[英]@ sign in C variable declaration
I found this header file for PIC microcontrollers by the name of pic1250.h and I'm unable to get the hang of some syntax used in it. 我找到了PIC微控制器的这个头文件,名称为pic1250.h,我无法理解其中使用的一些语法。
The source for the file is: 该文件的来源是:
/*
* Header file for the Microchip
* PIC 12c508 chip
* PIC 12c509 chip
* Baseline Microcontrollers
*/
static volatile unsigned char RTCC @ 0x01;
static volatile unsigned char TMR0 @ 0x01;
static volatile unsigned char PCL @ 0x02;
static volatile unsigned char STATUS @ 0x03;
static unsigned char FSR @ 0x04;
static volatile unsigned char OSCCAL @ 0x05;
static volatile unsigned char GPIO @ 0x06;
static unsigned char control OPTION @ 0x00;
static volatile unsigned char control TRIS @ 0x06;
/* STATUS bits */
static bit GPWUF @ (unsigned)&STATUS*8+7;
static bit PA0 @ (unsigned)&STATUS*8+5;
static bit TO @ (unsigned)&STATUS*8+4;
static bit PD @ (unsigned)&STATUS*8+3;
static bit ZERO @ (unsigned)&STATUS*8+2;
static bit DC @ (unsigned)&STATUS*8+1;
static bit CARRY @ (unsigned)&STATUS*8+0;
/* OPTION bits */
#define GPWU (1<<7)
#define GPPU (1<<6)
#define T0CS (1<<5)
#define T0SE (1<<4)
#define PSA (1<<3)
#define PS2 (1<<2)
#define PS1 (1<<1)
#define PS0 (1<<0)
/* OSCCAL bits */
static bit CAL7 @ (unsigned)&OSCCAL*8+7;
static bit CAL6 @ (unsigned)&OSCCAL*8+6;
static bit CAL5 @ (unsigned)&OSCCAL*8+5;
static bit CAL4 @ (unsigned)&OSCCAL*8+4;
/* GPIO bits */
static bit GP5 @ (unsigned)&GPIO*8+5;
static bit GP4 @ (unsigned)&GPIO*8+4;
static bit GP3 @ (unsigned)&GPIO*8+3;
static bit GP2 @ (unsigned)&GPIO*8+2;
static bit GP1 @ (unsigned)&GPIO*8+1;
static bit GP0 @ (unsigned)&GPIO*8+0;
#define CONFIG_ADDR 0xFFF
#define FOSC0 0x01
#define FOSC1 0x02
#define WDTE 0x04
#define CP 0x08
#define MCLRE 0x0F
I'm unable to understand the whole modifer-datatype @ declaration-something. 我无法理解整个modifer-datatype @ declaration-something。 Can someone please help me out? 有人可以帮帮我吗? I'm just a newbie at this. 我只是个新手。
It's a compiler extension. 这是一个编译器扩展。
From PIC MPLAB XC8 compiler documentation (emphasis mine): 从PIC MPLAB XC8编译器文档(强调我的):
5.5.4 Absolute Variables 5.5.4绝对变量
Most variables can be located at an absolute address by following its declaration with the construct @ address , where address is the location in memory where the variable is to be positioned. 大多数变量可以通过遵循构造@地址的声明来定位在绝对地址,其中address是存储器中要定位变量的位置。 Such a variables is known as an absolute variables. 这种变量称为绝对变量。
5.5.4.1 ABSOLUTE VARIABLES IN DATA MEMORY 5.5.4.1数据存储器中的绝对变量
Absolute variables are primarily intended for equating the address of a C identifier with a special function register, but can be used to place ordinary variables at an absolute address in data memory. 绝对变量主要用于将C标识符的地址与特殊功能寄存器等同,但可用于将普通变量放在数据存储器的绝对地址中。
For example: 例如:
volatile unsigned char Portvar @ 0x06; volatile unsigned char Portvar @ 0x06;
will declare a variable called Portvar located at 06h in the data memory. 将声明一个名为Portvar的变量,位于数据存储器中的06h。 The compiler will reserve storage for this object (if the address falls into general-purpose RAM) and will equate the variable's identifier to that address. 编译器将为此对象保留存储空间(如果地址属于通用RAM),并将变量的标识符等同于该地址。
Note that MPLAB XC8 is not the only compiler to have the same @
construct to place an object in the specific memory location. 请注意,MPLAB XC8不是唯一具有相同@
构造的编译器,用于将对象放置在特定的内存位置。
Another well known compiler is Freescale CodeWarrior (at least for HCS08). 另一个众所周知的编译器是Freescale CodeWarrior(至少对于HCS08而言)。
Another one is IAR C Compiler (at least for MSP430 and AVR). 另一个是IAR C编译器(至少对于MSP430和AVR)。
It's an extension in the PIC compiler, to place a variable at a specific memory position. 它是PIC编译器的扩展,用于将变量放在特定的内存位置。 No other compiler I know have that extension. 我知道没有其他编译器具有该扩展名。
It's a C language extension supported by the PIC compiler that allows assigning variables to specific RAM addresses. 它是PIC编译器支持的C语言扩展,允许将变量分配给特定的RAM地址。
exemples: exemples:
char a @ 0x25; /* place A at address 0x25 */
bit b @ 0x25.3; /* place B at the third bit of address 0x25 */
There are three uses for this: 这有三个用途:
Remember that embedded programming is all about total control of the hardware. 请记住,嵌入式编程完全是关于硬件的完全控制。
In addition to what has already been said, please note that the non-standard @
operator is a superfluous feature. 除了已经说过的内容之外,请注意非标准的@
运算符是一个多余的功能。 You can achieve exactly the same behavior with standard C: 您可以使用标准C实现完全相同的行为:
#define RTCC (*(volatile uint8_t*)0x0001u)
Since the variables in this case are hardware registers, you don't need to worry about allocation, they are already present in the hardware. 由于这种情况下的变量是硬件寄存器,因此您无需担心分配,它们已经存在于硬件中。 If you want to allocate a variable at a custom address, there should be a linker file of some kind to fix that (since the @ operator only solves specific allocation for variables, not for code). 如果要在自定义地址分配变量,应该有某种链接器文件来修复它(因为@运算符只解决变量的特定分配,而不是代码)。
The main reason why many embedded compilers come up with some non-standard operator like @
is because they can't think outside the box when designing the debugger. 许多嵌入式编译器提出像@
这样的非标准运算符的主要原因是因为在设计调试器时他们无法思考。 They expect some sort of variable to be present in the object file which is fed to the debugger, but if you use #define, no such "debug information object" is allocated. 他们期望在提供给调试器的目标文件中存在某种变量,但是如果使用#define,则不会分配这样的“调试信息对象”。
If the debugger looked at the source code instead, or better yet, had MCU awareness built-in, then non-standard code like this wouldn't be necessary. 如果调试器改为查看源代码,或者更好,但内置了MCU感知,那么就不需要像这样的非标准代码。 High quality tools from companies that focus solely on debuggers always come with built-in support for viewing register maps. 来自专注于调试器的公司的高质量工具总是内置支持查看寄存器映射。
A short extension: 简短的延期:
This is no longer working since xc8 2.0 and up. 自从xc8 2.0及更高版本开始,这已不再适用。 You now had to write: 你现在必须写:
unsigned char a __at(0x025);
to put a variable ( a
) at an absolute address ( 0x025
). 将变量( a
)放在绝对地址( 0x025
)。
With XC8 2.0 it is possible to compile your old code using the @ syntax if you set the compiler settings to use "C90" format. 使用XC8 2.0,如果将编译器设置设置为使用“C90”格式,则可以使用@语法编译旧代码。 The setting looks like this, it is under "XC8 Global Options" and is called "C standard". 设置如下所示,它位于“XC8 Global Options”下,称为“C标准”。
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