数字电子技术Lecture12.pptVIP

EI205 Lecture 12 Dianguang Ma Fall 2008 12MEMORY ANDSTORAGE 12-1 BASICS OF SEMICONDUCTOR MEMORY What’s Memory? Memory is the portion of a system for storing binary data in large quantities. Semiconductor memories consist of arrays of storage elements (cells) that are either latches or capacitors. The basic Semiconductor Memory Array Memories are made up of arrays of cells. Units of Data The smallest unit of binary data is the bit. A 4-bit unit is called a nibble. An 8-bit unit is called byte. A complement unit of information is called a word. Address The location of unit of data in a memory is called its address. An address is represented by a binary code, (address code). Capacity The capacity of a memory is the total number of data units that can be stored. A memory is identified by the number of words it can store times the word size, e.g., a 16k x 8 memory. Basic Memory Operations The write operation puts data into a specified address in the memory. The read operation takes data out of a specified address in the memory. Addressing The addressing operation, which is part of both the write and the read operations, selects the specified memory address. Address Bus The address code is placed on a set of lines called the address bus. Data Bus Data units go into the memory or out of the memory on a set of lines called the data bus. The Write Operation Data units go into the memory. The Read Operation Data units go out of the memory. Random Access Memory (RAM) RAM is a type of memory in which all addresses are accessible in an equal amount of time and can be selected in any order for a read or write operation. All RAMs have both read and write capability. RAMs are volatile memories because they lose stored data when the power is turned off. Read-Only Memory (ROM) ROM is a type of memory in which data are stored permanently or semipermanently. All ROMs have only read capability. ROMs are nonvolatile memories because they retain stored data when the power is turne