计算机专业文献翻译---微处理器(编辑修改稿)

计算机专业文献翻译---微处理器(编辑修改稿)内容摘要：

s 0 and 128 bytes of RAM starting at address 128. ROM stands for readonly memory. A ROM chip is programmed with a permanent collection of preset bytes. The address bus tells the ROM chip which byte to get and place on the data bus. When the RD line changes state, the ROM chip presents the selected byte onto the data bus. RAM stands for randomaccess memory. RAM contains bytes of information, and the microprocessor can read or write to those bytes depending on whether the RD or WR line is signaled. One problem with today’s RAM chips is that they forget everything once the power goes off. That is why the puter needs ROM. By the way, nearly all puters contain some amount of ROM (it is possible to create a simple puter that contains no RAMmany microcontrollers do this by placing a handful of RAM bytes on the processor chip itselfbut generally impossible to create one that contains no ROM). On a PC, the ROM is called the BIOS (Basic Input/Output System). When the microprocessor starts, it begins executing instructions it finds in the BIOS. The BIOS instructions do things like test the hardware in the machine, and then it goes to the hard disk to fetch the boot sector. This boot sector is another small program, and the BIOS stores it in RAM after reading it off the disk. The microprocessor then begins executing the boot sector’s instructions from RAM. The boot sector program will tell the microprocessor to fetch something else from the hard disk into RAM, which the microprocessor then executes, and so on. This is how the microprocessor loads and executes the entire operating system. Microprocessor Instructions Even the incredibly simple microprocessor shown here will have a fairly large set of instructions that it can perform. The collection of instructions is implemented as bit patterns, each one of which has a different meaning when loaded into the instruction register. Humans are not particularly good at remembering bit patterns, so a set of short words are defined to represent the different bit patterns. This collection of words is called the assembly language of the processor. An assembler can translate the words into their bit patterns very easily, and then the output of the assembler is placed in memory for the microprocessor to execute. If you use C language programming, a C piler will translates the C code into assembly language. So now the question is, “How do all of these instructions look in ROM?” Each of these assembly language instructions must be represented by a binary number . These numbers are known as opcodes. The instruction decoder needs to turn each of the opcodes into a set of signals that drive the different ponents inside the microprocessor. Let’s take the ADD instruction as an example and look at what it needs to do: During the first clock cycle, we need to actually load the instruction. Therefore the instruction decoder needs to: Activate the tristate buffer for the program counter Activate the RD line Activate the datain tristate buffer Latch the instruction into the instruction register During the second clock cycle, the ADD instruction is decoded. It needs to do very little:。