Behavioral Design

1
Behavioral Design

• Outline
• Design Specification
• Behavioral Design
• Behavioral Specification
• Hardware Description Languages
• Behavioral Simulation
• Behavioral Verification
• Behavior-Implementation Linkage
• Goal
• Understand behavioral design
• Overview HDLs
• Overview simulation and verification

2
Design Specification

• Describe desired system
• signal processing - frequency response
• protocols - state diagrams, Petri nets
• bus interface - timing diagram
• processor - instruction set
• be as precise as possible
• avoid ambiguity
• help clarify design goals
• Validation - are we building the right product?
• rapid prototyping
• quickly try out idea
• let users play with it
• No real formal hardware design spec languages
• Problems very similar to those of software systems

3
Behavioral Design

• Map design spec to formal behavioral spec
• algorithmic description of design
• examples
• low-pass filter gt finite impulse response filter
• CPU architecture gt fetch-decode-execute cycle
• Approach
• write behavioral hardware description language
  (HDL)
• use off-the-shelf libraries
• verify it has same function as design
  specification
• usually through simulation
• synthesize structural description
• feedback to design spec, from structural spec

X(i) W(i-1)X(i-1) W(i-2)X(i-2) ...
4
Hardware Description Languages

• Special-purpose languages
• special constructs for hardware description
• timing, concurrency, bit vectors, etc.
• examples
• ISP
• Verilog - see Weste Sec 1.6.1, 1.7.2
• VHDL
• Standard programming languages
• support advantage
• add data types and objects for hardware
  description
• examples
• C
• C
• Smalltalk
• LISP
• Simula - original OO language, CSP sim. support

5
Model Libraries

• Model - behavioral description of component
• Components
• vendor-supplied chip
• e.g. 68040 behavioral description
• module within a chip
• e. g. LSI Logic microprocessor core
• Model suppliers
• chip vendor
• CAD tool vendor
• model library company - e.g. Logic Automation
  Inc.
• Problem
• more than one HDL
• more than one behavioral simulator
• solutions
• multi-HDL simulator - e.g. Redwood VHDL-Verilog
  simulator
• common data flow interchange language - benchmarks

6
Behavioral Simulation

• Methods of simulation
• event-driven simulation
• variable changes and signals are events
• (compile and) execute program
• observation
• debugger
• dump all variables and browse
• Simulation model uses
• verify with design specification
• develop manufacturing tests
• develop software to execute on hardware
• e.g. compiler for new CPU architecture

7
Behavioral Verification

• Verification - are we building the product right?
• Is description self-consistent?
• static analysis - e.g. lint syntax, semantics
• dynamic analysis - type checking, access
  violations, etc.
• Does behavioral function match design spec?
• static analysis - same number of I/Os,
  instruction encodings
• simulation - observe behavior and compare
• primary limitation is vagueness of design spec
• cannot exhaustively simulate
• can miss design corners
• page fault while taking interrupt after
  floating-point error

8
Behavioral Synthesis

• Map behavior to structure
• Optimize - like compiler
• loop unrolling - pipelining
• code motion - moving between clock cycles
• strength reduction - multiply gt add
• Map to structure - like code generation
• assign instructions to RTL components
• partitioning into chips
• limited package bandwidth
• assign to clock cycles in sequential design
• meet timing constraints
• More complex than software compiler
• more complicated constraints
• more like compiling for heterogeneous parallel
  processor

if (a b) out 1 else out 0
9
Behavior-Implementation Linkage

• Behavioral spec influences implementation
• shows up in structural design
• hard to completely eliminate behavioral spec
  biases
• analogous to programming
• compiling program for parallel vs. serial machine
• Structural design style known - no problem
• e.g. CSP behavior gt pipelined structure
• 11 behavior-structure mapping
• Remapping old behavior - could be hard
• VAX behavior to superscalar implementation
• Feedback from structural, physical design
• were design goals met during synthesis
• May have to modify behavior
• to avoid influencing tools
• to meet design goals

10
Hardware/Software Co-Design

• Most products are hardware software
• current practice
• agree on hardware design specification
• build hardware and software separately
• integrate them
• problems
• potentially long feedback path
• forces all HW/SW tradeoffs to hardware design
  spec
• error-prone
• Solution CAD tools synthesize HW and SW
• in limited domains
• controller - state diagram, control equations
• DSP - transfer characteristic
• new research area - see 91-93 DAC, ICCAD
  proceedings

SW
Mem
CPU
HW
I/O
world