Memory •Memory technologies • Static RAM (SRAM): Flip-Flops – Fast, expensive, used for caches • Dynamic RAM (DRAM): Charge stored in capacitor – Leackage requires periodic refreshing (< 2ms), slower – High density, cheap and used for main memory. •EPROM, EEPROM: Charge stored in an isolated gate • Storage of charge with high voltage • Erase: via ultraviolet light (EPROM), or electrically (EEPROM, Flash ROM) • Non-volatile memory •PROM: Burning of fuses •ROM: contents inserted during production
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Memory Memory technologies Static RAM (SRAM): Flip-Flops –Fast, expensive, used for caches Dynamic RAM (DRAM): Charge stored in capacitor –Leackage requires.
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• One bit is implemented by 6 MOSFET transistors• No refresh• Very fast access times. • Expensive compared to DRAM • Used for caches
FLASH Memories
• Write• A high voltage (10-13V) between gate and source lets electrons
tunnel into the floating gate.
• Read• The charge of the floating gate partially cancels the electric field
from the control gate. • Thus, a higher voltage is required to make the channel conduct.• With a certain threshold voltage, the state of the transistor can
be sensed.
• High negative voltage removes the charge
• Reset is done for 16 KB blocks.• Isolation is damaged by reset.
•more compact since less wires, although more transistors
•read: offset power for other FETs
NOR
Single and Multi Level Cells (SLC / MLC)
• SLCs store one bit• MLCs store up to four bits
• Instead of only checking the presence of the current, the strenght is sensed. Thus, more presice measurement is required.
• The states are determined by the amount of charge in the floating gate. Thus, precise control of the charge deposit is required.
• Higher density, lower cost• Larger bit error ratio• Lower write speeds, lower number of program-erase cycles
and higher power consumption
NAND Flash Performance
• Organized in pages (512 or 2048 bytes) • Writes are performed to entire pages.• 200-300µs
• Reset done in larger blocks• 1-2 ms
• Reads are fast• 25 µs for 4KB
NAND Flash Durability
• 10.000 – 1.000.000 writes for each cell• Solution
• Wear leveling: distribution of writes to same address over multiple cells.
• Spare cells
FLASH Usage
• Solid State Disc (SSD)• Up to 512 GB (300 €), 1TB (800 €)• Up to 520 MB/s Lesen und 400 MB/s Schreiben• Lower energy consumtion in idle and active mode as normal
discs• Comparison with HDD see resources in Mindmap
• Hybrid Disc• Nonvolatile buffer for write accesses• Or used as permanent cache controlled by the OS
• Turbo Speicher• PCIe-MiniCard from Intel to speedup boot process.