Physical Memory and Physical AddressingTypes of RAMSRAMDRAMProblems with DRAMStructure2-1/2D One-Bit RamExample of RAM OrganizationRAM PropertiesRAM Size and DensityFlash MemorySlide 12SDRAMVirtual MemoryMemory OrganizationPhysical Memory and Physical Addressing By Alex AmesTypes of RAM Static Ram Dynamic Ram SRAM DRAMSRAMSRAM uses a flip flop to store a bitVery fastConsume a lot of powerLarger than DRAM, so less densityTypically used for memory applications that are small but fastDRAMDRAM 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 packedSlower than Flip FlopsLow cost, low power, high densityGenerally used for main memoryProblems with DRAMThere is an odd problem with DRAM where they can be prematurely discharged by naturally occurring gamma raysEarly 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 dayNow most computers have error correction algorithms because many computers run for very long periods of time, and errors would accumulate.StructureThe processor sends data to the memory controller, which modifies the RAM2-1/2D One-Bit RamExample of RAM OrganizationDuring a read operation, an entire row is fed to the column MUX which selects the bit for readingDuring 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 toThis example shows a square configuration, but usually a 2x1 rectangle is used to reduce the pin countRAM PropertiesSize – Determines the density. Speed – Determines how fast read and write operations areRead operations and write operations often have different speeds and should be measured separatelyRAM Size and DensityThe size of RAM determines how much can be packed on to one chipHigher density chips have more memory in the same spaceHowever, they use power power and generate more heatFlash MemoryEach flash cell stores a single bit in a floating gate as a negative chargeBefore updating, the charge must be cleared by applying high voltage to it (flashing)Flash MemoryMade of dielectric materialPoor conductors of electricityGood at holding a chargeThat's why flash memory doesn't need powerHowever, flash memory can only survive about 100000 write operationsSDRAMSDRAM 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 MemoryVirtual Memory allows a program to treat segmented memory as contiguous memoryThis makes programming much easier, as the operating system will ensure that the program believes it has all the memory it needsIt can even extend to more memory than is available, by moving some of the data in RAM to the hard driveAllows for much more main memoryHowever, it is much slower than regular RAMMemory
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