At the simplest level, computing environments are thought to consist of hardware, an operating system that runs on the hardware and applications that run on the OS The OS is aware of all the capacity and capability of the underlying hardware and controls it directly.

If another layer of software were placed between the OS and the CPU, then the OS would know only what that extra layer of software told it. Its understanding of the capacity and capability of the underlying hardware would depend on the intervening software layer, and it would be able to control the underlying hardware only in ways the intervening layer of software allowed it to.

在大系统开发过程中，测试包含几个阶段。首先，常常将每个程序模块与系统中的其他程序分开，作为单个程序测试，这样的测试称为模块测试或单元测试。它验证模块在设计时期望的输入类型情况下是否正确运行。只要有可能，都要在受控环境下进行模块测试，以便测试小组可以给在测模块输入一个预定的数据集，观察产生什么样的输出数据。另外，测试小组应检查内部数据结构、逻辑和输入、输出数据的边界条件影音翻译。

In the development of a large system，testing involves several stages．First，each program
module is tested as a single program，usually isolated from the other programs in the system．Such testing，known as module testing or unit-testing，verifies that the module functions properly with the types of input expected from studying the module design．Unit testing is done in a controlled environment whenever possible so that the test team can feed a predetermined set of data to the module being tested and observe what output data are produced[1]．In addition，the test team checks the internal data structures，the logic，and the boundary conditions for the input and output data．