《计算机系统要素》学习笔记：第四章机器语言

1.学习要点
1）机器语言可以被看作是一种约定的形式，它利用处理器和寄存器来操控内存。
2）汇编语言可分为三类：内存访问（直接寻址，立即寻址，间接寻址），算术逻辑运算，分支跳转语句（流程控制）
3）hack语言，共有A-指令和C-指令两种指令，A指令负责寻址，C指令负责运算和跳转，与第五章的CPU结构对应。
4）乘法程序用多次加法来实现。
5）I/O处理程序没看懂，网络找到答案，但是模拟器上效果不对。。。（+_+）

2.代码实现
（1）乘法程序

// This file is part of www.nand2tetris.org// and the book "The Elements of Computing Systems"// by Nisan and Schocken, MIT Press.// File name: projects/04/Mult.asm// Multiplies R0 and R1 and stores the result in R2.// (R0, R1, R2 refer to RAM[0], RAM[1], and RAM[2], respectively.)@0  D=M  @2  M=0  (LOOP)  @1  D=M  @END  D;JEQ  @1  M=M-1  @0  D=M  @2  M=M+D  @1  D=M  @LOOP  D;JGT  @END  (END)  0;JMP 

（2）I/O处理程序

// This file is part of www.nand2tetris.org// and the book "The Elements of Computing Systems"// by Nisan and Schocken, MIT Press.// File name: projects/04/Fill.asm// Runs an infinite loop that listens to the keyboard input.// When a key is pressed (any key), the program blackens the screen,// i.e. writes "black" in every pixel;// the screen should remain fully black as long as the key is pressed. // When no key is pressed, the program clears the screen, i.e. writes// "white" in every pixel;// the screen should remain fully clear as long as no key is pressed.// Put your code here.(BEGIN)  @KBD  D = M  @CLEAN  D;JEQ  @SCREEN          //涂黑   D = A  @Bcount  A = M + D  M = -1  @Bcount          //键盘按下  M = M + 1  D = M  @8192            //8K  D = D - A  @Bcount  M = D  @END  D;JGE  @8192  D = D + A  @Bcount  M = D  @END  0;JMP  (CLEAN)          //无键盘按下   @SCREEN          //涂白   D = A  @Wcount  A = M + D  M = 0  @Wcount          //键盘按下  M = M + 1  D = M  @8192            //8K  D = D - A  @Wcount  M = D  @END  D;JGE  @8192  D = D + A  @Wcount  M = D  (END)  @BEGIN  0;JMP