Notes

[1] A key component common to all CPUs is the program counter, a special memory cell that keeps track of which location in memory the next instruction is to be read from.

本句中，a special memory cell that keeps track of which location in memory the next instruction is to be read from是一个名词性短语，对the program counter进行补充说明。在该短语中，that keeps track of which location in memory the next instruction is to be read from是一个定语从句，修饰和限定a special memory cell。keep track of的意思是“跟踪；与…保持联系”。

[2] It is noticeable that the sequence of operations that the control unit goes through to process an instruction is in itself like a short computer program.

本句中，it是形式主语，真正的主语是that引导的从句the sequence of operations that the control unit goes through to process an instruction is in itself like a short computer program。在该从句中，the sequence of operations是主语；is like a short computer program是谓语；that the control unit goes through to process an instruction是一个定语从句，修饰和限定the sequence of operations，动词不定式短语to process an instruction作目的状语，修饰goes through。in itself的意思是“本身，实质上”。

[3] And indeed, in some more complex CPU designs, there is another yet smaller computer called a microsequencer that runs a microcode program that causes all of these events to happen.

本句中，called a microsequencer是一个过去分词短语，作定语，修饰和限定another yet smaller computer。that runs a microcode program是一个定语从句，修饰和限定a microsequencer; that causes all of these events to happen也是一个定语从句，修饰和限定a microcode program。

[4] Therefore, any computer can be programmed to perform any arithmetic operation, although it will take more time to do so if its ALU does not directly support the operation.

本句中，although it will take more time to do so if its ALU does not directly support the operation是一个让步状语从句。在该从句中，if its ALU does not directly support the operation是一个条件状语从句。so指perform any arithmetic operation。

[5] Since the CPU does not differentiate between different types of information, it is up to the software to give significance to what the memory sees as nothing but a series of numbers.

本句中，句型It is up to sb. to do sth．的意思是“由某人来做某事”。请看下例：

It is not up to you to decide what you should do next.

下一步该做什么你们说了不算。

significance的意思是“意义，重要性”;nothing but等于nothing else than，意思是“只，不过”。请看下例：

What he said is of no significance, nothing but nonsense.

他说的话毫无意义，简直就是一派胡言。

[6] Since data is constantly being worked on, reducing the need to access main memory, which is often slow compared to the ALU and control units, greatly increases the computer's speed.

本句中，since data is constantly being worked on是一个原因状语从句；reducing the need to access main memory是动名词短语作主语；which is often slow compared to the ALU and control units是一个非限定性定语从句，对main memory进行补充说明，which指main memory。