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14755-123

技巧7 – 避免易失存储器分配

不习惯在资源有限环境下工作的工程师，可能会试图使用其编程语言的特性，这种语言让他们可以使用易失存储器分配。毕竟，这是一种常在计算器系统中使用的技术，在计算器系统中，只有在有必要时，内存才会被分配。例如，以C开发时，工程师可能倾向于使用malloc来分配在堆（heap）上的空间。有一个操 作会执行，一旦完成，可以使用free将被分配的内存返回，以便堆的使用。

在资源受限的系统，这可 能是一场灾难！使用易失存储器分配的其中一个问题是，错误或不当的技术可能会导致内存泄漏或内存碎片。如果出现这些问题时，大多数的嵌入式系统并没有 资源或知识来监视堆或妥善地处理它。而当它们发生时，如果应用程序提出对空间的要求，但却没有所请求的空间可以使用，会发生什么事呢？

使用易失存储器分配所产生的问题是很复杂的，要妥善处理这些问题，可以说是一个噩梦！一种替代的方法是，直接以静态的方式，简化内存的分配。例如，只要在 程序中简单地建立一个大小为256字节长的缓冲区，而不是经由malloc请求这样大小的内存缓冲区。此一分配的内存可在整个应用程序的生命周期期 间保持，且不会有堆或内存碎片问题方面的顾虑。

这些都只是一些可以让开发人员开始建立更可靠嵌入式系统的方法。另外还有很多其他技术，例如利用良好的编码标准、位翻转的监测、执行数组和指针边界检查，及使用断言等。所有这些技术都是让设计者可以开发出可靠性更高嵌入式系统的秘诀。

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先介绍几点经验。
1、先通读一遍“系统手册”（不管能不能看懂，先了解个大概，以便学习编程时查找）；
2、由位控逻辑入手，逐渐增加定时、计数等条件，把或、与、非关系理顺；
3、进而练习整数运算、实数运算、类型转换；
4、量程转换、PID调节回路的建立及P、I、D参数的理解（最好是有个物理回路验证，比如变频器+电机+测速传感器）；
5、循环、顺序、跳转等程序控制（尽量避免跳转，非跳不可时也是跳的越近越好）；
6、中断、通讯、位置控制....

本人以ABB工业机器人配件DSQC系列和Motorola MVME系列，GE 卡件 模块 驱动器系列为主攻。各位电工朋友可根据自己的条件选择适合自己的PLC品牌，由于PLC的基本框架大同小异，学会一种至于其它的学习起来也就简单了。

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