The instruction cycle is the procedure a microprocessor goes through to process an instruction.First the microprocessor fetches,or reads,the instruction from memory.Then it decodes the instruction,determining which instruction it has fetched. Finally, it performs the operations necessary to execute the instruction. (Some people also include an additional element in the instruction cycle to store results.Here,we include that operation as part of the execute function.) Each of these functions–fetch,decode,and execute–consists of a sequence of one or more operations.指令周期是微处理器完成一条指令处理的步骤。首先，微处理器从存储器读取指令，然后将指令译码，辩明它取的是哪一条指令。最后，它完成必要的操作来执行指令（有人认为在指令周期中还要包括一个附加的步骤来存储结果，这里我们把该操作当作执行功能的一部分）。每一个功能——读取、译码和执行都包括一个或多个操作。
Let’s start where the computer starts,with the microprocessor fetching the instruction from memory.First,themicroprocessor places the address of the instruction on to the address bus.The memory subsystem inputs this address and decodes it to access the sired memory location.(We look at how this decoding occurs when we examine the memory subsystem in more detail later in this chapter.)我们从微处理器从存储器中取指令开始讲述。首先，微处理器把指令的地址放到地址总线上，然后，存储器子系统从总线上输入该地址并予以译码，去访问指定的存储单元。（译码是如何进行的，我们将在后面的章节中介绍存储器子系统是更为详细的讨论。）
After the microprocessor places the address of the instruction on to the address bus.The memory subsystem inputs this address and decodes it to access the sired memory location.(We look at how this decoding occurs when we examine the memory subsystem in more detail later in this chapter.)我们从微处理器从存储器中取指令开始讲述。首先，微处理器把指令的地址放到地址总线上，然后，存储器子系统从总线上输入该地址并予以译码，去访问指定的存储单元。（译码是如何进行的，我们将在后面的章节中介绍存储器子系统是更为详细的讨论。）
When the READ signal is asserted,the memory subsystem places the instruction code to be fetched onto the computer system’s data bus,The microprocessor then inputs this data from the bus and stores it in one of its internal registers.At this point,the microprocessor has fetched the instruction.读信号发出后，存储器子系统就把要取的指令码放到计算机的数据总线上，微处理器就从数据总线上输入该数据并且将它存储在其内部的某个寄存器中。至此，微处理器已经取得了指令。
Next, the microprocessor decodes the instruction.Each instruction may require a different sequence of operations to execute the instruction.When the microprocessor decodes the instruction,it determines which instruction it is in order to select the correct sequence of operations to perform.This is done entirely within the microprocessor;it does not use the system buses.接下来，微处理器对这条指令译码。每一条指令可能要有不同的操作序列来执行。当微处理器对该指令译码是，它确定处理的是哪一条指令以便选择正确的操作序列去执行。这一步完全在微处理器内完成，不需要使用系统总线。
Finally,the microprocessor executes the instruction.The sequence of operations to execute the instruction varies from instruction to instruction.The execute routine may read data from memory,write data to memory,read data from or write data to an I/O device,perform only operations within the CPU,or perform some combination of these operations.We now look at how the computer performs these operations from a system perspective.最后，微处理器执行该指令。指令不同，执行的操作序列也不同。执行过程可以是从存储器读取数据，写数据到存储器，读或写数据到I/O设备，执行CPU内部操作或者执行多个上述操作的组合。下面我们从系统的角度来看计算机是怎样执行这些操作的。
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