通过机器码程序理解冯诺依曼体系
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首先介绍 冯诺依曼周期
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冯诺依曼体系结构
简介
现今所有的商用计算机都是基于冯诺依曼体系的计算机。
冯·诺依曼体系结构冯·诺依曼理论的要点是：计算机的数制采用二进制；计算机应该按照程序顺序执行。存储程序原理，把程序本身当作数据来对待，程序和该程序处理的数据用同样的方式储存，二者地位是相等的。
通过机器码程序理解 冯诺依曼 计算机
下面将通过基于Pep8虚拟计算机运行的机器码程序理解冯诺依曼计算机。（Pep8是用于教学的开源小型计算机，可以执行机器码和汇编程序 下载链接 http://computersystemsbook.com/5th-edition/pep9/ ）
首先介绍 冯诺依曼周期
一个冯诺依曼周期包含 5 个操作：
- 取指
- 译码
- 增加PC（程序计数器）
- 执行指令
-
重复
如图：加载机器语言到内存
初始化寄存器（PC，SP,IR等）
do {
取下一条指令
解码 指令指示符
递增PC
执行取出的指令
} while（没有执行停止指令）
指令和寄存器
每种计算机都有自己的指令集，固化在cpu中。Pep8有39条指令，这些指令分为两类，由一个字节（8位二进制）组成的 指令指示符，或者由一个 指令指示符 和一个由一个字（两个字节）组成的 操作数指示符 组成。
Pep8中包含不限于PC（程序指令计数器），SP（栈指针计数器），IR（24位指令寄存器（3字节））等寄存器。
将要用到的指令:
0000 0000 停止执行
0100 1aaa 字符输入
0101 0aaa 字符输出
注（aaa表示一个字段，ａ可为0或1，表示寻址方式)
一个字符输出程序
地址 机器语言（bin）
---- -------
0000 0101 0001 0000 0000 0000 0111
0003 0101 0001 0000 0000 0000 1000
0006 0000 0000
0007 0100 1000
0008 0110 1001
地址 机器语言（hex）
---- --------
0000 510007
0003 510008
0006 00
0007 48
0008 69
输出
----
Hi
这两段代码是等价的，只不过是前者更利于计算机识别和读取，后者利于人类，但是计算机最终是要保存二进制的序列，而非其他任何形式的结果。（注：地址为了便于表示使用四位十六进制数表示）
分析程序
首先将上面代码输入Pep8虚拟机后，首先要做的是，装载。即将每条指令（每行右侧序列）装载到左侧序列标识的地址。如果装载的指令是一元指令即 指令指示符 ，那么他占据一个内存单元即一个字节。如果指令是一个二元指令也即 操作数指示符 那么他占据三个内存单元，所以第一条指令被装载到 0000、0001、 0002 所标识的内存。
当所有的指令装载完成后，也就意味着一个程序在内存中，并处于可执行状态。注意一点，这个程序的指令在内存中是逐条紧邻的。
执行程序
根据 冯诺依曼周期，程序是顺序执行的，所以初始化程序指令计数器PC,
PC: 0000 CPU根据PC的值作为内存地址索引，找到该内存并将该内存的值（二进制序列）复制，通过总线传输到CPU的指令寄存器IR中，
IR：Mem[PC] // 类比 数组索引值 a[i];此时CPU解释IR中的指令，指令：0101 0001 0000 0000 0000 0111
我们来解释一下这个指令，首先这是一个二元指令，所以前一个字节是指令指示符，后两个字节是操作数指示符。第一个字节 0101 0001 （原指令模式 0101 0aaa）代表以直接寻址的方式输出字符，后两个字节指示要输出的字符的地址，可以看出，这块地址就是之前我们装载程序时 0007 这块内存，而内存中的值正是 ‘H’ 的ASCII码值 48 。到此我们可以看出，其实 程序 和 程序的数据 对于计算机来说没有区别，都是保存在内存中的二进制数，计算机只单纯的将他们作为二进制数处理，而是作为 指令 被解释还是作为 数据 被解释是需要我们来定义。
接下来继续程序的执行，当获取到 操作数指示符 标识内存中的值后，将这个值发送到输出设备，此时由输出设备负责将该二进制数解释为字符。
下一步，根据取出指令的长度进行PC的自增。重复上述过程，直到
PC：0006索引值为指令
0000 0000 ；结束指令程序结束。
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