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
DRAM cell
• Architecture of a DRAM cell
• Read destroys information, therefore it has to be written back.
• Cycle time (time between two accesses) is larger than access
time.
Architecture of a DRAM Chip
Memory Organization of PCs
• SDRAM (Synchronous DRAM)
• Synchron: All signals (RAS, Adresse …) are combined with a
fixed clock cycle (PC100, PC133 …).
• Access via North Bridge or an integrated memory controller
Frontside Bus
Chip Select
• Data width is 64 Bit oder 8 Bytes
• Chip Select determines the row
• The row provides 8 Bytes
Address
Data
Control
DIMM
1 or 2 rows
(North Bridge)
DIMM
1 or 2 rows
Prozessor
Chip
Satz
Memory Organization of PCs
• Each row of a DIMM has 8 chips.
• To provide 1 GB per row we use eight 1 Gbit SDRAM
chips.
• Each chip
• 8 data wires
• internally organized in 4 banks with 256 Mbit. Each bank is
organized in eight memory arrays.
Internal Organization of a GBit DRAM
• 1 GBit or 128 MByte chips
• 128 M = 227
• 27 adress lines
• 11 (2K) column
• 14 (16K) row
• 2 (4) memory field
8 Bit width
1 Bit width
16K rows, 2K columns
Access to DDR-SDRAM
• Read , Burst length=8
• Burst Mode: transmission of 8 Bytes
• DDR (Double Data Rate): Transmission with rising and falling
edge of the clock signal
• RAS-to-CAS-Delay (tRCD), CAS latency (tCL), RAS-to-PrechargeLatency (tRAS), Read-Cycle-time (tRC)
• Precharge necessary because of differential lines
Access to DDR-SDRAM, Page Hit
• Read, Burst length=8, Page Hit
• Page: Data in the amplifiers of a row
• All amplifiers of all blocks in the same bank
• Hit: Data from the same row are accessed.
Access to DDR-SDRAM, Bank Overlap
• Access to another bank can already start when the first burst is
trasmitted.
Example Configuration
Frontside Bus (FSB800)
• 200 MHz
• Quadpumped
• 4*64 Bit  6,4 GB/s
DDR400
• 200 MHz
• 400 MB/s
DIMM (DDR400 chips)
DIMM (PC3200)
3,2GB/s
DIMM
DIMM (PC3200)
Prozessor
i875P
Zwei Kanäle mit je 3,2 GB/s
RAMBUS
• Rambus DRAM (RDRAM) is internally very similar to
DDR SDRAM.
• A point-to-point channel with higher clock rate and less
pins in the memory controller is used.
• Since Intel switched in 2003 to DDR RAM, RAMBUS
was almost eliminated.
• XDR-DRAM is used in the
Sony PlayStation 3
• XDR2-DRAM is used in
high-end graphics cards
and 3D TVs.
12,8 GBytes/s bandwidth
Static Memory
• 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.
NOR vs NAND
Bit Line
GroundSelectTransistor
Bit LineSelectTransistor
WordLine 0 WordLine 1WordLine 2 WordLine 3 WordLine 4 WordLine 5WordLine 6 WordLine 7
NAND
• more compact since
less wires, although
more transistors
N
N
N
N
N
N
N
N
N
N
N
• read: offset power for
other FETs
P
Bit Line
WordLine 0
WordLine 1
WordLine 2
WordLine 3
WordLine 4
WordLine 5
NOR
N
N, GND
N
N, GND
P
N
N, GND
N
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.