英文翻译存储器子系统的组成与接口.doc

附录10：英文原文 Memory Subsystem Organization and Interfacing In this section we examine the construction and functions of the memory subsystem of a computer. We review the different types of physical memory and the internal organization of their chips. We discuss the construction of the memory subsystem, as well as multibyte word organizations and advanced memory organizations. 1 Types of Memory There are two types of memory chips ; read only memory(ROM) and random access memory(RAM). Read Only Memory(ROM) chips are designed for applications in which data is only read. (This data can include program instructions.)These chips are programmed with data by an external programming unit before they are added to the computer system. Once this is done, the data usually does not change. A ROM chip always retains its data, even when power to the chip is turned off. As an example, an embedded controller for a microwave oven might continuously run one program that does not change. That program would be stored in a ROM. Random Access Memory (RAM), also called read/write memory, can be used to store data that change. This is the type of memory referred to as X MB of memory in ads for PCs. Unlike ROM,RAM chips lose their data once power is shut off. Many computer system, including personal computers, include both ROM and RAM. 2 Internal Chip Organization The internal organizations of ROM and RAM chips are similar. To illustrate the simplest organization, a linear organization, consider an 8x2 ROM chip. For simplicity, programming components are not shown。 This chip has three address inputs and two data outputs, and 16 bits of internal storage arranged as eight 2-bit locations. The three address bits are decoded to select one of the eight locations, but only if the chip enable is active. If CE=0, the decoder id disabled and no location is selected. The tri-state buffers for that location‘s cells are enabled, allowing data to pass to the output buffers. If both CE and OE set to 1, these