[经济学]Basic Assembly Language.ppt

Basic Assembly Language Outline Working with integers Control structures Translating standard control structures Example: finding prime numbers Working with integers Integer representation Sign extension Two’s complement arithmetic Example program Extended precision arithmetic Integer representation Integers come in two flavors: unsigned and signed. Unsigned integers are represented in a very straightforward binary manner. Three general techniques that have been used to represent signed integers. All of these methods use the most significant bit of the integer as a sign bit. This bit is 0 if the number is positive and 1 if negative. Signed magnitude One’s complement Two’s complement Signed magnitude(原码) The first method is the simplest and is called signed magnitude. E.g., 56 would be represented as the byteand ?56 would be drawbacks two possible values of zero, +0 and ?0 . general arithmetic is also complicated. E.g., If 10 is added to ?56, this must be recast as 10 subtracted by 56. One’s complement(反码) 正数的反码=正数的原码 负数的反码=原码（符号位除外）各位取反而得到的 As for the first method, there are two representations of zero:(+0) and(?0). Two’s complement(补码) 正数的补码=正数的原码 负数的补码=负数的反码加1 原码、反码和补码 十进制 原码 反码 补码 + 0 0000 0000B 0000 0000B 0000 0000B +4 0000 0100B 0000 0100B 0000 0100B +127 0111 1111B 0111 1111B 0111 1111B -0 1000 0000B 1111 1111B 0000 0000B -4 1000 0100B 1111 1011B 1111 1100B -127 1111 1111B 1000 0000B 1000 0001B -128 字长8位无法表示 1000 0000B Sign extension: Decreasing size of data The rule for unsigned numbers is that all the bits being removed must be 0 for the conversion to be correct. The rule for signed numbers is that the bits being removed must be either all 1’s or all 0’s. The first bit not being removed must have the same value as the removed bits. Sign extension: Increasing size of data In general, to extend an unsigned number, one makes all the new bits of the expanded number 0. e.g., F