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是一個名詞性短語，對the program counter進(jìn)行補充說明。在該短語中，that keeps track of which location in memory the next instruction is to be read from是一個定語從句，修飾和限定a special memory cell。keep track of的意思是“跟蹤；與…保持聯(lián)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是形式主語，真正的主語是that引導(dǎo)的從句the sequence of operations that the control unit goes through to process an instruction is in itself like a short computer program。在該從句中，the sequence of operations是主語；is like a short computer program是謂語；that the control unit goes through to process an instruction是一個定語從句，修飾和限定the sequence of operations，動詞不定式短語to process an instruction作目的狀語，修飾goes through。in itself的意思是“本身，實質(zhì)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是一個過去分詞短語，作定語，修飾和限定another yet smaller computer。that runs a microcode program是一個定語從句，修飾和限定a microsequencer; that causes all of these events to happen也是一個定語從句，修飾和限定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是一個讓步狀語從句。在該從句中，if its ALU does not directly support the operation是一個條件狀語從句。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“由某人來做某事”。請看下例：

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

下一步該做什么你們說了不算。

significance的意思是“意義，重要性”;nothing but等于nothing else than，意思是“只，不過”。請看下例：

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

他說的話毫無意義，簡直就是一派胡言。

[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是一個原因狀語從句；reducing the need to access main memory是動名詞短語作主語；which is often slow compared to the ALU and control units是一個非限定性定語從句，對main memory進(jìn)行補充說明，which指main memory。