Physical Memory and Physical Addressing By Alex Ames

Types of RAM Static Ram Dynamic Ram SRAM DRAM

SRAM SRAM uses a flip flop to store a bit Very fast Consume a lot of power Larger than DRAM, so less density Typically used for memory applications that are small but fast

DRAM DRAM use a capacitor to store a charge representing if it is set or cleared. Capacitors are smaller than flip flops, so it can be more densely packed Slower than Flip Flops Low cost, low power, high density Generally used for main memory

Problems with DRAM There is an odd problem with DRAM where they can be prematurely discharged by naturally occurring gamma rays Early PC's did not have error correction, but it was not a large problem because it was a rare event and PCs were usually turned off at the end of the day Now most computers have error correction algorithms because many computers run for very long periods of time, and errors would accumulate.

Structure The processor sends data to the memory controller, which modifies the RAM

2-1/2D One-Bit Ram

Example of RAM Organization During a read operation, an entire row is fed to the column MUX which selects the bit for reading During a write operation, the bit to be written is distributed by the DEMUX to the target column, while the row decoder selects which row to write to This example shows a square configuration, but usually a 2x1 rectangle is used to reduce the pin count

RAM Properties Size – Determines the density. Speed – Determines how fast read and write operations are  Read operations and write operations often have different speeds and should be measured separately

RAM Size and Density The size of RAM determines how much can be packed on to one chip Higher density chips have more memory in the same space However, they use power power and generate more heat

Flash Memory Each flash cell stores a single bit in a floating gate as a negative charge Before updating, the charge must be cleared by applying high voltage to it (flashing)‏

Flash Memory Made of dielectric material  Poor conductors of electricity  Good at holding a charge  That's why flash memory doesn't need power However, flash memory can only survive about write operations

SDRAM SDRAM is synchronized to the clock, meaning that it waits for a clock signal before performing operations. This allows the chip to have a more complex pattern of operation than DRAM which does not have synchronizing control circuits.

Virtual Memory Virtual Memory allows a program to treat segmented memory as contiguous memory This makes programming much easier, as the operating system will ensure that the program believes it has all the memory it needs It can even extend to more memory than is available, by moving some of the data in RAM to the hard drive  Allows for much more main memory  However, it is much slower than regular RAM

Memory Organization