lab1：kern/console.c: kbd_proc_data

    */

        if (data == 0xE0) {
                // E0 escape character，扩展按键，扫描码为2字节；E0后面还跟有一个字节值

                shift |= E0ESC;
                return 0;
        } else if (data & 0x80) {    
                // Key released，除去E0，最高位为1（+0x80）的，应该是键释放的扫描码

         // 如果该扫描码是E0 escape的，那么保持值不变；否则清除最高位的1，其值

         // 变为建按下是的码值。这么做的目的就是为了处理左右ctrl,alt和shift键，

         // 即在建释放时清除其shiftcode值，因为它们的shiftcode值会影响其他字符

         // 的解析

                data = (shift & E0ESC ? data : data & 0x7F);
                shift &= ~(shiftcode[data] | E0ESC);    // 清除之前的shiftcode,表示

                                // 释放了ctrl,alt或者shift

                                // 如果该扫描码是E0 escape的

                                // 清除E0 escape标记

                return 0;
        } else if (shift & E0ESC) {
                 // Last character was an E0 escape; or with 0x80

         // 上一个字节值为E0，那么该字节应该是键被按下时扫描码的第二字节，

         // 让其与上0x80，用以后面处理时区分单字节扫描码

                data |= 0x80;
                shift &= ~E0ESC;    // 清除E0 escape

        }

        shift |= shiftcode[data];    // 键被按下时，置ctrl,alt和shift键被按下的状态

        shift ^= togglecode[data];    // CAPSLOCK，NUMLOCK，SCROLLLOCK这三个键没按一次，

                    // 它们的状态就改变(0,1变化)，这里用异或巧妙的实现

                    // 它们的状态也会影响后面按键字符的解析


     // static uint8_t *charcode[4] = {normalmap,shiftmap,ctlmap,ctlmap};

     // CTL = 1 (01); SHIFT = 2 (10). CTL | SHIFT = 3 (11)

     // CTL和SHIFT二者组合值为0~3, 对应charcode里面的四个不同字符值map

        c = charcode[shift & (CTL | SHIFT)][data];
        if (shift & CAPSLOCK) {    // CAPSLOCK 影响字母字符解析

                if ('a' <= c && c <= 'z')    // 小写->大写

                        c += 'A' - 'a';
                else if ('A' <= c && c <= 'Z')    // 大写(shift键按下时)->小写

                        c += 'a' - 'A';
        }

        // Process special keys

        // Ctrl-Alt-Del: reboot

        if (!(~shift & (CTL | ALT)) && c == KEY_DEL) {
                cprintf("Rebooting!\n");
                outb(0x92, 0x3); // courtesy of Chris Frost

        // PS/2 POS, 0092: bit 1 = 1 indicates A20 active

        // bit 0 = 0 system reset or write

        //      1 pulse alternate reset pin (alternate CPU reset)

        }
        return c;
}