Internal Memory 5.1 Semiconductor Main Memory • In earlier computers, the most common form of random-access storage for computer main memory employed an array of doughnut-shaped ferromagnetic loops referred to as cores. • Today, the use of semiconductor chips for main memory is almost universal. • Properties • exhibit two stable (or semistable) states, which can be used to represent binary 1 and 0. • capable of being written into (at least once), to set the state. • capable of being read to sense the state. Memory Cell Operation • Select terminal selects a memory cell for a read or write operation. • Control terminal indicates read or write. • For writing, the other terminal provides an electrical signal that sets the state of the cell to 1 or 0. • For reading, that terminal is used for output of the cell’s state. Types of Memory Volatile Memory • Loses its contents when power is turned off. • RAM is an example of volatile memory Non-volatile Memory • Does not lose contents when power is removed. • ROM, Flash memory, and CMOS RAM – Random Access Memory • Misnamed as all semiconductor memorty is random access. • Random access means individual words of memory are directly accessed through wired-in addressing logic. • Read/Write • Volatile • Must be provided with a constant power supply. If the power is interrupted, then the data are lost. • Can only be used as temporary storage. • DRAM and SRAM DRAM – Dynamic RAM • Stores data as charge in capacitors. • The presence or absence of charge in a capacitor is interpreted as a binary 1 or 0. • Capacitors have a natural tendency to discharge. • Dynamic refers to this tendency of the stored charge to leak away, even with power continuously applied. DRAM vs SRAM Simpler construction More complex construction Smaller area per bit Requires larger area per bit Less Expensive More expensive Slower than SRAM Faster than DRAM Used in main memory Used in cache memory Need refreshed circuits No charges to leak (uses flipflops) DRAM vs SRAM Types of ROM • ROM – Read Only Memory • Holds data when power is off. • Two Problems of ROM: • The data insertion step includes a relatively large fixed cost, whether one or thousands of copies of a particular ROM are fabricated. • There is no room for error. If one bit is wrong, the whole batch of ROMs must be thrown out. PROM vs EPROM vs EEPROM PROM A ROM that can be modified only once by a user EPROM EEPROM A programmable ROM A user-modifiable ROM that can be erased and that can be ROM that reused can be erased and reprogrammed repeatedly through a normal electrical voltage Stands for Stands for Erasable Stands for Electronically Programmable Read Programmable Read Erasable Programmable Only Memory Only Memory Read Only Memory Developed by Wen Tsing Developed by Dov Developed by George Chow in 1956 Frohman in 1971 Perlegos in 1978 Reprogrammable only Can be reprogrammed Can be reprogrammed once using ultraviolet light. using electronic charge.
