[Emulate] Untimed TLM in SystemC

Published: June 14, 2020 by SingularityKChen (Last updated: June 16, 2020 )

• Categories:
• Emulate 12

• Tags:
• SystemC 15
• TLM 6

• Untimed TLM in SystemC
  • Untimed TLM – Data Flow Modeling
  • A Simple Adder
    • Dead Lock
    • FIFO Sizes
    • Insertion of Functional Delay
    • Stopping Untimed TLM Simulation

Untimed TLM in SystemC¹

Untimed TLM – Data Flow Modeling

• Purposes
  • Create initial draft of the specification model
• Focus
  • Data flow and functionality modeling
• Kahn process network
  • Deterministic, untimed model of computation
• How?
  • Untimed processes are modeled with SC_THREAD
  • Processes communicate through FIFO channels using blocking read/write
  • Implicit system synchronization is achieved by the blocking read/write
    • Blocking read is suspended until data is available in the FIFO channel
    • Blocking write is suspended until more space is available in the FIFO channel

A Simple Adder

Dead Lock

• Missing initial values of the feedback channel
  • The adder would never be able to read from its first input
• Unbalanced production and consumption rate in a feedback loop
  • A producer and a consumer connected to a FIFO channel need to move the same number of tokens on the average
  • If DF_fork produces two values on each output for every data sample consumed, the simulation would stall with the folk module attempting to write to its first output port
• A quest for empty and full FIFOs can help to identify the causes
  • Access suspicious FIFOs after returning from sc_start()

FIFO Sizes

• FIFO sizes play a key role
  • Determine whether a system deadlocks
  • Have an impact on system behavior (value sequences)
  • Have an impact on simulation speed
• Larger sizes cause less context switches and faster simulation

Insertion of Functional Delay

• Inserting functional delay in untimed TLM can be achieved by annotating the delay with wait statements
• SC_THREAD is more preferable than SC_METHOD
  • Adding delay information in method processes is nontrivial
  • Maintaining blocking read/write semantics in method processes is next to impossible

Stopping Untimed TLM Simulation

• Simulation time will never advance in a purely untimed TLM
  • Processes are simulated with advance of delta-cycles
  • Using sc_start with a positive argument does not lead to the termination of the simulation
• How can one terminate the simulation?
  • Solution I
    • Use process termination and hope for data backlog
      • Simulations are run until a sufficient amount of samples are obtained
      • Example: the print_process of the DF_printer module
    • It is not guaranteed to be successful
      • The process may be in some dead branch of the system
    • There could be multiple instances of DF_printer
      • Logic and-combination of exit conditions is a must
  • Solution II
    • Simply indicate the fact and continue to consume data
    • Create a terminator module and monitor different exit conditions
  • Combination of a given simulation time and data-dependent exit conditions can often be the best solution

1. IOC5080(5940) System Model Design and Verification, Department of Computer Science, National Chiao-Tung University ↩