[问答题,简答题]MCS-51的并行接口的扩展有多种方式，在什么情况下，采用扩展8155H比较合适？什么情况下，采用扩展8255A比较适合？
A．源端
B．核心交换机
C．路由器
D．目标端
A．ΠA(R)×ΠD(S)
B．R∪S
C．ΠC(R)∩ΠC(S)
D．R｜＞＜｜S
User programs interact with the kernel through a set of standard(71). They request services to be provided by the kernel. Such services would include accessing a file: open close, read, write, link, or execute a file; starting or(72)accounting records; changing ownership of a file or(73); changing to a new directory; creating,(74), or killing a process; enabling access to hardware devices; and setting limits on system resources. Unix is a multi -user, multi -tasking operating system. You can have many users logged into a system simultaneously, each running many programs. It's the kernel's job to keep each process and user separate and to regulate access to(75), including cpu, memory, disk and other L/O devices.
A．system commands
B．system transfer
C．system calls
D．system rings
一座办公大楼内的计算机网络系统可看作(27)。
A．LAN
B．MAN
C．NAN
D．WAN