第十一的讲运算部件.pdfVIP

第6章计算机的运算方法 ( 7 ) The digital computer is a digital system that performs various computational tasks. The word digital implies that the information in the computer is represented by variables that take a limited number of discrete values. These values are processed internally by components that can maintain a limited number of discrete states. An ALU must process numbers using the same format as the rest of the digital circuit. For modern processors, that almost always is the twos complement binary number representation. 1 计算机中的运算器是对数据进行加工处理的部件，有定点运 算器和浮点运算器之分。 定点运算部件 定点运算器主要用于实现对定点整数或定点小数的算术运算， 对二进制位串的逻辑运算，以及对主存的地址计算等。 浮点运算部件 Floating Point Unit 每个浮点部件都包含阶码运算和尾数运算两个定点运算器。 A Floating Point Unit also performs arithmetic operations between two values, but they do so for numbers in floating point representation, which is much more complicated than the twos complement representation used in a typical ALU. 只有定点运算器的计算机可以用软件实现浮点运算。 2 运算器由算术逻辑运算单元、通用寄存器组、状态标志位、 输入数据选择电路和输出数据控制电路等组成。高档计算机的运 算器还有专门的乘法和除法部件。 通用寄存器组用来暂存参与运算的数据和某些中间运算结果。 运算器能同时处理的二进制位数称为计算机的字长，它与通 用寄存器的位数是一致的。字长影响到计算机的运算速度快慢， 计算精度和指令功能的强弱等，是计算机性能的重要指标。 The inputs to the ALU are the data to be operated on (called operands) and a code from the control unit indicating which operation to perform. Its output is the result of the computation. In many designs the ALU also takes or generates as inputs or outputs a set of condition codes from or to a status register. These codes are used to indicate cases such as carry-in or carry-out, overflow, divide-by-zero, etc. 3 状态标志位用来记录算术/逻辑运算或测试操作的结果状态。这 些状态通常用作程序条件转移指令的判断条件，所以又称为 “程 序状态字”（PSW ）， “条件码寄存器”（Condition Code