键盘程序 出口：A,为键码值 占用：R0，R2，R4，R6，R7，2字节堆栈 此子程序没有键盘扫描功能，要实现键盘扫描，需要在主程序中循环调用-Keyboard program exit: A, for keycode occupation: R0, R2, R4, R6, R7, 2 bytes of stack not the keyboard scanning function of this subroutine to achieve the keyboard scanni