嵌入式Linux下完成LCD屏文字显示(帧缓冲框架)

1.前言

帧缓冲框架是linux下专门为显示类设备设计的接口，目的是将硬件和软件层分离开，方便应用层的编程，也方便应用层程序移植。帧缓冲框架向驱动层和应用层分别提供了一套标准接口，驱动层按照框架编写驱动，应用层按照框架编写应用程序。帧缓冲在/dev目录下生成的标准节点是fb，比如：/dev/fb0,/dev/fb1等等。

这篇文章就介绍在应用层 如何利用帧缓冲框架接口封装LCD屏的画点函数，获取LCD屏的硬件信息，完成对LCD屏编程，实现文字、数字显示。当期的文字采用点阵方式取模来完成显示，比较简单，与单片机上的LCD编程思路一样，可以更方便快速学习帧缓冲编程。后续正常开发中一般采用矢量字库完成字体显示，大小调整方便，字体更换方法，在前面文章有介绍过嵌入式Linux如何交叉编译freetype库。

2. 编程思路

下面是帧缓冲框架图：

帧缓冲设备是标准的字符设备，通过open函数打开设备，再通过ioctl接口获取LCD屏的一些硬件参数信息，在利用mmap函数映射LCD屏的地址到应用层。映射的这个地址就相当于是LCD屏的显存地址，对这个地址里写入数据就可以在LCD屏硬件上实时显示出来。

代码语言：javascript代码运行次数：0运行复制

int fd=open("/dev/fb0",O_RDWR);if(fd<0){perror("设备文件打开失败");return 0;}/*1. 获取LCD屏的可变形参*/ioctl(fd,FBIOGET_VSCREENINFO,&var);printf("分辨率:%d*%dn",var.xres,var.yres);printf("像素点位数:%dn",var.bits_per_pixel);/*2. 获取LCD屏的固定形参*/ioctl(fd,FBIOGET_FSCREENINFO,&fix);printf("映射的长度:%dn",fix.smem_len);printf("一行的字节数:%dn",fix.line_length);/*3. 映射LCD缓冲区地址到进程空间*/fb_mem=mmap(NULL,fix.smem_len,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);if(fb_mem==NULL){perror("空间映射失败!n");return 0;}

在应用mmap函数将驱动的DMA缓冲区地址映射到进程空间之后，如何控制LCD呢？ 为了方便对LCD屏进行操作，需要封装一个画点函数。然后后续的图片显示，文字显示，其他图形显示都基于这个画点函数来完成，程序就很好设计。

当前采用的LCD屏是800*480分辨率，24位像素，通过这些参数就可以编写一个公式，封装画点函数。

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下面是封装好的函数原型：fb_mem是LCD屏映射的地址，后面的参数是获取的LCD屏硬件参数信息。

代码语言：javascript代码运行次数：0运行复制

/*函数功能: 画点*/void Show_Pixel(int x,int y,int color){unsigned int *lcd=(unsigned int *)(fb_mem+y*var.xres*var.bits_per_pixel/8+x*var.bits_per_pixel/8);*lcd=color; //颜色赋值}

有了画点函数，就可以封装中文显示函数： 下面这个函数是针对横向取模的点阵字模进行描点的，其他取模方式根据情况修改即可。

代码语言：javascript代码运行次数：0运行复制

/*函数功能: 显示中文说明:  取模的字体是按照横向取模进行取点阵码。       取模的字体宽度是8的倍数。*/void ShowFont(int x,int y,int size,unsigned char *data){int i,j,x0=x;unsigned char tmp;for(i=0;i<size/8*size;i++){tmp=data[i];for(j=0;j<8;j++){if(tmp&0x80)Show_Pixel(x0,y,0xFF0033);//else 画背景色x0++;tmp<<=1;}if(x0-x==size){y++;x0=x;}}}

中文可以显示，照着思路再封装一个字母数字的显示接口：

代码语言：javascript代码运行次数：0运行复制

/*函数功能:  显示一个ASCII码。“空格开始 到  ~ 结束” !"#$%&'()*+,-./0123456789:;?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxyz{|}~说明: ASCII码取模 宽度和高度必须是8的倍数，传入的size(高) */void ShowASCII(int x,int y,int size,unsigned char c_data){int i,j,x0=x;unsigned char data;int cnt;if(c_data '~')return; cnt=c_data-' ';for(i=0;i<size/2/8*size;i++){ switch(size) {case 16:break; case 32:data=ASCII_16_32[cnt][i];break; } for(j=0;j<8;j++) { if(data&0x80)Show_Pixel(x0,y,0xFF0033); x0++; data<<=1; } if(x0-x==size/2) { x0=x; y++; }}}

3. 完整示例代码3.1 中文显示

下面这份代码是单个汉字取模进行显示的，代码较少，整体比较简单。

代码语言：javascript代码运行次数：0运行复制

#include #include #include #include #include #include #include #include #include struct fb_var_screeninfo var;//可变参数struct fb_fix_screeninfo fix;//固定参数unsigned char *fb_mem=NULL; //LCD屏的首地址extern unsigned char font[];/*函数功能: 画点*/void Show_Pixel(int x,int y,int color){unsigned int *lcd=(unsigned int *)(fb_mem+y*var.xres*var.bits_per_pixel/8+x*var.bits_per_pixel/8);*lcd=color; //颜色赋值}/*函数功能: 显示中文说明:  取模的字体是按照横向取模进行取点阵码。       取模的字体宽度是8的倍数。*/void ShowFont(int x,int y,int size,unsigned char *data){int i,j,x0=x;unsigned char tmp;for(i=0;i<size/8*size;i++){tmp=data[i];for(j=0;j<8;j++){if(tmp&0x80)Show_Pixel(x0,y,0xFF0033);//else 画背景色x0++;tmp<<=1;}if(x0-x==size){y++;x0=x;}}}int main(int argc,char **argv){int fd=open("/dev/fb0",O_RDWR);if(fd<0){perror("设备文件打开失败");return 0;}/*1. 获取LCD屏的可变形参*/ioctl(fd,FBIOGET_VSCREENINFO,&var);printf("分辨率:%d*%dn",var.xres,var.yres);printf("像素点位数:%dn",var.bits_per_pixel);/*2. 获取LCD屏的固定形参*/ioctl(fd,FBIOGET_FSCREENINFO,&fix);printf("映射的长度:%dn",fix.smem_len);printf("一行的字节数:%dn",fix.line_length);/*3. 映射LCD缓冲区地址到进程空间*/fb_mem=mmap(NULL,fix.smem_len,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);if(fb_mem==NULL){perror("空间映射失败!n");return 0;}/*4. 控制显示屏*/memset(fb_mem,0xFFFFFF,fix.smem_len); //将屏幕清屏为白色ShowFont(100+0*56,100,56,font);ShowFont(100+1*56,100,56,font+7*56*1);ShowFont(100+2*56,100,56,font+7*56*2);munmap(fb_mem,fix.smem_len);close(fd);return 0;}unsigned char font[]={ /*--  文字:  嵌  --*//*--  幼圆42;  此字体下对应的点阵为：宽x高=56x56   --*/0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x38,0x00,0x00,0x00,0x03,0x80,0x00,0x7C,0x00,0x03,0x80,0x07,0xC0,0x00,0x7C,0x00,0x03,0xC0,0x07,0xC0,0x00,0x7C,0x00,0x03,0xC0,0x07,0xC0,0x00,0x7C,0x00,0x03,0xC0,0x07,0xC0,0x00,0x7C,0x00,0x03,0xC0,0x07,0xC0,0x00,0x7C,0x00,0x03,0xC0,0x07,0xC0,0x00,0x7C,0x00,0x03,0xC0,0x07,0xFF,0xFF,0xFF,0xFF,0xFF,0xC0,0x03,0xFF,0xFF,0xFF,0xFF,0xFF,0x80,0x01,0xFF,0xFF,0xFF,0xFF,0xFF,0x80,0x00,0xFF,0xFF,0xFF,0xFF,0xFC,0x00,0x00,0x00,0x00,0x00,0x60,0x00,0x00,0x00,0xE0,0x07,0x00,0xF0,0x00,0x00,0x01,0xE0,0x0F,0x01,0xF0,0x00,0x00,0x01,0xE0,0x0F,0x01,0xF0,0x00,0x00,0x01,0xE0,0x0F,0x01,0xE0,0x00,0x00,0x01,0xE0,0x0F,0x01,0xE0,0x00,0x00,0x1F,0xFF,0xFF,0xF3,0xFF,0xFF,0xF0,0x3F,0xFF,0xFF,0xFB,0xFF,0xFF,0xF8,0x3F,0xFF,0xFF,0xFB,0xFF,0xFF,0xF8,0x1F,0xFF,0xFF,0xE7,0xC3,0x80,0x7C,0x01,0xE0,0x0F,0x07,0x87,0x80,0x7C,0x01,0xE0,0x0F,0x0F,0x87,0x80,0x7C,0x01,0xE0,0x0F,0x0F,0x87,0x80,0x78,0x01,0xE0,0x0F,0x1F,0x07,0x80,0x78,0x01,0xE0,0x0F,0x1F,0x07,0x80,0xF8,0x01,0xE0,0x0F,0x3E,0x07,0x80,0xF8,0x01,0xE0,0x0F,0x3E,0x07,0x80,0xF0,0x01,0xE0,0x0F,0x3C,0x07,0x81,0xF0,0x01,0xE0,0x0F,0x3C,0x07,0x81,0xE0,0x01,0xE0,0x0F,0x00,0x07,0x83,0xE0,0x01,0xFF,0xFF,0x00,0x07,0xC3,0xC0,0x01,0xFF,0xFF,0x00,0x07,0xC0,0x00,0x01,0xFF,0xFF,0x00,0x0F,0xE0,0x00,0x01,0xFF,0xFF,0x00,0x0F,0xE0,0x00,0x01,0xE0,0x0F,0x00,0x0F,0xF0,0x00,0x01,0xE0,0x0F,0x00,0x1E,0xF0,0x00,0x01,0xE0,0x0F,0x00,0x1E,0xF8,0x00,0x01,0xE0,0x0F,0x00,0x3E,0x78,0x00,0x01,0xE0,0x0F,0x00,0x7C,0x7C,0x00,0x01,0xE0,0x0F,0x00,0x78,0x3E,0x00,0x01,0xE0,0x0F,0x00,0xF8,0x1E,0x00,0x01,0xE0,0x0F,0x01,0xF0,0x1F,0x00,0x01,0xE0,0x0F,0x03,0xE0,0x0F,0x80,0x01,0xE0,0x0F,0x07,0xE0,0x07,0xC0,0x01,0xE0,0x0F,0x0F,0xC0,0x03,0xE0,0x01,0xE0,0x0F,0x1F,0x80,0x01,0xF0,0x01,0xFF,0xFF,0x3F,0x00,0x01,0xF8,0x00,0xFF,0xFF,0x7E,0x00,0x00,0xFC,0x00,0x7F,0xFE,0x78,0x00,0x00,0x3C,0x00,0x00,0x00,0x70,0x00,0x00,0x18,/*--  文字:  入  --*//*--  幼圆42;  此字体下对应的点阵为：宽x高=56x56   --*/0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x0F,0x00,0x00,0x00,0x00,0x00,0x00,0x1F,0xC0,0x00,0x00,0x00,0x00,0x00,0x0F,0xE0,0x00,0x00,0x00,0x00,0x00,0x07,0xF0,0x00,0x00,0x00,0x00,0x00,0x01,0xF8,0x00,0x00,0x00,0x00,0x00,0x00,0xFC,0x00,0x00,0x00,0x00,0x00,0x00,0x7C,0x00,0x00,0x00,0x00,0x00,0x00,0x3E,0x00,0x00,0x00,0x00,0x00,0x00,0x1F,0x00,0x00,0x00,0x00,0x00,0x00,0x1F,0x00,0x00,0x00,0x00,0x00,0x00,0x3F,0x00,0x00,0x00,0x00,0x00,0x00,0x3F,0x80,0x00,0x00,0x00,0x00,0x00,0x3F,0x80,0x00,0x00,0x00,0x00,0x00,0x7F,0xC0,0x00,0x00,0x00,0x00,0x00,0x7F,0xC0,0x00,0x00,0x00,0x00,0x00,0x7B,0xC0,0x00,0x00,0x00,0x00,0x00,0xFB,0xE0,0x00,0x00,0x00,0x00,0x00,0xF1,0xE0,0x00,0x00,0x00,0x00,0x00,0xF1,0xE0,0x00,0x00,0x00,0x00,0x01,0xF1,0xF0,0x00,0x00,0x00,0x00,0x01,0xE0,0xF0,0x00,0x00,0x00,0x00,0x03,0xE0,0xF8,0x00,0x00,0x00,0x00,0x03,0xC0,0xF8,0x00,0x00,0x00,0x00,0x07,0xC0,0x7C,0x00,0x00,0x00,0x00,0x07,0x80,0x7C,0x00,0x00,0x00,0x00,0x0F,0x80,0x3C,0x00,0x00,0x00,0x00,0x0F,0x00,0x3E,0x00,0x00,0x00,0x00,0x1F,0x00,0x1E,0x00,0x00,0x00,0x00,0x1E,0x00,0x1F,0x00,0x00,0x00,0x00,0x3E,0x00,0x0F,0x00,0x00,0x00,0x00,0x7C,0x00,0x0F,0x80,0x00,0x00,0x00,0x7C,0x00,0x07,0xC0,0x00,0x00,0x00,0xF8,0x00,0x07,0xC0,0x00,0x00,0x01,0xF0,0x00,0x03,0xE0,0x00,0x00,0x01,0xE0,0x00,0x01,0xE0,0x00,0x00,0x03,0xE0,0x00,0x01,0xF0,0x00,0x00,0x07,0xC0,0x00,0x00,0xF8,0x00,0x00,0x0F,0x80,0x00,0x00,0xF8,0x00,0x00,0x1F,0x00,0x00,0x00,0x7C,0x00,0x00,0x1F,0x00,0x00,0x00,0x3E,0x00,0x00,0x3E,0x00,0x00,0x00,0x3E,0x00,0x00,0x7C,0x00,0x00,0x00,0x1F,0x00,0x00,0xF8,0x00,0x00,0x00,0x0F,0x80,0x01,0xF0,0x00,0x00,0x00,0x07,0xC0,0x03,0xE0,0x00,0x00,0x00,0x03,0xE0,0x07,0xC0,0x00,0x00,0x00,0x03,0xF0,0x0F,0x80,0x00,0x00,0x00,0x01,0xF8,0x1F,0x00,0x00,0x00,0x00,0x00,0xFC,0x1E,0x00,0x00,0x00,0x00,0x00,0x7C,0x1C,0x00,0x00,0x00,0x00,0x00,0x3C,0x00,0x00,0x00,0x00,0x00,0x00,0x00,/*--  文字:  式  --*//*--  幼圆42;  此字体下对应的点阵为：宽x高=56x56   --*/0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x06,0x00,0x00,0x00,0x00,0x00,0x00,0xEF,0x80,0x00,0x00,0x00,0x00,0x01,0xEF,0xE0,0x00,0x00,0x00,0x00,0x01,0xEF,0xF0,0x00,0x00,0x00,0x00,0x01,0xE3,0xFC,0x00,0x00,0x00,0x00,0x01,0xE0,0xFF,0x00,0x00,0x00,0x00,0x01,0xF0,0x7F,0x80,0x00,0x00,0x00,0x01,0xF0,0x1F,0xC0,0x00,0x00,0x00,0x00,0xF0,0x07,0xC0,0x00,0x00,0x00,0x00,0xF0,0x03,0xC0,0x0F,0xFF,0xFF,0xFF,0xFF,0xFF,0xF0,0x1F,0xFF,0xFF,0xFF,0xFF,0xFF,0xF8,0x1F,0xFF,0xFF,0xFF,0xFF,0xFF,0xF8,0x0F,0xFF,0xFF,0xFF,0xFF,0xFF,0xF0,0x00,0x00,0x00,0x00,0xF0,0x00,0x00,0x00,0x00,0x00,0x00,0xF8,0x00,0x00,0x00,0x00,0x00,0x00,0x78,0x00,0x00,0x00,0x00,0x00,0x00,0x78,0x00,0x00,0x00,0x00,0x00,0x00,0x78,0x00,0x00,0x00,0x00,0x00,0x00,0x78,0x00,0x00,0x00,0x00,0x00,0x00,0x78,0x00,0x00,0x00,0x00,0x00,0x00,0x7C,0x00,0x00,0x0F,0xFF,0xFF,0xFE,0x3C,0x00,0x00,0x0F,0xFF,0xFF,0xFE,0x3C,0x00,0x00,0x0F,0xFF,0xFF,0xFE,0x3C,0x00,0x00,0x00,0x00,0xF0,0x00,0x3C,0x00,0x00,0x00,0x00,0xF0,0x00,0x3E,0x00,0x00,0x00,0x00,0xF0,0x00,0x3E,0x00,0x00,0x00,0x00,0xF0,0x00,0x1E,0x00,0x00,0x00,0x00,0xF0,0x00,0x1E,0x00,0x00,0x00,0x00,0xF0,0x00,0x1E,0x00,0x00,0x00,0x00,0xF0,0x00,0x1F,0x00,0x00,0x00,0x00,0xF0,0x00,0x0F,0x00,0x00,0x00,0x00,0xF0,0x00,0x0F,0x00,0x00,0x00,0x00,0xF0,0x00,0x0F,0x80,0x00,0x00,0x00,0xF0,0x00,0x0F,0x80,0x18,0x00,0x00,0xF0,0x00,0x07,0x80,0x3C,0x00,0x00,0xF0,0x00,0x07,0x80,0x3C,0x00,0x00,0xF0,0x00,0x07,0xC0,0x3C,0x00,0x00,0xF0,0x00,0x03,0xC0,0x3C,0x00,0x00,0xF0,0x03,0xC3,0xE0,0x3C,0x00,0x00,0xF0,0x1F,0xC1,0xE0,0x3C,0x00,0x00,0xF1,0xFF,0xC1,0xF0,0x3C,0x00,0x00,0xFF,0xFF,0x01,0xF0,0x3C,0x00,0x01,0xFF,0xF8,0x00,0xF8,0x3C,0x00,0x0F,0xFF,0x80,0x00,0xF8,0x7C,0x01,0xFF,0xFC,0x00,0x00,0x7C,0x7C,0x1F,0xFF,0xC0,0x00,0x00,0x3F,0x78,0x3F,0xFC,0x00,0x00,0x00,0x1F,0xF8,0x3F,0xC0,0x00,0x00,0x00,0x1F,0xF0,0x00,0x00,0x00,0x00,0x00,0x07,0xF0,0x00,0x00,0x00,0x00,0x00,0x01,0xE0};

3.2 调用字库显示

单个汉字取模太麻烦，可以使用点阵字库取模软件，然后程序里打开字库，读取字模进行绘制，下面的示例代码就是采用字库读取字模完成汉字显示。

代码语言：javascript代码运行次数：0运行复制

#include #include #include #include #include #include #include #include #include #include #include #include struct fb_var_screeninfo var;//可变参数struct fb_fix_screeninfo fix;//固定参数unsigned char *fb_mem=NULL; //LCD屏的首地址unsigned char *fb_GBK=NULL; //GBK字库的首地址extern unsigned char font[];extern const unsigned char ASCII_16_32[95][64];/*函数功能: 画点*/void Show_Pixel(int x,int y,int color){unsigned int *lcd=(unsigned int *)(fb_mem+y*var.xres*var.bits_per_pixel/8+x*var.bits_per_pixel/8);*lcd=color; //颜色赋值}/*函数功能: 显示中文说明:  取模的字体是按照横向取模进行取点阵码。       取模的字体宽度是8的倍数。*/void ShowFont(int x,int y,int size,unsigned char *font){unsigned char L,H;    unsigned int Addr;    unsigned int font_size=size*size/8; //得到该字节的总字节大小    unsigned char FontData[128];          //最大可以存放32*32尺寸字体点阵码unsigned char *GBK_p; //存放汉字点阵码首地址    /*计算该汉字在字库里的地址偏移量*/    L=*(font+1); //低字节    H=*font;     //高字节    if(L<0x7f)L=L-0x40;    else L=L-0x41;    H=H-0x81;    Addr=(190*H+L)*font_size; //得到当前汉字在字库里的偏移量GBK_p=(unsigned char *)(Addr+fb_GBK); //得到汉字点阵码在字库里首地址int i,j,x0=x;unsigned char tmp;for(i=0;i<size/8*size;i++){tmp=GBK_p[i];for(j=0;j<8;j++){if(tmp&0x80)Show_Pixel(x0,y,0xFF0033);//else 画背景色x0++;tmp<<=1;}if(x0-x==size){y++;x0=x;}}}/*函数功能:  显示一个ASCII码。“空格开始 到  ~ 结束” !"#$%&'()*+,-./0123456789:;?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[]^_`abcdefghijklmnopqrstuvwxyz{|}~说明: ASCII码取模 宽度和高度必须是8的倍数，传入的size(高) */void ShowASCII(int x,int y,int size,unsigned char c_data){int i,j,x0=x;unsigned char data;int cnt;if(c_data '~')return; cnt=c_data-' ';for(i=0;i<size/2/8*size;i++){ switch(size) {case 16:break; case 32:data=ASCII_16_32[cnt][i];break; } for(j=0;j<8;j++) { if(data&0x80)Show_Pixel(x0,y,0xFF0033); x0++; data<<=1; } if(x0-x==size/2) { x0=x; y++; }}}int main(int argc,char **argv){if(argc!=2){printf("./app n");return 0;}int fd=open("/dev/fb0",O_RDWR);if(fd<0){perror("设备文件打开失败");return 0;}/*1. 获取LCD屏的可变形参*/ioctl(fd,FBIOGET_VSCREENINFO,&var);printf("分辨率:%d*%dn",var.xres,var.yres);printf("像素点位数:%dn",var.bits_per_pixel);/*2. 获取LCD屏的固定形参*/ioctl(fd,FBIOGET_FSCREENINFO,&fix);printf("映射的长度:%dn",fix.smem_len);printf("一行的字节数:%dn",fix.line_length);/*3. 映射LCD缓冲区地址到进程空间*/fb_mem=mmap(NULL,fix.smem_len,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);if(fb_mem==NULL){perror("空间映射失败!n");return 0;}/*4. 映射GBK字库文件到进程空间*/int gbk_fb;struct stat gbk_buf;gbk_fb=open(argv[1],2);if(gbk_fb<0){perror("GBK字库文件打开失败!");return 0;}stat(argv[1],&gbk_buf);printf("GBK字库文件大小=%dn",gbk_buf.st_size);fb_GBK=mmap(NULL,gbk_buf.st_size,PROT_READ,MAP_SHARED,gbk_fb,0);if(fb_GBK==NULL){perror("空间映射失败!n");return 0;}/*4. 控制显示屏*/memset(fb_mem,0xFFFFFF,fix.smem_len); //将屏幕清屏为白色ShowFont(100+0*32,100,32,"嵌");ShowFont(100+1*32,100,32,"入");ShowFont(100+2*32,100,32,"式");ShowFont(100+3*32,100,32,"开");ShowFont(100+4*32,100,32,"发");ShowASCII(100+0*16,200,32,'A');ShowASCII(100+1*16,200,32,'C');ShowASCII(100+2*16,200,32,'B');munmap(fb_mem,fix.smem_len);close(fd);return 0;}

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