A.ΠA(R)×ΠD(S)
B.R∪S
C.ΠC(R)∩ΠC(S)
D.R|><|S
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以下说法错误的是(20)。
A.对象具有很强的表达能力和描述功能
B.对象是人们要进行研究的任何事务
C.对象是封装的最基本单位
D.类封装比对象封装更具体、更细致
A.a+b+c*d
B.(a+b)*c+d
C.(a+b)*(c+d)
D.a+b*c+d
某系统总线的一个总线周期包含4个时钟周期,每个总线周期中可以传送64位数据。若总线的时钟频率为44MHZ,则总线带宽为(43)。
A.132MB/s
B.88MB/s
C.44MB/s
D.396MB/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
A.read
B.write
C.draw
D.fill