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