2.3.4. Loosely Timed, Approximately Timed and Untimed TLM

TLM 2.0 considers two levels of timing detail.

A loosely timed model uses transactions corresponding to a complete read or write across a bus or network in physical hardware. It provides timing at the level of the individual transaction.
An approximately timed model breaks down transactions into a number of phases corresponding much more closely to the phasing of particular hardware protocols (for example the address and data phases of an AHB read or write).

Typically loosely timed models are implemented with a blocking interface and approximately timed models with a non-blocking interface.

TLM 2.0 also introduces the concept of temporal decoupling. Standard SystemC keeps a single synchronized view of time, which is used by all threads in all modules. However with temporal decoupling, each thread can keep its own local view of time, allowing the thread to run ahead in simulation time, until it needs to synchronize with another thread. This improves performance in loosely timed models with blocking interfaces, by avoiding bottlenecks in processing.

To ensure that one thread doesn't run away hogging all the processing, TLM 2.0 temporal decoupling uses the concept of the quantum, the greatest amount that a thread may differ in timing from the central view of time. This allows other threads a chance to catch up

TLM 2.0 does not have an explicit concept of an untimed socket (something that was explicit in TLM 1.0). The standardization group took the view that in practice all models need some concept of time, so purely untimed models are of little value.

However, untimed models are easily implemented as loosely timed models which always set the timing parameter in transport calls to zero. The example in Chapter 4, Chapter 6 and Chapter 7 uses this approach to create an untimed model. This is then refined in Chapter 8 to add synchronous timing information and in Chapter 9 to add temporal decoupling.