C语言 我什么时候使用 xdata？

When do I use xdata?

提问by PICyourBrain

I am new at embedded system programming. I am working on a device that uses an 8051 chipset. I have noticed in the sample programs that when defining variables, sometimes they use the keyword xdata. like this...

我是嵌入式系统编程的新手。我正在研究使用 8051 芯片组的设备。我在示例程序中注意到，在定义变量时，有时会使用关键字 xdata。像这样...

static unsigned char xdata PatternSize;

静态无符号字符 xdata PatternSize;

while other times the xdata keyword is omitted.

而其他时候 xdata 关键字被省略。

My understanding is that the xdata keyword instructs the compiler that that variable is to be stored in external, flash, memory.

我的理解是 xdata 关键字指示编译器将该变量存储在外部、闪存、内存中。

In what cases should I store variables externally with xdata? Accessing those variables takes longer, right? Values stored using xdata do not remain after a hard reset of the device do they?

在什么情况下我应该使用 xdata 在外部存储变量？访问这些变量需要更长的时间，对吗？在硬重置设备后，使用 xdata 存储的值不会保留吗？

Also, I understand that the static keyword means that the variable will persist through every call to the function it is defined in. Do static and xdata have to be used together?

另外，我知道 static 关键字意味着变量将在每次调用它定义的函数时保持不变。 static 和 xdata 必须一起使用吗？

采纳答案by Clifford

The 8051 architecture has three separate address spaces, the core RAM uses an 8 bit address, so can be up to 256 bytes, XDATA is a 16bit address space (64Kbytes) with read/write capability, and the program space is a 16bit address space with execution and read-only data capability. Because of its small address range and close coupling to the core, addressing the core RAM is more efficient in terms of code space and access cycles

8051架构有三个独立的地址空间，核心RAM使用8位地址，所以可以达到256字节，XDATA是具有读/写能力的16位地址空间（64Kbytes），程序空间是16位地址空间具有执行和只读数据功能。由于其地址范围小且与内核耦合紧密，因此对内核 RAM 寻址在代码空间和访问周期方面更为高效

The original 8051 core had tiny-on-chip RAM (an address space of 256 bytes but some variants had half that in actual memory), and XDATA referred to off-chip data memory (as opposed to program memory). However most modern 8051 architecture devices have on-chip XDATA and program memory.

最初的 8051 内核具有微小的片上 RAM（256 字节的地址空间，但某些变体在实际内存中只有一半），而 XDATA 指的是片外数据内存（与程序内存相对）。然而，大多数现代 8051 架构设备都有片上 XDATA 和程序存储器。

So you might use the core memory when performance is critical and XDATA for larger memory objects. However the compiler should in most cases make this decision for you (check your compilr's manual, it will describe in detail how memory is allocated). The instruction set makes it efficient to implement the stack in core memory, whereas static and dynamically allocated data would usually be more sensibly allocated in XDATA. If the compiler has an XDATA keyword, then it will override the compiler's strategy, and should only be used when the compiler's strategy somehow fails since it will reduce the portability of the code.

因此，当性能至关重要时，您可能会使用核心内存，而对于较大的内存对象，您可能会使用 XDATA。然而，在大多数情况下编译器应该为你做出这个决定（检查你的编译器手册，它会详细描述如何分配内存）。指令集使得在核心内存中实现堆栈更高效，而静态和动态分配的数据通常更明智地分配在 XDATA 中。如果编译器有一个 XDATA 关键字，那么它将覆盖编译器的策略，并且只应在编译器的策略以某种方式失败时使用，因为它会降低代码的可移植性。

[edit] Note also that the core memory includes a 32byte bit-addressable region, the bit-addressing instructions use an 8bit address into this region to access individual bits directly. The region exists within the 256byte byte addressable core memory, so is both bit and byte addressable[/edit]

[编辑] 还要注意，核心存储器包括一个 32 字节的位寻址区域，位寻址指令使用进入该区域的 8 位地址直接访问单个位。该区域存在于 256 字节字节可寻址内核内存中，因此位和字节均可寻址[/edit]

回答by Robert

xdata tells the compiler that the data is stored in external RAM so it has to use a different instruction to read and write that memory instead of internal RAM.

xdata 告诉编译器数据存储在外部 RAM 中，因此它必须使用不同的指令来读取和写入该内存，而不是内部 RAM。

Accessing external data does take longer. I usually put interrupt variables in internal RAM and most large arrays in external RAM.

访问外部数据确实需要更长的时间。我通常将中断变量放在内部 RAM 中，并将大多数大型数组放在外部 RAM 中。

As to the state of the external RAM after a hard reset (not power cycle): That would depend on the hardware setup. Does a reset line go to the external chip? Also some chips come with XDATA within the CPU chip. Read that again. Some chips have an 8051 CPU plus some amount of XDATA within the IC.

至于硬复位（不是电源循环）后外部 RAM 的状态：这取决于硬件设置。复位线是否连接到外部芯片？还有一些芯片在 CPU 芯片中带有 XDATA。再读一遍。一些芯片有一个 8051 CPU 加上一些 IC 内的 XDATA。

static and xdata do not overlap. Static tells the compiler how to allocate a variable (on a stack or at a memory location). Xdata tells the compiler how to get to that variable. Static can also restrict the name space of that variable to just that file. You can have an xdata static variable that is local to just a function, and have a static variable that is local to a function but uses internal RAM.

静态和 xdata 不重叠。静态告诉编译器如何分配变量（在堆栈上或内存位置）。Xdata 告诉编译器如何获取该变量。静态还可以将该变量的名称空间限制为该文件。您可以有一个仅对函数本地的 xdata 静态变量，也可以有一个对函数本地但使用内部 RAM 的静态变量。

回答by supercat

An important point not yet mentioned is that because different instructions are used to access different memory areas, the hardware has no unified concept of a "pointer". Any address which is known to be in DATA/IDATA space may be uniquely identified with a one-byte pointer; likewise any address which is known to be in PDATA space. Any address which is known to be in CODE space may be identified with a two-byte pointer; likewise any address which is known to be in XDATA space. In many cases, though, a routine like memcpywon't know in advance which memory space should be used with the passed-in pointers. To accommodate that, 8x51 compilers generally use a three-byte pointer type which may be used to access things in any memory space (one byte selects which type of instructions should be used with the pointer, and the other bytes hold the value). A pointer declaration like:

还没有提到的重要一点是，由于不同的指令用于访问不同的内存区域，因此硬件没有统一的“指针”概念。任何已知在 DATA/IDATA 空间中的地址都可以用一字节指针唯一标识；同样，任何已知位于 PDATA 空间中的地址也是如此。任何已知在 CODE 空间中的地址都可以用一个两字节的指针来标识；同样，任何已知位于 XDATA 空间中的地址也是如此。但是，在许多情况下，像这样的例程memcpy不会提前知道传入的指针应该使用哪个内存空间。为了适应这一点，8x51 编译器通常使用一个三字节的指针类型，它可以用来访问任何内存空间中的东西（一个字节选择应该与指针一起使用的指令类型，其他字节保存值）。一个指针声明，如：

char *ptr;

will define a three-byte pointer which can point to any memory space. Changing the declaration to

将定义一个可以指向任何内存空间的三字节指针。将声明更改为

char xdata *data ptr;

will define a two-byte pointer which is stored in DATA space, but which can only point to things in the XDATA space. Likewise

将定义一个两字节的指针，该指针存储在 DATA 空间中，但只能指向 XDATA 空间中的事物。同样地

char data * data ptr;

will define a two-byte pointer which is stored in DATA space, but which can only point to things in the DATA and IDATA spaces. Code which uses pointers that point to a known data space will be much faster (possibly by a factor of ten) than code which uses the "general-purpose" three-byte pointers.

将定义一个两字节的指针，该指针存储在 DATA 空间中，但只能指向 DATA 和 IDATA 空间中的内容。使用指向已知数据空间的指针的代码比使用“通用”三字节指针的代码快得多（可能快十倍）。

回答by Thomas Matthews

How and when to use xData memory area depends on the system architecture. Some systems may have RAM at this address while others could have ROM or Flash. In either case, access will be slower than accessing internal RAM, ROM or Flash.

如何以及何时使用 xData 内存区域取决于系统架构。一些系统可能在这个地址有 RAM，而其他系统可能有 ROM 或闪存。在任何一种情况下，访问都会比访问内部 RAM、ROM 或闪存慢。

Generally speaking, large items, constant items and lesser used items should go into xData. There are no standard rules as to what goes in xData, as it depends on the architecture.

一般来说，大项目、常量项目和较少使用的项目应该进入 xData。关于 xData 中的内容没有标准规则，因为它取决于体系结构。

回答by Andy Ross

The 8051 has a 128 byte range of scratch pad "pseudo-registers" that (most) compilers use as the default for declared variables. But obviously this area is very small, and you want to be able to put variables in the 16 bit memory address space too. That's what the xdata (i.e. "external data") specifier is for. What to put where depends, obviously, on what the data is and how you plan on using it.

8051 具有 128 字节范围的暂存器“伪寄存器”，（大多数）编译器将其用作声明变量的默认值。但显然这个区域很小，而且你也希望能够在 16 位内存地址空间中放置变量。这就是 xdata（即“外部数据”）说明符的用途。显然，将什么放在哪里取决于数据是什么以及您计划如何使用它。

Basically, I think this is the wrong question. You need to understand your CPU architecture first before learning how to use the C compiler's 8051-specific features.

基本上，我认为这是一个错误的问题。在学习如何使用 C 编译器的 8051 特定功能之前，您需要先了解您的 CPU 架构。