Transcript futures
CA406 Computer Architecture Futures VLIW - Very Long Instruction Word • Instruction word: multiple operations • n RISC-style instructions • Architecture: fixed set of functional units Each FU matched to a “slot” in the instruction VLIW - Very Long Instruction Word • Compiler responsible for allocating instructions to words • Burden squarely on compiler • Needs to produce near optimal schedule • Inevitable: large number of empty slots! Lower code density • Similar to superscalar • but instruction issue flexibility missing • VLIW simpler faster? • Re-compilation needed • Each new generation will have different functional unit mix Synchronous Logic Systems • Clock distribution • Major problem for chip architect • Clock skews < 100-200ps over whole die • 10% of cycle time • Small changes Re-engineer whole chip • Checking for data hazards & logic races Synchronous Logic Systems • Clock distribution • Power consumption • Major problem @ 30W+ per chip • CMOS logic consumes power only on switch but synch systems clock a lot of logic on every cycle Clock is distributed to every subsystem Even if the logic of the subsystem is disabled! Synchronous Logic Systems • Clock distribution • Power consumption • Worst case propagation delay • Determines maximum clock speed • Clock edge must wait until all logic has settled • Temperature and process fabrication Even slower clocks • Design is simpler • Logic designers have experience • Good tools Asynchronous Logic Systems • Clock distribution • No longer a problem • Synchronisation bundled with data • Circuits are composable • No global clock … No need to re-engineer a whole chip to change one section! • Known correct circuits can be combined • Power consumption • Circuits switch only when they’re computing Potentially very low power consumption • May be the biggest attraction of asynch systems! Asynchronous Logic Systems • • • • Clock distribution problem removed Circuits are composable Power consumption Average case propagation delay • Completion signal generated when result is available • Independent of • Temperature and process fabrication • Design is harder • Experience will remove this? Optical Technology - Bandwidth • Electrical signal in Cu • ? GHz • On a single wire • Gbits / second possible • One channel at 1 Gbit / second • 10 channels at 100 Mbits / second, • etc • Optical • Visible light • ? Hz • On a single beam (or in a single fibre) • ? Gbits / second • ? 100 Mbit channels Optical Technology - Bandwidth • Electrical signal in Cu • ? GHz • On a single wire • Gbits / second possible • One channel at 1 Gbit / second • 10 channels at 100 Mbits / second, • Optical • Visible light • 1014 Hz • On a single beam (or in a single fibre) • 105 Gbits / second • 106 100 Mbit channels • but allow for channel separation, etc Optical Technology - Interconnect • Fibre Optic • Used extensively for long-haul serial links • Connection density • Currently lower than Cu • Drivers - Solid-state lasers • Lower density than CMOS transistors • Mechanical problems • Fibre alignment, etc • No E-M interference - negligible cross-talk • Switching / Modulation • Conventional techniques at source Optical Technology - Interconnect • Free space • Mechanical alignment required • Precise alignment possible • But on an optical bench! • Connection density • Multiple beams can share same path • No E-M interference - negligible cross-talk • Potential for nx103 bit buses • Source & receiver matrices available • Currently slow!! • MHz at best? • Switching / Steering • Variety of electro-optical devices available Optical Technology - Switching • Techniques • Amplitude modulation • On / off switching • Polarisation rotation • Block or steer beam • Kerr Cells • Pockels Cells • Fast, ns • Liquid Crystals • Slow, ms • Refractive index manipulation • Beam steering • Generic Name: Spatial Light Modulators Optical Technology - Computing • Switching is the key • Digital computers compute by switching! • High density spatial light modulators • Highly parallel computations Speed? Control? • Ideal for regular computations • Image processing • Signal processing Optical Technology - Computing • Vector-Matrix multiplication Optical Technology - Memory • CD ROM • Already well entrenched • High density • Low speed • Mechanical access! • Holographic Memory • Photographic film • Very high bit density! • First applications? • Read only (program) memory • Parallel read of millions of bits Very high data transfer rates Molecular Technology • Single molecule storage • cis-, trans- isomers • Very High density • A bit in 10nm x 10nm • 0.1m = 100nm! • Low speed • Speed of sound • Switches • Alternating single- / double- bonds • Block / transmit electrons Quantum Computing • Quantum system • superposition of infinite number of states • observation resolves system • Potential to perform parallel computation on all states simultaneously • Only one state can be read at end • Fine for some problems! • Possible to solve certain intractable problems in finite time