4.1. 割込み信号の役割－The Role of Interrupt Signals

As the name suggests, interrupt signals provide a way to divert the processor to code outside the normal flow of control. When an interrupt signal arrives, the CPU must stop what it’s currently doing and switch to a new activity; it does this by saving the current value of the program counter (i.e., the content of the eip and cs registers) in the Kernel Mode stack and by placing an address related to the interrupt type into the program counter.

割込みが着く時、ＣＰＵが現在のやっていることを止めて、新しい活動に変えらなければなりません、カーネルモードスタックにプログアムカウンターのカレント値を保管し、割込み型に連関するアドレスがプログラムカウンターに置くことでやります。

There are some things in this chapter that will remind you of the context switch described in the previous chapter, carried out when a kernel substitutes one process for another. But there is a key difference between interrupt handling and process switching: the code executed by an interrupt or by an exception handler is not a process. Rather, it is a kernel control path that runs at the expense of the same process that was running when the interrupt occurred (see the later section “Nested Execution of Exception and Interrupt Handlers“). As a kernel control path, the interrupt handler is lighter than a process (it has less context and requires less time to set up or tear down).

先の章に記述したコンテーススイッチを思う出させることが本章にあり、カーネルが一つのプロセスを他のプロセスに代用する時に実行します。しかし、割込みハーンドルとプロセススイッチと違うキーがあります；割込みと例外に実行されるコードはプロセスではありません。というより、これは割込みが起こる時に実行していたプロセスと同じな消費で実行するカーネル制御パスである。

Interrupt handling is one of the most sensitive tasks performed by the kernel, because it must satisfy the following constraints:

割込みハーンドルは一つのカーネルに実行されるものである、これは以下の強制を満足しなければなりません：

• Interrupts can come anytime, when the kernel may want to finish something else it was trying to do. The kernel’s goal is therefore to get the interrupt out of the way as soon as possible and defer as much processing as it can. For instance, suppose a block of data has arrived on a network line. When the hardware interrupts the kernel, it could simply mark the presence of data, give the processor back to whatever was running before, and do the rest of the processing later (such as moving the data into a buffer where its recipient process can find it, and then restarting the process). The activities that the kernel needs to perform in response to an interrupt are thus divided into a critical urgent part that the kernel executes right away and a deferrable part that is left for later.

• カーネルが他のやっていることを完成したがるうちに、割込みが起これる。カーネルの目標はディレイできるのをディレイし、許可があり次第に割込みハーンドルを出ます。例えば、ネットワークライン上データブロックを着くことを想定します。ハードウェアがカーネルを割り込む時、これは現在のデータを保管だけで、前に実行されることにプロセッサを返し、後で残すことを実行します（受領プロセスが過程を再起動して、探せするバッファにデータを移植する）。カーネルが割込みに応答におけて実行しなければならない行動は

• Because interrupts can come anytime, the kernel might be handling one of them while another one (of a different type) occurs. This should be allowed as much as possible, because it keeps the I/O devices busy (see the later section “Nested Execution of Exception and Interrupt Handlers“). As a result, the interrupt handlers must be coded so that the corresponding kernel control paths can be executed in a nested manner. When the last kernel control path terminates, the kernel must be able to resume execution of the interrupted process or switch to another process if the interrupt signal has caused a rescheduling activity.

• Although the kernel may accept a new interrupt signal while handling a previous one, some critical regions exist inside the kernel code where interrupts must be disabled. Such critical regions must be limited as much as possible because, according to the previous requirement, the kernel, and particularly the interrupt handlers, should run most of the time with the interrupts enabled.