

do not consider a car to be an electronic device. However, it certainly has many complex, smart electronic systems,

such as the anti-lock brakes and the fuel-injection system. Each of these systems is controlled by a microprocessor.

Yes, even the black, hardened blob that looks like a dried-up and pressed-down piece of gum inside a musical

greeting card is a microprocessor.

There are generally two types of microprocessors: general-purpose microprocessors and dedicated

microprocessors. General-purpose microprocessors, such as the Pentium CPU, can perform different tasks under

the control of software instructions. General-purpose microprocessors are used in all personal computers.

Dedicated microprocessors, also known as application-specific integrated circuits (ASICs), on the other

hand, are designed to perform just one specific task. For example, inside your cell phone, there is a dedicated

microprocessor that controls its entire operation. The embedded microprocessor inside the cell phone does nothing

else but control the operation of the phone. Dedicated microprocessors are, therefore, usually much smaller and not

as complex as general-purpose microprocessors. However, they are used in every smart electronic device, such as

the musical greeting cards, electronic toys, TVs, cell phones, microwave ovens, and anti-lock break systems in your

car. From this short list, I’m sure that you can think of many more devices that have a dedicated microprocessor

inside them. Although the small dedicated microprocessors are not as powerful as the general-purpose

microprocessors, they are being sold and used in a lot more places than the powerful general-purpose

microprocessors that are used in personal computers.

Designing and building microprocessors may sound very complicated, but don’t let that scare you, because it is

not really all that difficult to understand the basic principles of how microprocessors are designed. We are not trying

to design a Pentium microprocessor here, but after you have learned the material presented in this book, you will

have the basic knowledge to understand how it is designed.

This book will show you in an easily understandable approach, starting with the basics and leading you through

to the building of larger components, such as the arithmetic logic unit (ALU), register, datapath, control unit, and

finally to the building of the microprocessor — first dedicated microprocessors, and then general-purpose

microprocessors. Along the way, there will be many sample circuits that you can try out and actually implement in

hardware using the optional Altera UP2 development board. These circuits, forming the various components found

inside a microprocessor, will be combined together at the end to produce real, working microprocessors. Yes, the

exciting part is that at the end, you actually can implement your microprocessor in a real IC, and see that it really

can execute software programs or make lights flash!

1.1 Overview of a Microprocessor

The Von Neumann model of a computer, shown in Figure 1.1, consists of four main components: the input, the

output, the memory, and the microprocessor (or CPU). The parts that you purchased for your computer can all be

categorized into one of these four groups. The keyboard and mouse are examples of input devices. The CRT

(cathode ray tube) and speakers are examples of output devices. The different types of memory (cache, read-only

memory (ROM), random-access memory (RAM), and the disk drive) are all considered part of the memory box in

the model. In this book, the focus is not on the mechanical aspects of the input, output, and storage devices. Rather,

Digital Logic and Microprocessor Design with VHDL Chapter 1 - Designing Microprocessors

the focus is on the design of the digital circuitry of the microprocessor, the memory, and other supporting digital

logic circuits.

The logic circuit for the microprocessor can be divided into two parts: the datapath and the control unit, as

shown in Figure 1.1. Figure 1.2 shows the details inside the control unit and the datapath. The datapath is

responsible for the actual execution of all data operations performed by the microprocessor, such as the addition of

two numbers inside the arithmetic logic unit (ALU). The datapath also includes registers for the temporary storage

of your data. The functional units inside the datapath, which in our example includes the ALU and the register, are

connected together with multiplexers and data signal lines. The data signal lines are for transferring data between

two functional units. Data signal lines in the circuit diagram are represented by lines connecting two functional

units. Sometimes, several data signal lines are grouped together to form a bus. The width of the bus (that is, the

number of data signal lines in the group) is annotated next to the bus line. In the example, the bus lines are thicker

and are 8-bits wide. Multiplexers, also known as MUXes, are for selecting data from two or more sources to go to

one destination. In the sample circuit, a 2-to-1 multiplexer is used to select between the input data and the constant

‘0’ to go to the left operand of the ALU. The output of the ALU is connected to the input of the register. The output

of the register is connected to three different destinations: (1) the right operand of the ALU, (2) an

OR gate used as a

comparator for the test “not equal to 0,” and (3) a tri-state buffer. The tri-state buffer is used to control the output of

the data from the register.

Input

Microprocessor

Memory

Output

Control

Unit

Datapath

Figure 1.1. Von Neumann model of a computer.

Control

Signals

Status

Signals

MUX

'0'

Data

Inputs

Data

Outputs

Datapath

ALU

Output

Logic

Next-

state

Logic

Control

Inputs

Control

Outputs

State

Memory

Control Unit

Figure 1.2. Internal parts of a microprocessor.

Even though the datapath is capable of performing all of the data operations of the microprocessor, it cannot,

however, do it on its own. In order for the datapath to execute the operations automatically, the control unit is

required. The control unit, also known as the controller, controls all of the operations of the datapath, and therefore,

the operations of the entire microprocessor. The control unit is a finite state machine (FSM) because it is a machine

that executes by going from one state to another and that there are only a finite number of states for the machine to

go to. The control unit is made up of three parts: the next-state logic, the state memory, and the output logic. The

purpose of the state memory is to remember the current state that the FSM is in. The next-state logic is the circuit for

Digital Logic and Microprocessor Design with VHDL Chapter 1 - Designing Microprocessors

determining what the next state should be for the machine. And the output logic is the circuit for generating the

actual control signals for controlling the datapath.

Every digital logic circuit, regardless of whether it is part of the control unit or the datapath, is categorized as

either a combinational circuit or a sequential circuit. A combinational circuit is one where the output of the circuit

is dependent only on the current inputs to the circuit. For example, an adder circuit is a combinational circuit. It

takes two numbers as inputs. The adder evaluates the sum of these two numbers and outputs the result.

A sequential circuit, on the other hand, is dependent not only on the current inputs, but also on all the previous

inputs. In other words, a sequential circuit has to remember its past history. For example, the up-channel button on a

TV remote is part of a sequential circuit. Pressing the up-channel button is the input to the circuit. However, just

having this input is not enough for the circuit to determine what TV channel to display next. In addition to the up-

channel button input, the circuit must also know the current channel that is being displayed, which is the history. If

the current channel is channel 3, then pressing the up-channel button will change the channel to channel 4.

Since sequential circuits are dependent on the history, they must therefore contain memory elements for

remembering the history; whereas combinational circuits do not have memory elements. Examples of combinational

circuits inside the microprocessor include the next-state logic and output logic in the control unit, and the ALU,

multiplexers, tri-state buffers, and comparators in the datapath. Examples of sequential circuits include the register

for the state memory in the controller and the registers in the datapath. The memory in the Von Neuman computer

model is also a sequential circuit.

Irregardless of whether a circuit is combinational or sequential, they are all made up of the three basic logic

gates:

AND, OR, and NOT gates. From these three basic gates, the most powerful computer can be made.

Furthermore, these basic gates are built using transistors — the fundamental building blocks for all digital logic

circuits. Transistors are just electronic binary switches that can be turned on or off. The on and off states of a

transistor are used to represent the two binary values: 1 and 0.

Figure 1.3 summarizes how the different parts and components fit together to form the microprocessor. From

transistors, the basic logic gates are built. Logic gates are combined together to form either combinational circuits or

sequential circuits. The difference between these two types of circuits is only in the way the logic gates are

connected together. Latches and flip-flops are the simplest forms of sequential circuits, and they provide the basic

building blocks for more complex sequential circuits. Certain combinational circuits and sequential circuits are used

as standard building blocks for larger circuits, such as the microprocessor. These standard combinational and

sequential components usually are found in standard libraries and serve as larger building blocks for the

microprocessor. Different combinational components and sequential components are connected together to form

either the datapath or the control unit of a microprocessor. Finally, combining the datapath and the control unit

together will produce the circuit for either a dedicated or a general microprocessor.

Digital Logic and Microprocessor Design with VHDL Chapter 1 - Designing Microprocessors

Combinational

Circuits

Flip-flops

Sequential

Components

Combinational

Components

Datapath

Control Unit

Gates

Transistors

910

Dedicated

Microprocessor

General

Microprocessor

Sequential

Circuits

Figure 1.3. Summary of how the parts of a microprocessor fit together. The numbers in each box denote the chapter

number in which the topic is discussed.

1.2 Design Abstraction Levels

Digital circuits can be designed at any one of several abstraction levels. When designing a circuit at the

transistor level, which is the lowest level, you are dealing with discrete transistors and connecting them together to

form the circuit. The next level up in the abstraction is the gate level. At this level, you are working with logic gates

to build the circuit. At the gate level, you also can specify the circuit using either a truth table or a Boolean equation.

In using logic gates, a designer usually creates standard combinational and sequential components for building

larger circuits. In this way, a very large circuit, such as a microprocessor, can be built in a hierarchical fashion.

Design methodologies have shown that solving a problem hierarchically is always easier than trying to solve the

entire problem as a whole from the ground up. These combinational and sequential components are used at the

level, we are concerned with how the data is transferred between the various registers and functional units to realize

or solve the problem at hand. Finally, at the highest level, which is the behavioral level, we construct the circuit by

describing the behavior or operation of the circuit using a hardware description language. This is very similar to

writing a computer program using a programming language.

1.3 Examples of a 2-to-1 Multiplexer

As an example, let us look at the design of the 2-to-1 multiplexer from the different abstraction levels. At this

point, don’t worry too much if you don’t understand the details of how all of these circuits are built. This is intended

just to give you an idea of what the description of the circuits look like at the different abstraction levels. We will get

to the details in the rest of the book.

An important point to gain from these examples is to see that there are many different ways to create the same

functional circuit. Although they are all functionally equivalent, they are different in other respects such as size (how

big the circuit is or how many transistors it uses), speed (how long it takes for the output result to be valid), cost

Digital Logic and Microprocessor Design with VHDL Chapter 1 - Designing Microprocessors

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