[翻译]ARM汇编简介（五）条件执行指令&&Thumb模式下的条件执行指令&&分支指令
今晚拼一枪！再翻译一节。原文链接https://azeria-labs.com/arm-conditional-execution-and-branching-part-6/
CONDITIONAL EXECUTION 条件执行指令
We already briefly touched the conditions’ topic while discussing the CPSR register. We use conditions for controlling the program’s flow during it’s runtime usually by making jumps (branches) or executing some instruction only when a condition is met. The condition is described as the state of a specific bit in the CPSR register. Those bits change from time to time based on the outcome of some instructions. For example, when we compare two numbers and they turn out to be equal, we trigger the Zero bit (Z = 1), because under the hood the following happens: a – b = 0. In this case we have equal condition. If the first number was bigger, we would have a GreaterThan condition and in the opposite case –LowerThan. There are more conditions, like Lower orEqual (LE),Greater orEqual (GE) and so on.
在讨论CPSR寄存器时，我们已经简要地讨论了条件的话题。当特定条件满足时，借助条件指令， 通过跳转（分支）或执行某些特定指令来控制程序的流动方向。相关条件被描述为CPSR寄存器中的特定位的状态，这些位根据指令计算后的结果实时改变。比如，如果我们比较两个数并且他们相等，就将零标志位置位 （Z=1） ，因为在系统底层发生了a-b=0。在这个例子里两个数是相等的，但如果第一个数字比第二个大，会得出大于结论。而相反的情况下得出小于结论。当然还有很多其他的条件，比如小于等于（LE），大于等于（GE）等等。
The following table lists the available condition codes, their meanings, and the status of the flags that are tested.
下表列出了可能的条件指令，他们的含义以及被检测的状态标志位
We can use the following piece of code to look into a practical use case of conditions where we perform conditional addition.
我们使用如下代码来实现条件相加指令：
.global main main: mov r0, #2 /* setting up initial variable */ cmp r0, #3 /* comparing r0 to number 3. Negative bit get's set to 1 R0与数字3比较，N位置一*/ addlt r0, r0, #1 /* increasing r0 IF it was determined that it is smaller (lower than) number 3 如果R0比3小就将R0自增1*/ cmp r0, #3 /* comparing r0 to number 3 again. Zero bit gets set to 1. Negative bit is set to 0 再次比较r0和3，Z标志位置位 */ addlt r0, r0, #1 /* increasing r0 IF it was determined that it is smaller (lower than) number 3 如果r0小于3给r0自增1*/ bx lr
The first CMP instruction in the code above triggers Negative bit to be set (2 – 3 = -1) indicating that the value in r0 is Lower Than number 3. Subsequently, the ADDLT instruction is executed because LT condition is full filled when V != N (values of overflow and negative bits in the CPSR are different). Before we execute second CMP, our r0 = 3. That’s why second CMP clears out Negative bit (because 3 – 3 = 0, no need to set the negative flag) and sets theZero flag (Z = 1). Now we have V = 0 and N = 0 which results in LT condition to fail. As a result, the second ADDLT is not executed and r0 remains unmodified. The program exits with the result 3.
代码中，第一个CMP比较指令执行后触发了N标志位的置位（2-3=-1），这表明r0的值比数字3要小。随后，由于LT条件满足， 即V != N（在CPSR里溢出标志位和负标志位不是同一个），所以执行了addlt指令。由于第二个cmp指令将N标志位清空（因为3-3=0，不需要置位N标志位），将零标志位置位（Z=0），现在 V = 0且 N = 0，从而导致LT条件不成立，结果就是第二个addlt没有执行，r0也没改变，程序退出并返回结果3
CONDITIONAL EXECUTION IN THUMB Thumb模式下的条件执行
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