Stored Program Model Memory Output Input CPU Stored-program computer model
Types of Memory Random-Access Memory (RAM) Volatile Store data or programs that can be conveniently reloaded Read-Only Memory (ROM) Non-volatile Programming once store essential programs
Random-Access Memory (RAM) Any of its contents may be accessed at any time. (Why this notation is used?) Two technologies are used: Static RAM (SRAM) Dynamic RAM (DRAM)
Static RAM A static RAM stores a bit of information within a flip-flop. 6-Transistor SRAM Cell bit word
Dynamic RAM A DRAM stores a bit of information as a charge. row select bit
DRAM vs SRAM DRAM is much higher density (less transistor is used for a cell). DRAM needs to be refreshed every now and then (to prevent memory loss). Must incorporate with extra (complicated) circuit for refreshing.
ROM Technologies Pure ROM PROM (User-Programmable ROM) EPROM (Erasable PROM) EAROM (Electrically-Alterable ROM)
Pure ROM A mask-programmed ROM made from manufacturer. High cost – need to be produced for large volumes.
PROM Can be programmed (once) by user. Using PROM programmer. It’s a fusible-link PROM each cell is fuse (nichrome or polysilicon). Originally, all rows and columns are connected (logical 0s). Once a cell is ‘burnt’, it becomes logical 1. “Grow back” problem reliability problem. Once programmed, it can not be altered ever again.
EPROM Can be reprogrammed a number of times using a programmer machine. Two main types: UV-erasable Electrically-erasable
EAROM Can be read from and written to. It actually is non-volatile RAM. Now, we use Flash Memory
Accessing Memory There must be a control bus for the following operations: Read data from memory Write data to memory Do not access memory