理解计算机编程基础：从汇编语言开始

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"Programming from the Ground Up" 本书《Programming from the Ground Up》由Jonathan Bartlett撰写，Dominick Bruno, Jr.编辑，旨在帮助学生深入理解汇编语言和计算机编程的工作原理。该书以初学者为目标读者，不追求成为详尽的x86汇编语言参考书，而是注重引导读者理解编程基础。在汇编语言的学习过程中，作者有意省略了一些复杂的细节，以使初学者能更顺畅地掌握基础知识。书中的重点是解释计算机如何执行指令，以及程序设计的基本概念。通过这种方式，读者可以建立起对计算机底层运作机制的直观认识。书中涵盖的主题可能包括： 1. 计算机体系结构：介绍计算机硬件的基本组成部分，如CPU、内存、输入/输出设备，以及它们如何协同工作。 2. 数据类型和内存：讲解二进制、十六进制、ASCII码等数据表示方式，以及如何在内存中存储和访问这些数据。 3. 指令集和汇编语言：介绍x86架构下的基本指令，如数据处理、流程控制和内存操作，并讲解如何编写简单的汇编程序。 4. 寄存器与运算：探讨CPU寄存器的作用，如何使用它们进行快速计算和数据传递。 5. 子程序和堆栈：介绍如何通过子程序调用来组织代码，以及堆栈在其中的角色。 6. 输入/输出操作：讲解如何与外部设备进行通信，包括键盘、屏幕和磁盘。 7. 链接和加载：阐述程序如何被编译、链接成可执行文件，并在内存中加载执行。 8. 实模式和保护模式：简述x86处理器的不同工作模式，以及它们对程序设计的影响。 9. 高级主题：可能涉及中断处理、多任务、系统调用等更复杂的内容，帮助读者进一步深化理解。此外，书中的代码示例允许自由使用，无需特别署名，这为读者提供了实践和学习编程的便利。虽然这本书不完全适合专业级别的汇编语言编程学习，但它为理解计算机科学的基础和进入更高级的编程领域打下了坚实的基础。为了获得更全面的汇编语言知识，读者可能需要结合其他资源，如参考书籍、在线教程或实际项目经验。但作为入门书籍，《Programming from the Ground Up》无疑是一个优秀的起点，它将帮助读者建立起计算机科学的坚实根基。

Chapter 2. Computer Architecture

decoder which ﬁgures out what the instruction means. This includes what process

needs to take place (addition, subtraction, multiplication, data movement, etc.) and

what memory locations are going to be involved in this process. Computer

instructions usually consist of both the actual instruction and the list of memory

locations that are used to carry it out.

Now the computer uses the data bus to fetch the memory locations to be used in

the calculation. The data bus is the connection between the CPU and memory. It is

the actual wire that connects them. If you look at the motherboard of the

computer, the wires that go out from the memory are your data bus.

In addition to the memory on the outside of the processor, the processor itself has

some special, high-speed memory locations called registers. There are two kinds

of registers - general registers and special-purpose registers. General-purpose

registers are where the main action happens. Addition, subtraction, multiplication,

comparisions, and other operations generally use general-purpose registers for

processing. However, computers have very few general-purpose registers. Most

information is stored in main memory, brought in to the registers for processing,

and then put back into memory when the processing is completed. special-purpose

registers are registers which have very speciﬁc purposes. We will discuss these as

we come to them.

Now that the CPU has retrieved all of the data it needs, it passes on the data and

the decoded instruction to the arithmetic and logic unit for further processing.

Here the instruction is actually executed. After the results of the computation have

been calculated, the results are then placed on the data bus and sent to the

appropriate location in memory or in a register, as speciﬁed by the instruction.

This is a very simpliﬁed explanation. Processors have advanced quite a bit in

recent years, and are now much more complex. Although the basic operation is

still the same, it is complicated by the use of cache hierarchies, superscalar

processors, pipelining, branch prediction, out-of-order execution, microcode

translation, coprocessors, and other optimizations. Don’t worry if you don’t know

what those words mean, you can just use them as Internet search terms if you want

Chapter 2. Computer Architecture

So what about if we need numbers larger than 255? We can simply use a

combination of bytes to represent larger numbers. Two bytes can be used to

represent any number between 0 and 65536. Four bytes can be used to represent

any number between 0 and 4294967295. Now, it is quite difﬁcult to write

programs to stick bytes together to increase the size of your numbers, and requires

a bit of math. Luckily, the computer will do it for us for numbers up to 4 bytes

long. In fact, four-byte numbers are what we will work with by default.

We mentioned earlier that in addition to the regular memory that the computer has,

it also has special-purpose storage locations called registers. Registers are what

the computer uses for computation. Think of a register as a place on your desk - it

holds things you are currently working on. You may have lots of information

tucked away in folders and drawers, but the stuff you are working on right now is

on the desk. Registers keep the contents of numbers that you are currently

manipulating.

On the computers we are using, registers are each four bytes long. The size of a

typical register is called a computer’s word size. x86 processors have four-byte

words. This means that it is most natural on these computers to do computations

four bytes at a time. This gives us roughly 4 billion values.

Addresses are also four bytes (1 word) long, and therefore also ﬁt into a register.

x86 processors can access up to 4294967296 bytes if enough memory is installed.

Notice that this means that we can store addresses the same way we store any

other number. In fact, the computer can’t tell the difference between a value that is

an address, a value that is a number, a value that is an ASCII code, or a value that

you have decided to use for another purpose. A number becomes an ASCII code

when you attempt to display it. A number becomes an address when you try to

look up the byte it points to. Take a moment to think about this, because it is

crucial to understanding how computer programs work.

Addresses which are stored in memory are also called pointers, because instead of

having a regular value in them, they point you to a different location in memory.

As we’ve mentioned, computer instructions are also stored in memory. In fact,

Chapter 2. Computer Architecture

they are stored exactly the same way that other data is stored. The only way the

computer knows that a memory location is an instruction is that a special-purpose

instruction pointer points to a memory word, it is loaded as an instruction. Other

than that, the computer has no way of knowing the difference between programs

and other types of data.

Interpreting Memory

Computers are very exact. Because they are exact, programmers have to be

equally exact. A computer has no idea what your program is supposed to do.

Therefore, it will only do exactly what you tell it to do. If you accidentally print

out a regular number instead of the ASCII codes that make up the number’s digits,

the computer will let you - and you will wind up with jibberish on your screen (it

will try to look up what your number represents in ASCII and print that). If you

tell the computer to start executing instructions at a location containing data

instead of program instructions, who knows how the computer will interpret that -

but it will certainly try. The computer will execute your instructions in the exact

order you specify, even if it doesn’t make sense.

The point is, the computer will do exactly what you tell it, no matter how little

sense it makes. Therefore, as a programmer, you need to know exactly how you

have your data arranged in memory. Remember, computers can only store

numbers, so letters, pictures, music, web pages, documents, and anything else are

just long sequences of numbers in the computer, which particular programs know

how to interpret.

For example, say that you wanted to store customer information in memory. One

way to do so would be to set a maximum size for the customer’s name and address

- say 50 ASCII characters for each, which would be 50 bytes for each. Then, after

2. Note that here we are talking about general computer theory. Some processors and op-

erating systems actually mark the regions of memory that can be executed with a special

marker that indicates this.

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