中文4200字
移植μC/OS-II
这篇文章介绍如何将μC/OS-II移植到不同的处理器上。所谓移植，就是使一个实时内核能在其他的微处理器上运行。为了方便移植，大部分μC/OS-II的代码是用C语言编写的：但是，仍需要用C语言和汇编语言编写一些与处理器硬件相关的代码，这是因为μC/OS-II在读/写处理器寄存器时，只能通过汇编语言来实现。由于μC/OS-II在设计前就已经考虑了可移植性，所以它的移植相对来说是比较容易的。要使μC/OS-II正常运行，处理器必须满足以下要求：
(1)处理器的C编译器能产生可重入型代码：
(2) 处理器支持中断，并且能产生定时中断(通常为10-100Hz)；
(3) 用C语言就可以开关中断；
(4) 处理器能支持一定数量的数据存储器的硬件堆栈；
(5) 处理器有将堆栈指针以及其他CPU寄存器的内容读出、并存储到堆栈或内存中去的指令；
如果已经了解处理器和C编译器的技术细节，那么移植的工作是非常容易的，测试一个像μC/OS-II这样的实时内核其实并不复杂，甚至可以在没有任何应用程序下测试，换句话说，就是让内核自己测试自己。有两种原因要这样做：第一，避免使本来就复杂的事情变的更加复杂化；第二，如果出现问题可以知道问题出在内核代码中，而不是应用程序中。刚开始时，可以运行一些简单的任务和时钟节拍中断程序。一旦多任务调度成功运行了，再添加应用程序的任务就更加容易了。
1.1 开发工具
如前所述移植μC/OS-II需要标准的C交叉编译器，并且是针对所使用的CPU 的；因为它是一个可剥夺的内核，只能通过C编译器来产生可重入型代码。同时C编译器还要支持汇编语言程序。绝大部分为嵌入式系统设计的C编译器都包括汇编器、链接器、定位器。链接器用来将不同的模块(编译过或汇编过的文件)链接成目标文件；定位器则允许将代码和数据放置在目标处理器的指定内存空间中。
所用的C编译器还提供另一种机制，能在C编译器中开中断和关中断。一些编译器允许在C语言源代码中直接插入汇编语句，这就使得插入相应的处理器中的指令开中断和关中断变得容易了。
1.2 文件
(1)INCLUDES.H文件
INCLUDES.H是一个头文件，它出现在每个.C文件的第一行，如下：
# include“ includes.h”
INCLUDES.H文件使得工程项目中的每个.C文件无需分别考虑它实际上需
要哪些头文件。使用INCLUDES.H文件的唯一缺点就是它可能包含一些与当前
要编译的.C文件实际上不相干的头文件。这意味着每个文件的编译时间都会增
加；但由于他增加了代码的可移植性，所以还是决定使用这种方法。也可以通过
重新编译INCLUDES.H文件来增加头文件，但是头文件必须添加在文件列表的
最后。
(2)OS_CPU.H文件
OS_CPU.H包含了用#define语句定义的、与处理器相关的常数、宏以及类
型。OS_CPU.H文件的大体结构如程序清单T1所列：
#ifdef
#define OS_CPU_EXT
#else
#define OS_CPU_EXT extern
#endif
typedef unsigned char BOOLEAN;
typedef unsigned char INT8U; /* 无符号8位整数*/
typedef signed char INT8S; /* 有符号8位整数*/
typedef unsigned int INT16U; /* 无符号16位整数*/
typedef signed int INT16S; /* 有符号16位整数*/
typedef unsigned long INT32U; /* 无符号32位整数 */ typedef signed long INT32S; /* 有符号32位整数*/
typedef float FP32; /* 单精度浮点数 */
typedef double FP64; /* 双精度浮点数 */
typedef unsigned int OS_STK; /* 堆栈入口宽度为16位*/
#define OS_ENTER_CRITICAL() ??? /* 关中断*/
#define OS_EXIT_CRITICAL() ??? /* 开中断s */ #define OS_STK_GROWTH 1 /* 定义堆栈方向：1=向下递减，0=向上递增
*/
#define OS_TASK_SW() ???
程序清单T1
①OS_CPU.H，与编译器相关的数据类型
因为不同的微处理器有不同的字长，所以μC/OS-II的移植包括了一系列的数据类型的定义，而确保其可移植性。尤其是，μC/OS-II代码从不使用C语言中的short, int 及long等数据类型，因为它们是与编译器相关的，是不可移植的。相反，定义的数据结构等既是可移植的，又很直观。
举例来说，INT16U数据类型总是代表16位的无符号整型数。这样μC/OS-II 就可以断定，声明为该数据类型变量的范围都是0～65535。将μC/OS-II移植到32位的处理器上，就意味着INT16U实际被声明为无符号短整型数，而不是无符号整数，但是，μC/OS-II处理的仍然是INT16U。
你必须将任务堆栈的数据类型告诉μC/OS-II。这是通过为OS_STK声明恰当的C数据类型来实现的。如果处理器的堆栈是32位的，那么就应该将OS_STK 声明:为：unsigned int,所有的任务堆栈都必须声明使用OS_STK作为它的数据类型。
用户需要做的只是查阅编译器文档，找出对应于μC/OS-II的标准的C的相应数据类型。
②OS_CPU.H，OS_ENTER_CRITICAL()和OS_EXIT_CRITICAL()
像其它的实时内核一样，μC/OS-II需要先关中断再处理临界段代码，并且在处理完毕后再重新开中段。这就能够保证临界段代码免受多任务或中断服务子程序的破坏。通常每个处理器都会提供一定的汇编指令来开关中断，C编译器必须有一定的机制直接从C语言中执行这些操作。有些编译器允许在C源代码中直接加入汇编语句，这就使得插入处理器指令来开关中断变的容易，有些其它的编译器提供语言扩展功能，可以直接从C语言中开关中断。为了隐藏编译器厂商提供的不同实现方法，以增加可移植性，μC/OS-II定义了2个宏，用来关开中断：OS_ENTER_CRITICAL()和OS_EXIT_CRITICAL()，如T1：
它们都是成对出现的，分别加在临界段代码的前面和后面；
μC/OS-II Service Function
{
OS_ENTER_CRITICAL();
/*临界段代码*/
OS_EXIT_CRITICAL();
}
T1
方法一：
实现OS_ENTER_CRITICAL()和OS_EXIT_CRITICAL()这两个宏的这种方法是最简单的方法，在中调用处理器指令关中断，以及在中调用相应处理器指令
开中断；但是这个过程还存在小小的问题：如果在禁止中断的情况下调用μC/OS-II函数，那么从μC/OS-II函数返回时，中断可能会变成允许的了。而实际上如果调用μC/OS-II之前中断是关掉的希望从μC/OS-II函数返回时，希望中断还是关掉的。在这种情况下，仅靠这种方法是不适宜的。
方法二：
执行OS_ENTER_CRITICAL()时，先将中断状态保存到堆栈中，然后关中断；而当执行OS_EXIT_CRITICAL()时，再从堆栈中恢复原来的中断开关状态。如果用这种方法，那么不管用户是在中断禁止，还是中断允许的情况下调用μC/OS-II 的功能函数，调用后都不会改变中断状态。应用程序可以调用OS_ENTER_CRITICAL()和OS_EXIT_CRITICAL()，以保护临界段代码；但是，在使用这种方法时需特别小心，因为如果在调用像OSTimeDly()之类的功能函数之前就关掉了中断，应用程序就会崩溃。发生这种情况的原因是，任务被挂起，知道延迟时间到，而中断是关掉的。OSTimeDly()实际上是依靠时钟节拍实现中断的，而因为中断是关掉的，程序不可能获得时钟节拍中断。明显的，所有的PEND调用都会涉及到这个问题，需十分小心。一个通用的办法是，应该在中断允许的情况下调用μC/OS-II的系统功能函数。
③OS_CPU.H, OS_STK_GROWTH
绝大多数微处理器和微控制器的堆栈都是从上往下递减的，但是也有某些处理器使用的是相反的方式，μC/OS-II被设计成对这两种情况都可以处理，只要再用配置常数OS_STK_GROWTH指定堆栈的方向就可以了：
置OS_STK_GROWTH为0，表示堆栈从下(低地址)往上(高地址)递增：
置OS_STK_GROWTH为1，表示堆栈从上(高地址)往下(低地址)递减。
④OS_CPU.H, OS_TASK_SW()
OS_TASK_SW()是一个宏，是在μC/OS-II从低优先级任务切换到高优先级任务时须用到的。OS_TASK_SW()总是在任务级代码中被调用。另一个函数OSIntExit()用在中断服务子程序ISR中。任务切换只是简单地将处理器的寄存器保存到将被挂起的任务的堆栈中，并且从堆栈中恢复要运行的更高优先级的任务。
在μC/OS-II中，处于就绪态任务的堆栈结构看起来就像刚刚发生过中断一样，所有的寄存器都保存在堆栈中。换句话说，μC/OS-II要运行处于就绪态的任务必须要做的事是，从任务堆栈中恢复所有的寄存器，并且执行中断返回指令。为了任务调度，可以通过执行OS_TASK_SW()模仿中断的产生。绝大多数处理器会提供软中断或指令陷阱来完成这项功能。中断服务子程序或指令陷阱处理函数(也叫做异常处理函数)的中断向量地址必须指向汇编语言函数OSCtxSw()。
例如，在Intel或者AMD80X86处理器上可以使用INT指令，但是中断向量
必须指向OSCtxSw()。有些处理器如Z80，并不提供软中断机制。在这种情况下需要想办法将堆栈结构设置成与软中断发生后的堆栈结构一样。在OS_TASK_SW()函数中调用OSCtxSw()，而不是将某个中断向量指向OSCtxSw()。实际上μC/OS-II已经被移植到了Z80处理器上，μC/OS-II也同样是可以的。(3)OS_CPU_A.ASM
μC/OS-II的移植实例要求用户编写4个简单的汇编程序：
OSStartHighRdy()
OSCtxSw()
OSIntCtxSw()
OSTickISR()
如果编译器支持插入行汇编代码，就可以将所有与处理器相关的代码放到OS_CPU.C文件中，而不必再有单独的汇编语言文件。
①OS_CPU_A.ASM, OSStartHighRdy()
OSStart()函数调用OSStartHighRdy()来使就绪态任务中优先级最高的任务开始运行，在调用它之前，要已经建立了至少一个应用任务。OSStartHighRdy()假设OSTCBHighRdy指向最高优先级任务的任务控制块。就像先前提到的，在μC/OS-II中处于就绪态任务的堆栈结构看起来就像刚发生过中断一样，所有的寄存器都保存在堆栈中。要想运行最高优先级任务，需将所有处理器按顺序从任务堆栈中恢复出来，并且执行中断返回指令。为简单起见，堆栈指针总是存储字任务控制块的开头。换句话说，也就是需要恢复的任务堆栈指针总是存储在任务控制块的偏移地址为0的位置。
注意到OSStartHighRdy()必须调用OSTaskSwHook()函数，但OSStartHighRdy()函数只是做了任务切换工作的一半——只是完成了高优先级任务寄存器的恢复，而并没有保存当前任务的寄存器。OSTaskSwHook()函数必须检查OSRunning位，以确定OSTaskSwHook()函数是被OSStartHighRdy()调用的(OSRunning是FALSE)，还是在正常的任务切换之中(OSRunning是TRUE)被调用的。
②OS_CPU_A.ASM, OSCtxSw()
任务级的切换是通过执行软中段指令，或者依据处理器的不同，执行TRAP 指令来实现的。中断服务子程序，陷阱或异常处理的向量地址必须指向OSCtxSw()。
如果当前任务调用μC/OS-II提供的功能函数，并使得更高优先级任务进入了就绪态，在系统服务调用的最后，μC/OS-II会调用OSSched()，并由此推断出当前任务不是需要运行的最重要的任务了，OSSched()先将最高优先级任务的地址装载到OSTCBHighRdy，再通过调用OS_TASK_SW()执行软中断或陷阱指令。注意变量OSTCBCur已经包含了指向当前任务控制块的指针。软中断指令会强
制将处理器的一些寄存器保存到当前任务的堆栈中并使处理器执行OSCtxSw()。
OSCtxSw()示意性代码如T2，这些代码必须用汇编语言编写，因为用户不能直接在C语言中访问CPU寄存器。
void OSCtxSw(void)
{
保存处理器寄存器；
在当前任务的任务控制块中保存当前任务的堆栈指针:
OSTCBCur->OSTCBStkPtr = Stack pointer;
Call user definable OSTaskSwHook();
OSTCBCur = OSTCBHighRdy;
OSPrioCur = OSPrioHighRdy;
得到将要重新开始运行的任务的堆栈指针:
Stack pointer = OSTCBHighRdy->OSTCBStkPtr;
从新任务的任务堆栈中恢复处理器所有寄存器的值；
执行中断返回指令；
}
T2 OSCtxSw()的示意性代码
③OS_CPU_A.ASM, OSIntCtxSw()
OSIntExit()通过调用OSIntCtxSw()，在ISR中执行任务切换功能。因为OSIntCtxSw()是在ISR中被调用的，所以假定所有的处理器都被正确地保存到了被中断任务的堆栈之中。OSIntCtxSw()的示意性代码如T3所示，这些代码必须用汇编语言编写，因为在C语言中不能直接访问CPU寄存器。如果编译器支持插入汇编语言代码，就可以将OSIntCtxSw()的代码放在OS_CPU_C.C文件中，而不放在OS_CPU_A.ASM文件中。实际上如果需要，可以跳转到OSCtxSw()中相同的代码，以减少代码量。
void OSIntCtxSw(void)
{
调用用户定义的OSTaskSwHook();
OSTCBCur = OSTCBHighRdy;
OSPrioCur = OSPrioHighRdy;
得到将要重新执行任务的堆栈指针:
Stack pointer = OSTCBHighRdy->OSTCBStkPtr;
从新任务堆栈中恢复所有处理器寄存器；
执行中断返回指令；
}
T3 OSIntCtxSw()的示意性代码
④OS_CPU_A.ASM, OSTickISR()
μC/OS-II要求用户提供一个周期性的时钟源，来实现时间的延迟和超时功能。时钟节拍应该每秒发生10~100次/秒。为了完成该任务，可以使用硬件定时器，也可以从交流点中获得50/60Hz的时钟频率。
必须在开始多任务后，即调用OSStart()后，启动时钟节拍中断；然后，可以在OSStart()运行后，μC/OS-II启动运行的第一个任务中初始化节拍中断。通常容易犯的错误是，在调用OSInit()和OSStart()之间打开了时钟节拍。(如程序清单T4所列)。
void main(void)
{
OSInit(); /* 初始化μC/OS-II */
/* 应用程序初始化代码*/
/* 调用OSTaskCreate() 建立至少1个任务*/
开时钟节拍中断; /* 千万不要在这里开中断*/
OSStart(); /* 开始多任务*/ }
T4 在不正确的位置启动节拍中断
Porting μC/OS-II
This article describes in general terms what needs to be done in order to adapt μC/OS-II to different processors. Adapting a real-time kernel to a microprocessor or a microcontroller is called a port. Most of μC/OS-II is written in C for portability, however, it is still necessary to write some processor specific code in C and assembly language. Specifically, μC/OS-II manipulates processor registers which can only be done through assembly language. Porting μC/OS-II to different processors is relatively easy because μC/OS-II was designed to be portable.
A processor can run μC/OS-II if it satisfies the following general requirements:
(1) You must have a C compiler for the processor and the C compiler must be able to produce reentrant code.
(2) You must be able to disable and enable interrupts from C.
(3) The processor must support interrupts and you need to provide an interrupt that occurs at regular intervals (typically between 10 to 100 Hz).
(4) The processor must support a hardware stack, and the processor must be able to store a fair amount of data on the stack (possibly many Kbytes).
(5) The processor must have instructions to load and store the stack pointer and other CPU registers either on the stack or in memory.
Porting μC/OS-II is actually quite straightforward once you understand the subtleties of the target processor and the C compiler you will be using. Depending on the processor, a port can consist of writing or changing between 50 and 300 lines of code! Porting μC/OS-II could take you anywhere between a few hours to about a week.
Once you have a port of μC/OS-II for your processor, you will need to verify its operation. Testing a multitasking real-time kernel such as μC/OS-II is not as complicated as you may think. You should test your port without application code. In other words, test the operations of the kernel by itself. There are two reasons to do this. First, you don?t want to complicate things anymore than they need to be. Second, if something doesn?t work, you know that the prob lem lies in the port as opposed to your application. Start with a couple of simple tasks and only the ticker interrupt service routine. Once you get multitasking going, it?s quite simple to add your application tasks.
1.1 Development Tools
As previously stated, you need a C compiler for the processor you intend to use in order to port μC/OS-II. Because μC/OS-II is a preemptive kernel, you should only use a C compiler that generates reentrant code. Your C compiler must also be able to support assembly language programming. Most C compiler designed for embedded systems will, in fact, also include an assembler, a linker, and a locator. The linker is used to combine object files (compiled and assembled files) from different modules while the locator will allow you to place the code and data anywhere in the memory map of the target processor. Your C compiler must also provide a mechanism to disable and enable interrupts from C. Some compilers will allow you to insert in-line assembly language statements in your C source code. This makes it quite easy to insert the proper processor instructions to enable and disable interrupts.
1.2 Files
(1) INCLUDES.H
INCLUDES.H is a MASTER include file and is found at the top of all .C files as follows:
# include “includes.h”
INCLUDES.H allows every .C file in your project to be written without concerns about which header file will actually be needed. The only drawback to having a master include file is that INCLUDES.H may include header files that are not pertinent to the actual .C file being compiled. This means that each file will require extra time to compile. This inconvenience is offset by code portability. You can edit INCLUDES.H to add your own header files but, your header files should be added at the end of the list.
(2) OS_CPU.H
OS_CPU.H contains processor and implementation specific #defines constants, macros, and typedefs. The general layout of OS_CPU.H is shown in listing #ifdef OS_CPU_GLOBALS
#define OS_CPU_EXT
#else
#define OS_CPU_EXT extern
#endif
typedef unsigned char BOOLEAN;
typedef unsigned char INT8U; /* Unsigned 8 bit quantity*/ (1)
typedef signed char INT8S; /* Signed 8 bit quantity*/
typedef unsigned int INT16U; /* Unsigned 16 bit quantity*/
typedef signed int INT16S; /* Signed 16 bit quantity*/
typedef unsigned long INT32U; /* Unsigned 32 bit quantity*/
typedef signed long INT32S; /* Signed 32 bit quantity*/
typedef float FP32; /*Single precision floating point*/ (2)
typedef double FP64; /* Double precision floating point */ typedef unsignedint OS_STK; /*Each stack entryis 16-bit wide*/
#define OS_ENTER_CRITICAL() ??? /* Disable interrupts*/ (3)
#define OS_EXIT_CRITICAL() ??? /* Enable interrupts*/
#define OS_STK_GROWTH 1 /* Define stack growth: 1 = Down, 0 = Up*/ (4)
#define OS_TASK_SW() ???
①OS_CPU.H, Compiler Specific Data Types
Because different microprocessors have different word length, the port of μC/OS-II includes a series of type definitions that ensures portability. Specifically, μC/OS-II?s code never makes use of C?s short, int an d, long data types because they are inherently non-portable. Instead, I defined integer data types that are both portable and intuitive
The INT16U data type, for example, always represents a 16-bit unsigned integer. μC/OS-II and your application code can now assume that the range of values for variables declared with this type is from 0 to 65535. A μC/OS-II port to a 32-bit processor could mean that an INT16U is actually declared as an unsigned short instead of an unsigned int. Where μC/OS-II is concerned, however, it still deals with an INT16U.
You must tell μC/OS-II the data type of a task?s stack. This is done by declaring the proper C data type for OS_STK. If stack elements on your processor are 32-bit and your compiler documentation specify that an int is 32-bit then, you would declare OS_STK as being of type unsigned int. All task stacks MUST be declared using OS_STK as its data type.
All you have to do is to consult the compiler?s manual and find the standard C data types that corresponds to the types expected by μC/OS-II.
②OS_CPU.H, OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL()
μC/OS-II like all real-time kernels need to disable interrupts in order to access critical sections of code, and re-enable interrupts when done. This allows μC/OS-II to protect critical code from being entered simultaneously from either multiple tasks or Interrupt Service Routines (ISRS). Every processor generally provide instructions to disable/enable interrupts and your C compiler must have a mechanism to perform these operations directly from C. Some compilers will allow you to insert in-line assembly language statements in your C source code. This makes it quite easy to insert processor instructions to enable and disable interrupts. Other compilers will actually contain language extensions to enable and disable interrupts directly from C. To hide the implementation method chosen by the compiler manufacturer, μC/OS-II defines two macros to disable and enable interrupts: OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL(), respectivelyT1.
μC/OS-II?s critical sections are wrapped with OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL() as shown below:
μC/OS-II Service Function
{
OS_ENTER_CRITICAL(); /* μC/OS-II critical code section */
OS_EXIT_CRITICAL();
}
T1
Method #1:
The first and simplest way to implement these two macros is to invoke the processor instruction to disable interrupts for OS_ENTER_CRITICAL() and the enable interrupts instruction for OS_EXIT_CRITICAL(). There is, however, a little problem with this scenario. If you called the μC/OS-II function with interrupts disabled then, upon return from μC/OS-II, interrupts would be enabled! If you had interrupts disabled, you may have wanted them to be disabled upon return from the μC/OS-II function. In this case, the above implementation would not be adequate. Method #2:
The second way to implement OS_ENTER_CRITICAL() is to save the interrupt disable status onto the stack and then, disable interrupts. OS_EXIT_CRITICAL() would simply be implemented by restoring the interrupt status from the stack. Using this scheme, if you called a μC/OS-II service with either interrupts enabled or disabled then, the status would be preserved across the call.Your application can use OS_ENTER_CRITICAL()and OS_EXIT_CRITICAL() to also protect critical sections of code. Be careful, however, because your application will …crash? if you have interrupts disabled before calling a service such as OSTimeDly(). This will happen because the task will be suspended until time expires but, because interrupts are disabled, you would never service the tick interrupt! Obviously, all the PEND calls are also subject to this problem so, be careful. As a general rule, you should always call μC/OS-II services with interrupts enabled!
③OS_CPU.H, OS_STK_GROWTH
The stack on most microprocessors and microcontrollers grows from high-memory to low-memory. There are, however, some processors that work the other way around. μC/OS-II has been designed to be able to handle either flavor. This is accomplished by specifying to μC/OS-II which way the stack grows through the configuration constant OS_STK_GROWTH as shown below:
Set OS_STK_GROWTH to 0 for Low to High memory stack growth.
Set OS_STK_GROWTH to 1 for High to Low memory stack growth.
④OS_CPU.H, OS_TASK_SW()
OS_TASK_SW() is a macro that is invoked when μC/OS-II switches from a low-priority task to the highest-priority task. OS_TASK_SW() is always called from task level code. Another mechanism, OSIntExit(), is used to perform a context switch when an ISR makes a higher priority task ready for execution. A context switch simply consist of saving the processor registers on the stack of the task being suspended, and restoring the registers of the higher-priority task from its stack.
In μC/OS-II, the stack frame for a ready task always looks as if an interrupt has just occurred and all processor registers were saved onto it. In other words, all that μC/OS-II has to do to run a ready task is to restore all processor registers from the task?s stac k and execute a return from interrupt. To switch context, you should implement OS_TASK_SW() so that you simulate an interrupt. Most processors provide either software interrupt or TRAP instructions to accomplish this. The ISR or trap handler (also called t he …exception handler?) MUST vector to the assembly language function OSCtxSw()
For example, a port for an Intel or AMD 80x86 processor would use an INT instruction. The interrupt handler would need to vector to OSCtxSw(). There are some processors like the Zilog Z80 that do not provide a software interrupt mechanism. In this case, you would need to simulate the stack frame as closely to an interrupt stack frame as you can. In this case, OS_TASK_SW() would simply call OSCtxSw() instead of vector to it. The Z80 is a processor that has been ported to μC/OS and thus would be portable to μC/OS-II.
(3) OS_CPU_A.ASM
A μC/OS-II port requires that you write four fairly simple assembly language functions:
OSStartHighRdy()
OSCtxSw()
OSIntCtxSw()
OSTickISR()
If your compiler supports in-line assembly language code, you could actually place all the processor specific code into OS_CPU_C.C instead of having a separate assembly language file.
①OS_CPU_A.ASM, OSStartHighRdy()
This function is called by OSStart() to start the highest priority task ready-to-run. Before you can call OSStart(), however, you MUST have created at least one of your tasks. OSStartHighRdy() assumes that OSTCBHighRdy points to the task control block of the task with the highest priority. As mentioned previously, in μC/OS-II, the stack frame for a ready task always looks as if an interrupt has just occurred and all processor registers were saved onto it. To run the highest priority task all you need to do is restore all processor registers f rom the task?s stack in the proper order and, execute a return from interrupt. To simplify things, the stack pointer is always stored at the beginning of the task control block (i.e. its OS_TCB). In other words, the stack pointer of the task to resume is always stored at offset 0 in the OS_TCB.
Note that OSStartHighRdy() MUST call OSTaskSwHook() because we are basically doing a …half? context switch – we are restoring the registers of the highest priority task. OSTaskSwHook() can examine OSRunning to tell it whether OSTaskSwHook() was called from OSStartHighRdy() (i.e. if OSRunning is FALSE) or from a regular context switch (i.e. OSRunning is TRUE).
②OS_CPU_A.ASM, OSCtxSw()
As previously mentioned, a task level context switch is accomplished by issuing a software interrupt instruction or, depending on the processor, executing a TRAP instruction. The interrupt service routine, trap or exception handler MUST vector to OSCtxSw().
The current task calls a service provided by μC/OS-II which causes a higher priority task to be ready-to-run. At the end of the service call, μC/OS-II calls the function OSSched() which concludes that the current task is no longer the most important task to run. OSSched() loads the address of the highest priority task into OSTCBHighRdy and then executes the software interrupt or trap instruction by invoking the macro OS_TASK_SW(). Note that the variable OSTCBCur already contains a pointer to the current task?s Task Control Block, OS_TCB. The software interrupt instruction (or trap) forces some of the processor registers (most likely the return address and the processor?s status word) onto the current task?s stack and the processor then vectors to OSCtxSw().
The pseudo code of what needs to be done by OSCtxSw() is shown in listingT2. This code must be written in assembly language because you cannot access CPU registers directly from C.
void OSCtxSw(void)
{
Save processor registers;
Save the current task?s stack pointer into the current task?s OS_TCB:
OSTCBCur->OSTCBStkPtr = Stack pointer;
Call user definable OSTaskSwHook();
OSTCBCur = OSTCBHighRdy;
OSPrioCur = OSPrioHighRdy;
Get the stack pointer of the task to resume:
Stack pointer = OSTCBHighRdy->OSTCBStkPtr;
Restore all processor registers from the new task?s stack;
Execute a return from interrupt instruction;
}
T2 Pseudo code for OSCtxSw()
③OS_CPU_A.ASM, OSIntCtxSw()
OSIntCtxSw() is a function that is called by OSIntExit() to perform a context switch from an ISR. Because OSIntCtxSw() is called from an ISR, it is assumed that all the processor registers are properly saved onto the interrupted task?s stack. The pseudo code in listing T3 shows what needs to be done by OSIntCtxSw(). This code must be written in assembly language because you cannot access CPU registers directly from C. If your C compiler supports in-line assembly, you can put the code for OSIntCtxSw() in OS_CPU_C.C instead of OS_CPU_A.ASM. As you can see, except for the first line, the code is identical to OSCtxSw(). You can thus reduce the amount of code in the port by …jumping? to the appropriate section of code in OSCtxSw().
void OSIntCtxSw(void)
{
Call user definable OSTaskSwHook();
OSTCBCur = OSTCBHighRdy;
OSPrioCur = OSPrioHighRdy;
Get the stack pointer of the task to resume:
Stack pointer = OSTCBHighRdy->OSTCBStkPtr;
Restore all processor registers from the new task?s stack;
Execute a return from interrupt instruction;
}
T3 Pseudo code for OSIntCtxSw()
④OS_CPU_A.ASM, OSTickISR()
μC/OS-II requires that you provide a periodic time source to keep track of time delays and timeouts. A …tick? should occur between 10 and 100 times per second, or Hertz. To accomplish this, you can either dedicate a hardware timer, or obtain 50/60 Hz from an AC power line.
You must enable ticker interrupts AFTER multitasking has started, i.e. after calling OSStart(). In other words, you should initialize and tick interrupts in the first task that executes following a call to OSStart(). A common mistake is to enable ticker interrupts between calling OSInit() and OSStart() as shown in listing T4 void main(void)
{
OSInit(); /* Initialize μC/OS-II */
/* Application initialization code */
/* Create at least on task by calling OSTaskCreate() */
Enable TICKER interrupts; /* DO NOT DO THIS HERE!!! */
OSStart(); /* Start multitasking */
}
T4 Incorrect place to start the tick interrupt.
Photoshop中英文对照 1、File 文件 New 新建 Open 打开 Open As 打开为 Open Recent 最近打开文件 Close 关闭 Save 存储 Save As 存储为 Save for Web 存储为Web所用格式Revert 恢复 Place 置入 Import 输入 PDF Image PDF图象导入Annotations 注释 Export 输出 Manage Workflow 管理工作流程Check In 登记 Undo Check Out 还原注销 Upload To Server 上载到服务器Add To Workflow 添加到工作流程 Open From Workflow 从工作流程打开 Automate 自动 Batch 批处理 Create Droplet 创建快捷批处理 Conditional Mode Change 条件模式更改 Contact Sheet 联系表 Fix Image 限制图像 Multi Page PDF to PSD 多页面PDF文件到PSD文件 Picture package 图片包 Web Photo Gallery Web照片画廊File Info 文件简介 Print Options 打印选项 Page Setup 页面设置 Print 打印 Jump to 跳转到 Exit 退出 2、Edit 编辑 Undo 还原 Step Forward 向前 Step Backward 返回 Fade 消退 Cut 剪切 Copy 拷贝 Copy Merged 合并拷贝 Paste 粘贴 Paste Into 粘贴入 Clear 清除 Fill 填充 Stroke 描边 Free Transform 自由变形Transform 变换 Again 再次 Scale 缩放 Rotate 旋转 Skew 斜切 Distort 扭曲 Perspective 透视 Rotate 180°旋转180度 Rotate 90°CW 顺时针旋转90度Rotate 90°CCW 逆时针旋转90度
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A Wavelet Based Approach for Fast Detection of Internal Fault in Power Transformers The power transformer is one of the most expensive elements of power system and its protection is an essential part of the overall system protection strategy. The differential protection provides the best protection for power transformer. Its operation principle is based on this point that the differential current during an internal fault is higher than normal condition. But, a large transient current (inrush current) can cause mal-operation of differential relays. Then, studies for the improvement of the transformer protection have focused on discrimination between internal short circuit faults and inrush currents in transformers. The magnetizing inrush current has a large second order harmonic component in comparison to internal faults. Therefore , some transformer protection systems are designed to halt operating during the inrush current by sensing this large second order harmonic. The second harmonic component in the magnetizing inrush currents tend to be relatively small in modern large power transformers because of improvements in the power transformer core materials. Also , it has been seen that the fault current can contain higher second order harmonics than the inrush current due to nonlinear fault resistance, CT saturation .the distributed capacitance in the transmission line, which transformer is connected to, or due to the use of extra high voltage underground cables. Various methods have been suggested for overcoming this protection system mal-operation. This paper presents a wavelet based method for discrimination among inrush current, internal short circuit ,external short circuit and energizing and it is not affected by CT saturation and it is able to detect internal faults while transformer energization. Unlike Artificial Neural Network and Fuzzy logic based algorithms. This approach is not system dependent. The operating time of the scheme is less than 10ms. The Daubechies mother wavelet is used with a sample rate of 5 kHz. Then , the differential currents of the three phases are decomposed into two details and only the second level will be considered by using db5 mother wavelet. Discrete Wavelet Transform The wavelet transform is a powerful tool to extract information from the non-stationary signals simultaneously in both time and frequency domains. The ability of the wavelet transform to focus on short time intervals for high-frequency components and long intervals for low-frequency components improves the analysis
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例1.Winners do not dedicate their lives to a concept of what they imagine they should be, rather, they are themselves and as such do not use their energy putting on a performance, maintaining pretence and manipulating（操纵） others . They are aware that there is a difference between being loved and acting loving, between being stupid and acting stupid, between being knowledgeable and acting knowledgeable. Winners do not need to hide behind a mask. 1.dedicate to 把时间，精力用于 2.pretence 虚伪，虚假 6 ．1 斤斤于字比句次，措辞生硬 例2.Solitude is an excellent laboratory in which to observe the extent to which manners and habits are conditioned by others. My table manners are atrocious( 丑恶)—in this respect I've slipped back hundreds of years in fact, I have no manners whatsoever（完全，全然）. If I feel like it, I eat with my fingers, or out of a can, or standing up —in other words, whichever is easiest. 孤独是很好的实验室，正好适合观察一个人的举止和习惯在多大程度上受人制约。如今我吃东西的举止十分粗野；这方面一放松就倒退了几百年，实在是一点礼貌也没有。我高兴就用手抓来吃，（eat out of a can）开个罐头端着吃，站着吃；反正怎么省事就怎么吃。 3.Whatsoever 完全，全然 1.Be conditioned by 受……制约 2.Atrocious 丑恶 6 ．2 结构松散，表达过于口语化 例3．有一次，在拥挤的车厢门口，我听见一位男乘客客客气气地问他前面的一位女乘客：“您下车吗？”女乘客没理他。“您下车吗？”他又问了一遍。女乘客还是没理他。他耐不住了，放大声问：“下车吗？”，那女乘客依然没反应。“你是聋子，还是哑巴？”他急了，捅了一下那女乘客，也引起了车厢里的人都往这里看。女乘客这时也急了，瞪起一双眼睛，回手给了男乘客一拳。（庄绎传，英汉翻译教程，1999 ：练习 3 ） 译文1：Once at the crowded door of the bus, I heard a man passenger asked politely a woman passenger before him: “Are you getting off?” The woman made no
1、Just as Darwin discovered the law of development of organic nature, so Marx discovered the law of development of human history: the simple fact, hitherto concealed by an overgrowth of ideology, that mankind must first of all eat, drink, have shelter and clothing, before it can pursue polities, science, art, religion, etc.; that therefore the production of the immediate material means of subsistence and consequently the degree of economic development attained by a given people or during a given epoch form the foundation upon which the state institutions, the legal conceptions, art, and even the ideas on religion, of the people concerned have been evolved, and in the light of which they must, therefore, be explained, instead of vice versa, as had hitherto been the case. 正像达尔文发现有机界的规律一样，马克思发现了人类历史的发展规律，即历来被繁茂芜杂的意识形态所掩盖着的一个简单事实：人们首先必须吃、喝、住、穿，然后才能从事政治、科学、艺术、宗教等活动；所以，直接的物质生活资料的生产和一个民族或一个时代的一定的经济发展程度，便构成为基础，人们的国家制度，法律的概念，艺术以至宗教概念，就是从这个基础上发展起来的，因而，也必须由这个基础来解释，而不像过去那样做得正好相反。 2、Behaviorists suggest that the child who is raised in an environment where there are many stimuli which develop his or her capacity for appropriate responses will experience greater intellectual development. 行为主义者认为, 如果儿童的成长环境里有许多刺激因素, 这些因素又有利于其适当反应能力的发展, 那么, 儿童的智力就会发展到较高的水平。 3、How it happened that a little rocky peninsula in a remote corner of the Mediterranean was able to provide our world in less than two centuries with the complete framework for all our present day experiments in politics, literature, drama, sculpture, chemistry, physics and Heaven knows what else, is a question
The finite element analysis Finite element method, the solving area is regarded as made up of many small in the node connected unit (a domain), the model gives the fundamental equation of sharding (sub-domain) approximation solution, due to the unit (a domain) can be divided into various shapes and sizes of different size, so it can well adapt to the complex geometry, complex material properties and complicated boundary conditions Finite element model: is it real system idealized mathematical abstractions. Is composed of some simple shapes of unit, unit connection through the node, and under a certain load. Finite element analysis: is the use of mathematical approximation method for real physical systems (geometry and loading conditions were simulated. And by using simple and interacting elements, namely unit, can use a limited number of unknown variables to approaching infinite unknown quantity of the real system. Linear elastic finite element method is a ideal elastic body as the research object, considering the deformation based on small deformation assumption of. In this kind of problem, the stress and strain of the material is linear relationship, meet the generalized hooke's law; Stress and strain is linear, linear elastic problem boils down to solving linear equations, so only need less computation time. If the efficient method of solving algebraic equations can also help reduce the duration of finite element analysis. Linear elastic finite element generally includes linear elastic statics analysis and linear elastic dynamics analysis from two aspects. The difference between the nonlinear problem and linear elastic problems: 1) nonlinear equation is nonlinear, and iteratively solving of general; 2) the nonlinear problem can't use superposition principle; 3) nonlinear problem is not there is always solution, sometimes even no solution. Finite element to solve the nonlinear problem can be divided into the following three categories: 1) material nonlinear problems of stress and strain is nonlinear, but the stress and strain is very small, a linear relationship between strain and displacement at this time, this kind of problem belongs to the material nonlinear problems. Due to theoretically also cannot provide the constitutive relation can be accepted, so, general nonlinear relations between stress and strain of the material based on the test data, sometimes, to simulate the nonlinear material properties available mathematical model though these models always have their limitations. More important material nonlinear problems in engineering practice are: nonlinear elastic (including piecewise linear elastic, elastic-plastic and viscoplastic, creep, etc. 2) geometric nonlinear geometric nonlinear problems are caused due to the nonlinear relationship between displacement. When the object the displacement is larger, the strain and displacement relationship is nonlinear relationship. Research on this kind of problem Is assumes that the material of stress and strain is linear relationship. It consists