什么是（（端口*）0x41004400UL）的意思是在这里吗？

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问题描述

我工作的一个开发板，上有一个32位基于ARM microntroller（即板是Atmel SAM D21J18A）。我仍然在学习阶段，我有很多去，但我真的到嵌入式系统中。

I'm working on a developing board that has a 32-bit ARM based microntroller on it (namely the board is Atmel SAM D21J18A). I'm still at the learning phase and I have a lot to go, but I'm really into embedded systems.

我在C.一些背景然而，这显然是不够的。我一直在寻找在$ C $由Atmel实例项目的CS，我并没有真正得到它的某些部分。这里是其中的一个：

I have some background in C. However, it's obviously not enough. I was looking at the codes of an example project by Atmel, and I didn't really get some parts of it. Here is one of them:

    #define PORT              ((Port     *)0x41004400UL) /**< \brief (PORT) APB Base Address */

端口被定义为：

    typedef struct {
        PortGroup             Group[2];    /**< \brief Offset: 0x00 PortGroup groups [GROUPS] */
    } Port;

和端口组的定义是：

typedef struct {
    __IO PORT_DIR_Type             DIR;         /**< \brief Offset: 0x00 (R/W 32) Data Direction */
    __IO PORT_DIRCLR_Type          DIRCLR;      /**< \brief Offset: 0x04 (R/W 32) Data Direction Clear */
    __IO PORT_DIRSET_Type          DIRSET;      /**< \brief Offset: 0x08 (R/W 32) Data Direction Set */
    __IO PORT_DIRTGL_Type          DIRTGL;      /**< \brief Offset: 0x0C (R/W 32) Data Direction Toggle */
    __IO PORT_OUT_Type             OUT;         /**< \brief Offset: 0x10 (R/W 32) Data Output Value */
    __IO PORT_OUTCLR_Type          OUTCLR;      /**< \brief Offset: 0x14 (R/W 32) Data Output Value Clear */
    __IO PORT_OUTSET_Type          OUTSET;      /**< \brief Offset: 0x18 (R/W 32) Data Output Value Set */
    __IO PORT_OUTTGL_Type          OUTTGL;      /**< \brief Offset: 0x1C (R/W 32) Data Output Value Toggle */
    __I  PORT_IN_Type              IN;          /**< \brief Offset: 0x20 (R/  32) Data Input Value */
    __IO PORT_CTRL_Type            CTRL;        /**< \brief Offset: 0x24 (R/W 32) Control */
    __O  PORT_WRCONFIG_Type        WRCONFIG;    /**< \brief Offset: 0x28 ( /W 32) Write Configuration */
    RoReg8                         Reserved1[0x4];
    __IO PORT_PMUX_Type            PMUX[16];    /**< \brief Offset: 0x30 (R/W  8) Peripheral Multiplexing n */
    __IO PORT_PINCFG_Type          PINCFG[32];  /**< \brief Offset: 0x40 (R/W  8) Pin Configuration n */
    RoReg8                         Reserved2[0x20];
} PortGroup;

所以在这里，我们正在寻找在地址0x41004400UL，得到的数据在那里，然后发生了什么？

So here, we are looking at the address 0x41004400UL, get the data in there, and then what happens?

我抬头一看这个，但找不到任何有用的东西。如果您有任何建议（教程，书籍等），请让我听到。

I looked up for this but couldn't find anything useful. If you have any suggestions (tutorials, books etc.), please let me hear.

推荐答案

一般来说，你可以访问硬件寄存器用C以这种方式：

Generally you can access a hardware register in C in this manner:

#define PORT  (*(volatile uint8_t*)0x1234)

0×1234 是寄存器地址

uint8_t有是寄存器的类型，在这种情况下，1个字节大。

挥发性是必需的，这样编译器知道它不能优化这样一个变量，但每次读或写在code规定的变量必须实际上是完成。

（挥发性uint8_t有*）注塑整数文字所需类型的地址。

最左边的 * 然后采取地址的内容，使宏可用于就像PORT是一个普通的变量。

0x1234 is the register address
uint8_t is the type of the register, in this case 1 byte large.
volatile is required so that the compiler knows it cannot optimize such a variable, but that each read or write to the variable stated in the code must actually be done.
(volatile uint8_t*) casts the integer literal to an address of the desired type.
The left-most * then take the contents of that address, so that the macro can be used just as if PORT was a regular variable.

请注意，这不分配任何东西！它只是假定有一个硬件寄存器present在给定的地址，可以通过指定的类型进行访问（ uint8_t有）。

Note that this does not allocate anything! It just assumes that there is a hardware register present at the given address, which can be accessed by the type specified (uint8_t).

使用，还可以有其他的C数据类型直接对应硬件寄存器的方法相同。例如，通过使用一个方便的结构，可以映射特定的硬件外设的整个寄存器区域。例如code然而有点危险和可疑的，因为它必须采取的东西像对齐/结构填充和走样帐户。

Using the same method you can also have other C data types to correspond directly hardware registers. For example by using a handy struct, you can map the whole register area of a particular hardware peripheral. Such code is however a bit dangerous and questionable, since it must take things like alignment/struct padding and aliasing in account.

至于在你的榜样具体code，它是一个典型的可怕的寄存器映射为一个特定的硬件外设（看起来像一个普通的通用I / O口）在某微控制器。一个这样的野兽一般具有每个编译器支持MCU。

As for the specific code in your example, it is a typical awful register map for a particular hardware peripheral (looks like a plain general-purpose I/O port) on a certain microcontroller. One such beast is typically provided with each compiler supporting the MCU.

这样的寄存器映射黯然总是写在可怕的，完全不可移植的方法。例如，两个下划线 __ 是C.禁标识符无论是编译器和程序员被允许这样的声明标识符（7.1.3）。

Such register maps are sadly always written in awful, completely non-portable ways. For example, two underscores __ is a forbidden identifier in C. Neither the compiler nor the programmer is allowed to declare such identifiers (7.1.3).

什么是真正奇怪的是，他们已经省略了挥发性关键字。这意味着，你这里有这些情形之一：

What's really strange is that they have omitted the volatile keyword. This means that you have one of these scenarios here:

volatile关键字隐藏在下方端口定义。最有可能的是这种情况，或者

寄存器映射充满了致命的错误，或者

编译器就是这样一个可怕的一块垃圾，它并没有在所有优化变量。这将使问题与挥发性走开。

The volatile keyword is hidden beneath the Port definition. Most likely this is the case, or
The register map is full of fatal bugs, or
The compiler is such an awful piece of crap that it doesn't optimize variables at all. Which would make the issues with volatile go away.

我将进一步展开调查。

至于结构填充和混叠，编译器厂商可能已经隐含地假定只有他们的编译器将被使用。他们在为您提供一个便携式寄存器映射，这样就可以切换竞争对手的编译器相同的MCU没有兴趣。

As for struct padding and aliasing, the compiler vendor has likely implicitly assumed that only their compiler is to be used. They have no interest in providing you with a portable register map, so that you can switch the the competitor's compiler for the same MCU.

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