Introduction to VLSI Programming Lecture 6: Resource sharing

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Introduction to VLSI Programming Lecture 6
Resource sharing

• (course 2IN30)
• Prof. dr. ir.Kees van Berkel

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Time table 2005
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Lecture 6

• Outline
• Recapitulation of Lecture 5
• Sharing of sequentially-used resources
• commands
• auxiliary variables
• expressions and operators
• Lab work assignment GCD

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Tangram

• Purpose programming language for asynchronous
  VLSI circuits.
• Creator Tangram team _at_ Philips Research Labs
  (proto-Tangram 1986 release 2 in 1998).
• Inspiration Hoares CSP, Dijkstras GCL.
• Lectures no formal introduction manual hand-out
  (learn by example, learn by doing).
• Main tools compiler, analyzer, simulator, viewer.

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VLSI programming of asynchronous circuits
behavior, area, time, energy, test coverage
Tangram program
feedback
compiler
simulator
Handshake circuit
expander
Asynchronous circuit
(netlist of gates)
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VLSI programming for

• Low costs
• introduce resource sharing.
• Low delay (high throughput)
• introduce parallelism.
• Low energy (low power)
• reduce activity

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VLSI programming for low costs

• Keep it simple!!
• Introduce resource sharing commands, auxiliary
  variables, expressions, operators.
• Enable resource sharing, by
• reducing parallelism
• making similar commands equal

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Command sharing

• S S

P proc(). S P() P()
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Command sharing example

• a?x a?x

ax proc(). a?x ax() ax()
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Two-place wagging buffer
byte type 0..255 wag2 main
proc(a?chan byte b!chan byte).begin
x,y var byte ax proc(). a?x ax()
forever do (a?y b!x) (ax()
b!y) odend
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Procedure definition vs declaration

• Procedure definition P proc (). S
• provides a textual shorthand (expansion)
• each call generates copy of resource, i.e. no
  sharing
• Procedure declaration P proc (). S
• defines a sharable resource
• each call generates access to this resource

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Command sharing

• Applies only to sequentially used commands.
• Saves resources, almost always(i.e. when command
  is more costly than a mixer).
• Impact on delay and energy often favorable.
• Introduced by means of procedure declaration.
• Makes Tangram program less well readable.
  Therefore, apply after program is
  correct sound.
• Should really be applied by compiler.

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Sharing of auxiliary variables

• xE is an auto assignment when E depends on x.
  This is compiled as auxE x aux ,
  where aux is a fresh auxiliary
  variable.
• With multiple auto assignments to x, as in
• xE ... xF
• auxiliary variables can be shared, as in
• auxE aux2x() ... auxF aux2x()
  with aux2x() proc(). xaux

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Expression sharing

• xE a!E

f func(). E xf() a!f()
e0

e1
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Expression sharing

• Applies only to sequentially used expressions.
• Often saves resources, (i.e. when expression is
  more costly than the demultiplexer).
• Introduced by means of function declarations.
• Makes Tangram program less well readable.
  Therefore apply after program is
  correct sound.
• Should really be applied by compiler.

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Operator sharing

• Consider x0 y0z0 x1 y1z1 .
• Operator can be shared by introducing
• add func(a,b? var T) T. ab
• and applying it as in x0 add(y0, z0)
  x1 add(y1,z1) .

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Operator sharing the costs

• Operator sharing may introduce multiplexers to
  (all) inputs of the operator and a demultiplexer
  to its output.
• This form of sharing only reduces costs when
• operator is expensive,
• some input(s) and/or output are common.

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Operator sharing example

• Consider x yz0 x yz1 .
• Operator can be shared by introducingadd2y
  proc(b? var T). xyb
• and applying it as inadd2y(z0) add2y(z1) .

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Making similar equal example

• Consider x yz x y-z .
• Use y-z y bitwise_complement(z) 1
• condinv func (fbool xint8) int8. ?f ? x
  f ? bwc(x)
• begin yval x cast bool8
• ltltfy.0,fy.1,fy.2,fy.3,fy.4,fy.5,fy.6,fy
  .7gtgt cast int8
• end
• addsub func (fbool xint8 yint8) int9.
  ?f ? xy f ? x-y
• (ltltf,xgtgt cast int9 ltltf,condinv(f, y)gtgt cast
  int9) cast ltltbool,int9gtgt.1

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Greatest Common Divisor

• gcd main proc (ab?chan ltltbyte,bytegtgt c!chan
  byte).begin x,y var byte forever do
  ab?ltltx,ygtgt do xlty then y y-x or
  xgty then x x-y od c!x odend

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Assigment 3 make GCD smaller

• Both assignments (y y-x and x x-y) are auto
  assignments and hence require an auxiliary
  variable.
• Program requires 4 arithmetic resources (twice lt
  and ) .
• Reduce costs of GCD by saving on auxiliary
  variables and arithmetic resources. (Beware the
  costs of multiplexing!)
• Use of ff variables not allowed for this
  exercise.

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Lab-work and report

• You are allowed to team up with a colleague.
  
• Report
• more than listing of functional Tangram
  programs
• analyze the specifications and requirements
• present design options, alternatives, trade-offs
• motivate your design choices
• explain functional correctness of your Tangram
  programs
• analyze explain area, time, energy of your
  programs.

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Next week lecture 7

• Outline
• Introduction to the DLX processor.
• Introduction to Tangram version of a sub-set DLX
  (executing a software GCD).
• Lab work extend instruction set of Tangram DLX
  and reduce its costs.