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**GCC编译器详解** GCC（GNU Compiler Collection）是由GNU项目开发的一套开源编译器套件，支持多种编程语言，包括C、C++、Objective-C、Fortran、Ada、Go等。它不仅是一个编译器，还包括了预处理器、汇编器和链接器等组成部分，是GNU/Linux操作系统及其他类UNIX系统中的标准编译工具。 **ProGMEM属性** 在GCC中，特别是针对嵌入式系统，ProGMEM是一个非常重要的属性，特别是在使用Arduino或其他基于AVR微控制器的平台上。ProGMEM（Program Memory）是一个关键字，用于指定变量存储在程序存储器（Flash Memory）而不是默认的数据存储器（SRAM）中。这样做的主要原因是程序存储器的容量通常大于数据存储器，且读取速度较慢，但可以减少对有限的SRAM的占用，从而节省宝贵的内存资源。 **ProGMEM的使用** 1. **声明变量**：要将变量存储在ProGMEM，你需要在声明时使用`PROGMEM`关键字，如`const char myString[] PROGMEM = "Hello, World!";` 2. **访问变量**：由于程序存储器和数据存储器之间的访问方式不同，访问ProGMEM变量需要特殊函数。例如，对于字符串，可以使用`pgm_read_byte()`或`pgm_read_word()`等函数来读取单个字节或字。对于数组，可以使用`PSTR()`宏来创建一个指向PGMN内存的指针。 3. **与函数参数交互**：当函数参数需要使用ProGMEM变量时，需要特别注意。GCC不支持直接将ProGMEM变量作为函数参数，因此通常需要先读取到SRAM中，再传递给函数。 **GCC编译过程** 1. **预处理**：GCC的预处理器处理源代码中的宏定义、条件编译指令以及包含的头文件，生成预处理后的.i文件。 2. **编译**：编译器将预处理后的文件转换为汇编代码，生成.asm文件。这个阶段会进行语法检查、类型检查和优化。 3. **汇编**：汇编器将汇编代码转化为机器码，生成.o目标文件。 4. **链接**：链接器将多个.o文件（包括库文件）合并，解决符号引用，生成可执行文件或库。 **GCC的优化选项** GCC提供了一系列的优化级别选项，如-O1、-O2、-O3和-Os，分别代表不同级别的优化强度。优化可以提高代码的运行效率，但可能会影响调试信息和代码大小。例如，-Os选项专注于减小程序大小，而-O3则倾向于最大化性能。 **总结** GCC和ProGMEM属性在嵌入式开发中扮演着关键角色。GCC提供了强大的编译和优化功能，而ProGMEM则是应对嵌入式系统内存限制的有效手段。理解并熟练运用这些工具和特性，能帮助开发者更高效地利用资源，创建出更加高效、可靠的嵌入式应用程序。

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GCC and the PROGMEM Attribute.pdf, 介绍 GCC 编译器.rar （1个子文件）

GCC and the PROGMEM Attribute.pdf 157KB

GCC and the PROGMEM Attribute

Tutorial (c) Dean Camera, 2006.

dean_camera@hotmail.com

PART 1: PROGMEM BASICS

Introduction:

The PROGMEM attribute is always a source of confusion for those beginning with AVR-GCC. The

PROGMEM attribute is a powerful one and holds the potential to save a lot of RAM, which is

something of a limited commodity on many AVRs. Before you can use the PROGMEM attribute,

you must first understand what it does and why it is useful.

When strings are used in a program, they are commonly "hard-coded" into the firmware source

code:

Code:

LCD_puts("Hardcoded String");

While this seems the most logical way of using strings, it is *not* the most optimal. Common sense

and intuition would dictate that the compiler would store the string in the program memory, and

read it out byte-by-byte inside the LCD_puts routine. But this is not what happens.

Because the LCD_puts routine (or other string routines) are designed to work with strings in RAM,

the compiler is forced to read out the entire string constant from program memory into RAM, and

then pass the string's RAM pointer to the routine (in this case LCD_puts). It certainly works but the

RAM wastage adds up to significant amounts with each string. Why?

Intial variable values and strings are copied out from program memory into RAM as part of the C

startup routines, which execute before your main() function. Those startup routines give your

globals their initial values, as well as ensure that all strings are inside RAM so they can be passed to

your desired string handling routines. As more strings are added to your program, more data must

be copied to RAM at startup and the more RAM is used up by holding static (unchanging) data.

The solution to the problem is forcing strings to stay in program memory and only be read out as

they are needed. This is not a simple task and requires string routines specifically designed to

handle strings kept solely inside the program memory space.

Using the "const" modifier on your string variables is a natural solution - but a wrong one. A

"const" variable can only not be modified by the program code, but it does not explicitly prevent the

varible value from being copied out to the AVRs RAM on startup. Some compilers implictly do this

for you, but GCC needs to be explicitly told that the contents must live in program memory for the

entire duration of the program's execution.

Entering pgmspace.h

The AVR-LibC library contains a header file, avr/pgmspace.h, which contains all the interfacing

information needed to allow you to specify data which is to be kept inside the AVR's flash memory.

To use the pgmspace.h functions, you need to include the header at the start of your C file(s):

Code:

#include <avr/pgmspace.h.h>

To force a string into program memory, we can now use the "PROGMEM" attribute modifier on

our string constants. An example of a global string which is stored into program memory and not

copied out at execution time is:

Code:

char FlashString[] PROGMEM = "This is a string held

completly in flash memory.";

Although it is not an absolute requirement, we can remind ourselves - and possibly prevent bugs

further down the track - that the string cannot be changed at all during execution by declaring it as a

constant:

Code:

const char FlashString[] PROGMEM = "This is a string held

completly in flash memory.";

Now that the PROGMEM string has a "const" modifier, we cannot try to modify it in our code. The

pgmspace.h header also exposes a neat little macro, PSTR, which by some GCC magic allows you

to create inline strings:

Code:

LCD_puts(PSTR("Program Memory String"));

This stops you from having to clutter your program up with hundreds of variables which hold one-

time-used strings. The downside to using the PSTR macro rather than a PROGMEM modified

string variable is that you can only use the PSTR string once.

However, we now have a problem - now all your code doesn't work! What's wrong?

The problem is that your string functions are expecting the string to be inside RAM. When you pass

a string to a routine, you are in fact just passing a pointer to the start of the string in RAM. The

string handling routine then just loads the bytes of the string one-by-one, starting from the pointer's

location. But our PROGMEM strings are not in RAM, so the pointer to them is invalid.

A pointer to a string stored in PROGMEM returns the address in flash memory at which the string

is stored. It's still a valid pointer, it's just pointing to a different memory space. To use PROGMEM

strings in our application, we need to make our string routines PROGMEM-pointer aware.

Again, pgmspace.h comes to our rescue. It contains several functions and macros which deal with

PROGMEM-based strings. First, you have all your standard string routines (memcpy, strncmp, etc.)

with a "_P" postfix denoting that the function deals with the FLASH memory space. For example,

to compare a RAM-based string with a PROGMEM-based string, you would use the strncmp_P

function:

Code:

strncmp_P("RAM STRING", PSTR("FLASH STRING"));

For a full list of the avaliable string functions, check out the AVRLibC documentation which is

installed with your WinAVR installation.

But what if you have your own string function, like a USART-transmitting routine? Lets look at a

typical example:

Code:

void USART_TxString(const char *data)

{

while (*data != '\0')

USART_Tx(*data++);

}

This relies on the routine USART_Tx, which for the purposes of this example we will assume to be

predefined as a function which transmits a single passed character through the AVR's USART.

Now, how do we change our routine to use a PROGMEM string?

pgmspace.h exposes a macros which is important for PROGMEM-aware routines; pgm_read_byte.

This macro takes a PROGMEM pointer as its argument, and returns the string located at that pointer

value. To mark that our new routine deals in PROGMEM strings, let's append a "_P" to it's name

just like the other routines in pgmspace.h:

Code:

void USART_TxString_P(const char *data)

{

while (pgm_read_byte(*data) != 0x00)

USART_Tx(pgm_read_byte(*data++));

}

Now we have our PROGMEM-aware routine, we can use PROGMEM-attributes strings:

Code:

USART_TxString_P(PSTR("FLASH STRING"));

Or:

Code:

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