为什么我的代码不画字符屏幕？

Question

我正在做一个小的操作系统，我已经到了我想要画字符到屏幕的部分。我在运行代码时遇到了问题(这可能是由于我生疏的C++技能造成的)，当我这么做的时候，屏幕上没有看到所要求的字符。我复制了一个8x8ASCII字符位图，并尝试使用OSDev维基中的代码作为我的文本显示功能的基础。到目前为止，这是我的C++代码：

#define LOCHAR 32
#define HICHAR 127
#define FONT_HEIGHT 8
//bitmap
unsigned char font[HICHAR-LOCHAR+1][FONT_HEIGHT] = {
{ 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00 },{ 0x00,0x00,0x00,0x00,0x02,0x02,0x00,0x02 },{ 0x00,0x00,0x00,0x00,0x05,0x05,0x00,0x00 },{ 0x00,0x00,0x00,0x00,0x06,0x0F,0x0F,0x06 },{ 0x00,0x00,0x00,0x00,0x02,0x07,0x07,0x02 },{ 0x00,0x00,0x00,0x00,0x09,0x04,0x02,0x09 },{ 0x00,0x00,0x00,0x00,0x03,0x0D,0x07,0x0B },{ 0x00,0x00,0x00,0x00,0x04,0x02,0x00,0x00 },{ 0x00,0x00,0x00,0x00,0x04,0x02,0x02,0x04 },{ 0x00,0x00,0x00,0x00,0x02,0x04,0x04,0x02 },{ 0x00,0x00,0x00,0x00,0x02,0x07,0x02,0x05 },{ 0x00,0x00,0x00,0x00,0x00,0x02,0x07,0x02 },{ 0x00,0x00,0x00,0x00,0x00,0x00,0x06,0x04 },{ 0x00,0x00,0x00,0x00,0x00,0x00,0x07,0x00 },{ 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x02 },{ 0x00,0x00,0x00,0x00,0x04,0x04,0x02,0x02 },{ 0x00,0x00,0x00,0x00,0x07,0x05,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x03,0x02,0x02,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x04,0x03,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x06,0x04,0x07 },{ 0x00,0x00,0x00,0x00,0x05,0x05,0x07,0x04 },{ 0x00,0x00,0x00,0x00,0x07,0x03,0x04,0x07 },{ 0x00,0x00,0x00,0x00,0x03,0x01,0x07,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x04,0x02,0x02 },{ 0x00,0x00,0x00,0x00,0x06,0x07,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x07,0x04,0x06 },{ 0x00,0x00,0x00,0x00,0x00,0x02,0x00,0x02 },{ 0x00,0x00,0x00,0x00,0x02,0x00,0x06,0x04 },{ 0x00,0x00,0x00,0x00,0x00,0x04,0x02,0x04 },{ 0x00,0x00,0x00,0x00,0x00,0x07,0x00,0x07 },{ 0x00,0x00,0x00,0x00,0x00,0x02,0x04,0x02 },{ 0x00,0x00,0x00,0x00,0x07,0x06,0x00,0x02 },{ 0x00,0x00,0x00,0x00,0x0F,0x09,0x0A,0x0F },{ 0x00,0x00,0x00,0x00,0x06,0x05,0x07,0x05 },{ 0x00,0x00,0x00,0x00,0x03,0x07,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x06,0x01,0x01,0x07 },{ 0x00,0x00,0x00,0x00,0x03,0x05,0x05,0x03 },{ 0x00,0x00,0x00,0x00,0x07,0x03,0x01,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x03,0x01,0x01 },{ 0x00,0x00,0x00,0x00,0x06,0x01,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x05,0x05,0x07,0x05 },{ 0x00,0x00,0x00,0x00,0x07,0x02,0x02,0x07 },{ 0x00,0x00,0x00,0x00,0x06,0x04,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x05,0x03,0x03,0x05 },{ 0x00,0x00,0x00,0x00,0x01,0x01,0x01,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x07,0x07,0x05 },{ 0x00,0x00,0x00,0x00,0x07,0x05,0x05,0x05 },{ 0x00,0x00,0x00,0x00,0x07,0x05,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x05,0x07,0x01 },{ 0x00,0x00,0x00,0x00,0x07,0x05,0x07,0x0F },{ 0x00,0x00,0x00,0x00,0x07,0x05,0x03,0x05 },{ 0x00,0x00,0x00,0x00,0x07,0x01,0x06,0x07 },{ 0x00,0x00,0x00,0x00,0x07,0x02,0x02,0x02 },{ 0x00,0x00,0x00,0x00,0x05,0x05,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x05,0x05,0x03,0x01 },{ 0x00,0x00,0x00,0x00,0x05,0x07,0x07,0x07 },{ 0x00,0x00,0x00,0x00,0x05,0x02,0x05,0x05 },{ 0x00,0x00,0x00,0x00,0x05,0x07,0x02,0x02 },{ 0x00,0x00,0x00,0x00,0x07,0x04,0x02,0x07 },{ 0x00,0x00,0x00,0x00,0x06,0x02,0x02,0x06 },{ 0x00,0x00,0x00,0x00,0x02,0x02,0x04,0x04 },{ 0x00,0x00,0x00,0x00,0x06,0x04,0x04,0x06 },{ 0x00,0x00,0x00,0x00,0x02,0x05,0x00,0x00 },{ 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x0F },{ 0x00,0x00,0x00,0x00,0x01,0x02,0x00,0x00 },{ 0x00,0x00,0x00,0x00,0x00,0x06,0x07,0x07 },{ 0x00,0x00,0x00,0x00,0x01,0x07,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x00,0x06,0x01,0x07 },{ 0x00,0x00,0x00,0x00,0x04,0x07,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x00,0x07,0x07,0x03 },{ 0x00,0x00,0x00,0x00,0x06,0x02,0x07,0x02 },{ 0x00,0x00,0x00,0x00,0x06,0x07,0x04,0x03 },{ 0x00,0x00,0x00,0x00,0x01,0x07,0x05,0x05 },{ 0x00,0x00,0x00,0x00,0x02,0x00,0x02,0x02 },{ 0x00,0x00,0x00,0x00,0x02,0x00,0x02,0x03 },{ 0x00,0x00,0x00,0x00,0x01,0x05,0x03,0x05 },{ 0x00,0x00,0x00,0x00,0x03,0x02,0x02,0x06 },{ 0x00,0x00,0x00,0x00,0x00,0x07,0x07,0x05 },{ 0x00,0x00,0x00,0x00,0x00,0x07,0x05,0x05 },{ 0x00,0x00,0x00,0x00,0x00,0x07,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x02,0x05,0x03,0x01 },{ 0x00,0x00,0x00,0x00,0x02,0x05,0x06,0x04 },{ 0x00,0x00,0x00,0x00,0x00,0x07,0x01,0x01 },{ 0x00,0x00,0x00,0x00,0x00,0x06,0x02,0x03 },{ 0x00,0x00,0x00,0x00,0x02,0x06,0x02,0x06 },{ 0x00,0x00,0x00,0x00,0x00,0x05,0x05,0x07 },{ 0x00,0x00,0x00,0x00,0x00,0x05,0x07,0x02 },{ 0x00,0x00,0x00,0x00,0x00,0x05,0x07,0x07 },{ 0x00,0x00,0x00,0x00,0x00,0x05,0x02,0x05 },{ 0x00,0x00,0x00,0x00,0x00,0x05,0x06,0x03 },{ 0x00,0x00,0x00,0x00,0x00,0x03,0x02,0x06 },{ 0x00,0x00,0x00,0x00,0x06,0x03,0x02,0x06 },{ 0x00,0x00,0x00,0x00,0x02,0x02,0x02,0x02 },{ 0x00,0x00,0x00,0x00,0x03,0x06,0x02,0x03 },{ 0x00,0x00,0x00,0x00,0x04,0x07,0x01,0x00 }};

void draw_pixel(int pos_x, int pos_y, unsigned char colour)
{
    unsigned char* location = (unsigned char*)0xA0000 + 320 * pos_y + pos_x;
    *location = colour;
}
 
void draw_char(unsigned char c, int x, int y, int color)
{
    int cx,cy;
    int mask[8]={1,2,4,8,16,32,64,128};
    unsigned char *glyph[8]=font[(int)c-LOCHAR];
 
    for(cy=0;cy<8;cy++){
        for(cx=0;cx<8;cx++){
            if((int)glyph[cy]&mask[cx]==true){
                draw_pixel(color,x+cx,y+cy-12);
            }
        }
    }
}

extern "C" void main(){

    for(int h = 0; h<200; h++)
    {
        for(int w = 0; w<320; w++)
        {    
            draw_pixel(w,h,h);
        }
    }
    draw_char(97, 100, 100, 4);
    
    return;
}

至于我到目前为止所做的故障排除，我已经尽可能地修改了最初的'draw_char‘函数，以满足我的需求，但是，凭借我对C++的一点经验，我确信我只是使它变得更糟了。

Answer 1

你误解了运算符优先。下面的声明不是您所期望的：

if((int)glyph[cy] & mask[cx] == true) ...

用括号积极隔离所有操作符，这意味着：

((int)(glyph[cy])) & ( (mask[cx]) == true )

换句话说，首先计算值mask[cx] == true，然后将其用于按位-和。这是因为运算符==比&具有更高的优先级。

你可以这样做，如果你必须(但不要，并继续读)：

if (((int)glyph[cy] & mask[cx]) == true) ...

这里的问题是，现在直接将整数与布尔值进行比较，依赖于编译器显式转换。不仅如此，您还比较真实的值，这是我们倾向于避免的东西(历史上，任何非零值都被认为是正确的)。您会发现，大多数生产代码要么完全忽略布尔比较，要么总是倾向于使用与虚值相关的比较。

这听起来可能有点迂腐，如果是这样的话，那是不幸的，但也很好。但结果是..。你真正需要的是：

if (glyph[cy] & mask[cx]) ...

现在，让我们来看看glyph。我真的不知道你的编译器为什么不对你大喊大叫。这是错误的，并且可能也对你的程序的怪异行为负责。您已经将其定义为：

// incorrect: you didn't want an array of 8 pointers!!
unsigned char *glyph[8] = font[(int)c - LOCHAR];

正确的定义是：

const unsigned char *glyph = font[(int)c - LOCHAR];

您的mask表上的最后一个注意事项。这并不是必要的，除非你打算用它做些花哨的事。您也可以轻松地通过轮班来实现它：

if (glyph[cy] & ((unsigned char)1U << cx)) ...