最近把手头的SD卡更新NK到NAND项目完成，总算抽空上网瞎逛游了。正好群里面有人推荐了一个系统，说是对小资源的单片机非常适合。这里我就推荐2篇网文：
这2篇网文一个讲系统简介，一个是里面宏的说明。熟悉一下是很不错的。
然后就是官网了：
~adam/pt/index.html
我翻译了里面的under the hood章节：How protothreads really work
What goes on behind the magical macros in the C protothreads library? Why are they macros? How do the macros work? Read on for the explanation.
In the C implementation of protothreads, all protothread operations are hidden behind C macros. The reason for building the protothread library on C macros rather than C functions is that protothreads alter the flow of control. This is typically difficult to do with C functions since such an implementation typically would require low-level assembly code to work. By implementing protothreads with macros, it is possible for them to alter the flow of control using only standard C constructs.
结构为他们改变程序的跳转。
This page explains how protothreads work under the hood. We do this by taking a look at how the C preprocessor expands the protothread macros, and by looking at how the resulting C code is executed.
下如何工作。我们来看看protothread C
First, we’ll introduce a simple example program written with protothreads. Since this is a simple program we can show the entire program, including the main() function from which the protothread is driven. The code, shown below, waits for a counter to reach a certain threshold, prints out a message, and resets the counter. This is done in a while() loop that runs forever. The counter is increased in the main() function.
Before we let the C preprocessor expand the above code, we’ll take a look at how the protothread macros are defined. In order to make things easier, we use a simpler definition than the actual definition from the protothreads source code. (The definition used here is a combined version of the protothread macros and the local continuation macros implemented with the C switch statement.) This definition looks like:
宏和本地延续宏与C switch语句结合的组合版本。）这个定义是这样的：
We see that the struct pt consists of a single unsigned short called lc, short for local continuation. This unsigned short variable is the source of the "two byte overhead" frequently mentioned on the protothread web pages. Furthermore, we see that the PT_THREAD macro simply puts a char before its argument. Also, we note how the PT_BEGIN and PT_END macros open and close a C switch statement, respectively. But the PT_WAIT_UNTIL macro is the most complex looking of them all. It contains one assignment, one case statement, one if statement, and even a return statement! Also, it uses the built-in __LINE__ macro twice. The __LINE__ macro is a special macro that the C preprocessor will expand to the line number at which the macro is issued. Finally, the PT_INIT macro simply initializes the lc variable to zero.
unsigned short 类型。这unsigned short宏是如何打开与关闭一个C swithc
Many of the statements used in the protothread macros are not commonly used in macros. The return statement used in the PT_WAIT_UNTIL macro breaks the flow of control in the function the macro is used. For this reason, many people dislike the use return statements in macros. The PT_BEGIN macro opens a switch statement, but does not close it. The PT_END macro closes a compound statement that it has not itself opened. These things does look weird when looked at without the perspective of protothreads. However, in the context of protothreads these things are absolutely essential to the correct operation of protothreads: the macros has to change the flow of control in the C function in which they are used. This is indeed the whole point of protothreads.
也只有懂的接受自己的失败，才能更好的去发挥自身优势，也才能够更好的去实现自我;
