题目内容

8255A控制字的最高位为（）时，表示该控制字为方式控制字。

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方式1是有联络信号的输入/输出，端口A、B都可以设定成该方式，此时三个端口的信号线分成了A、B两组，PC7~PC4用作()组的联络信号，PC3~PC0用作( )组的联络信号。适用于高速CPU与低速I/O设备的数据传输；

方式2是双向传输，只有()组端口可以设定成该方式，适用于双机之间数据的并行传送。

如果8255A的端口地址为300H～303H，A组和B组均为方式0，端口A为输出，端口B为输入，PC3～PC0为输入，PC7～PC4为输出，填空完成出8255A的初始化程序段；要求实现将从端口C低4位读入的值从高4位送出。MOV DX,（）MOV AL,（）OUT DX,ALMOV DX,（）IN AL,DXMOV CL,()() AL,CLOUT DX,AL

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