SystemC Reference Implementation

The SimpleBus is a bus model designed for TLM (Transaction Level Modeling) simulations. It provides flexibility by allowing users to switch between Loosely Timed (LT) and Approximately Timed (AT) modes at runtime, provided there are no pending transactions during the switch.

Key Features

• Dual Mode Support: Switchable between LT and AT modes.
• Transaction Handling: Supports an unlimited number of pending transactions (targets must support multiple transactions if they return false to nb_transport_req).
• Extended Support: Includes support for Direct Memory Interface (DMI) and debug transactions.

Operating Modes

Loosely Timed (LT) Mode

In LT mode, the bus acts as a simple forwarder:

• It forwards nb_transport calls directly to initiators and targets.
• It enforces a constraint where only one active request/response phase can occur at a time.

Approximately Timed (AT) Mode

In AT mode, the bus provides more realistic timing behavior:

• Incoming transactions are placed into a queue.
• The AT protocol is executed within a dedicated SC_THREAD.
• Targets are notified immediately upon the completion of a transaction using timing annotations. This ensures that initiators can safely reuse transaction objects, as the target uses the transaction pointer for identification.

The SystemC/TLM reference implementation follows the IEEE Std. 1666-2023 scheme based on Major.Minor.Patch numbers. Note that no compatibility guarantees are attached to these version numbers to allow flexibility across different implementations.

Versioning Criteria

• Major version: Updated for IEEE Standard 1666 updates or major new language features.
• Minor version: Updated for new Accellera standard releases or significant language extension proposals.
• Patch version: Updated for minor API changes.
• SYSTEMC_VERSION macro: Updated after every pull-request merge using ISO8601 format (YYYYMMDD).

Component Versioning

SystemC and TLM versions are incremented separately:

• SystemC: Managed in src/sysc/kernel/sc_ver.h
• TLM: Managed in src/tlm_core/tlm_2/tlm_version.h

Note: Changes affecting only documentation, examples, or the build system do not trigger a version change.

SystemC enforces specific extension rules when converting between data types of different widths (e.g., using to_XXXX() methods or implicit conversions). The behavior depends on whether the source type is classified as signed or unsigned:

• Sign Extension: For signed types, the upper bit (sign bit) is copied to fill the new width.
• Zero Extension: For unsigned types, a zero bit is used to fill the new width.

Signed Types

These types perform sign extension:

• sc_bigint
• sc_signed
• sc_int
• sc_int_base
• C++ signed integer types (e.g., int, long long)

Unsigned Types

These types perform zero extension:

• sc_biguint
• sc_unsigned
• sc_uint
• sc_uint_base
• sc_bv
• sc_bv_base
• sc_lv
• sc_lv_base
• sc_signed_subref / sc_signed_subref_r
• sc_unsigned_subref / sc_unsigned_subref_r
• sc_int_subref / sc_int_subref_r
• sc_uint_subref / sc_uint_subref_r
• C++ unsigned integer types (e.g., unsigned int, unsigned long)

The force() and suppress() methods provide a way to temporarily override the global sc_report_handler configuration.

• force(actions): Forces specific actions to be taken for subsequent reports, regardless of the current configuration. These override any settings in suppress().
• suppress(actions): Prevents specific actions from being taken for subsequent reports.
• force() / suppress() (no arguments): Restores the default behavior by clearing previous calls to force or suppress.

Example Scenario: If you are in a code block that is not C++ exception-safe, you can use suppress(SC_THROW) to prevent reports from throwing exceptions, and then call suppress() to restore normal behavior once the block is exited.

// Force all reports to be logged during a specific debug section
sc_report_handler::force(SC_LOG);
// ... perform operations ...
sc_report_handler::force(); // Restore default