EEL 5708 Memory technology Lotzi Bölöni

EEL 5708 Acknowledgements All the lecture slides were adopted from the slides of David Patterson (1998, 2001) and David E. Culler (2001), Copyright , University of California Berkeley

EEL 5708 Standing on shoulders of giants “Ideally one would desire an indefinitely large memory capacity such that any particular… word would be immediately available… We are… forced to recognize the possibility of constructing a hierarchy of memories, each of which has a greater capacity than the preceding but which is less quickly accessible.” A.W.Burks, H.H.Goldstine and J. von Neumann Preliminary Discussion of the Logical Design of an Electronic Computing Instrument (1946)

EEL 5708 Elements of Memory Organization The technologies (SRAM, DRAM etc) The components –Cache (L1,L2) –Main memory –Virtual memory

EEL 5708 Main Memory Background Random Access Memory (vs. Serial Access Memory) Different flavors at different levels –Physical Makeup (CMOS, DRAM) –Low Level Architectures (FPM,EDO,BEDO,SDRAM) Cache uses SRAM: Static Random Access Memory –No refresh (6 transistors/bit vs. 1 transistor Size: DRAM/SRAM ­ 4-8, Cost/Cycle time: SRAM/DRAM ­ 8-16 Main Memory is DRAM: Dynamic Random Access Memory –Dynamic since needs to be refreshed periodically (8 ms, 1% time) –Addresses divided into 2 halves (Memory as a 2D matrix): »RAS or Row Access Strobe »CAS or Column Access Strobe

EEL 5708 Static RAM (SRAM) Six transistors in cross connected fashion –Provides regular AND inverted outputs –Implemented in CMOS process Single Port 6-T SRAM Cell

EEL 5708 SRAM cells exhibit high speed/poor density DRAM: simple transistor/capacitor pairs in high density form Dynamic RAM Word Line Bit Line C Sense Amp......

EEL 5708 DRAM Operations Write –Charge bitline HIGH or LOW and set wordline HIGH Read –Bit line is precharged to a voltage halfway between HIGH and LOW, and then the word line is set HIGH. –Depending on the charge in the cap, the precharged bitline is pulled slightly higher or lower. –Sense Amp Detects change Explains why Cap can’t shrink –Need to sufficiently drive bitline –Increase density => increase parasitic capacitance Word Line Bit Line C Sense Amp......

EEL 5708 DRAM logical organization (4 Mbit) Square root of bits per RAS/CAS Column Decoder SenseAmps & I/O MemoryArray (2,048 x 2,048) A0…A10 … 11 D Q Word Line Storage Cell Row Decoder …

EEL 5708 So, Why do I care? By it’s nature, DRAM isn’t built for speed –Response times dependent on capacitive circuit properties which get worse as density increases DRAM process isn’t easy to integrate into CMOS process –DRAM is off chip –Connectors, wires, etc introduce slowness –IRAM efforts looking to integrating the two Memory Architectures are designed to minimize impact of DRAM latency –Low Level: Memory chips –High Level memory designs. –You will pay $$$$$$ and then some $$$ for a good memory system.

EEL 5708 So, Why do I care? : Speed = ƒ(no. operations) 1990 –Pipelined Execution & Fast Clock Rate –Out-of-Order execution –Superscalar Instruction Issue 1998: Speed = ƒ(non-cached memory accesses) What does this mean for –Compilers?,Operating Systems?, Algorithms? Data Structures?

EEL 5708 DRAM Performance A 60 ns (t RAC ) DRAM can –perform a row access only every 110 ns (t RC ) –perform column access (t CAC ) in 15 ns, but time between column accesses is at least 35 ns (t PC ). »In practice, external address delays and turning around buses make it 40 to 50 ns These times do not include the time to drive the addresses off the microprocessor nor the memory controller overhead! Can it be made faster? Many techniques are trading higher bandwidth, but have higher latency –The idea that the latency will be taken care of by the cache.

EEL 5708 Synchronous DRAM Has a clock input. –Data output is in bursts w/ each element clocked Flavors: SDRAM, DDR PC100: Intel spec to meet 100MHz memory bus designs. Introduced w/ i440BX chipset Write Read

EEL 5708 RAMBUS “Intellectual property company”. –Located in Los Altos, CA –Designed a memory architecture –Licenced to manufacturers –They have no factories. Picked up by Intel, who signed an exclusive deal with them for Pentium 4 motherboards. Litigation regarding the intellectual property.

EEL 5708 RAMBUS (RDRAM) Protocol based RAM w/ narrow (16-bit) bus –High clock rate (400 Mhz), but long latency –Pipelined operation Multiple arrays w/ data transferred on both edges of clock RAMBUS BankRDRAM Memory System

EEL 5708 RDRAM Timing

EEL 5708 DRAM History DRAMs: capacity +60%/yr, cost –30%/yr –2.5X cells/area, 1.5X die size in ­3 years ‘98 DRAM fab line costs $2B –DRAM only: density, leakage v. speed Rely on increasing no. of computers & memory per computer (60% market) –SIMM or DIMM is replaceable unit => computers use any generation DRAM Commodity, second source industry => high volume, low profit, conservative –Little organization innovation in 20 years –Don’t want to be chip foundries (bad for RDRAM) Order of importance: 1) Cost/bit 2) Capacity –First RAMBUS: 10X BW, +30% cost => little impact

EEL 5708 Read-only memory (ROM) Programmed at time of manufacture –Can not be written by the computer –It is not erased by loss of power –Some of them can be erased and rewritten by special hardware (EEPROM) One transistor / bit. Used in: –BIOS of desktop computers –Embedded devices (also serves as a code protection device)

EEL 5708 FLASH Memory Floating gate transitor –Presence of charge => “0” –Erase Electrically or UV (EPROM) Performance –Reads like DRAM (~ns) –Writes like DISK (~ms). Write is a complex operation