Event Driven Simulation Kernel

Datastructure for the model (flattened netlist form):

(*  A net has a string name and a width.
 *  A net may be high z, dont know or contain an integer from 0 up to 2**width - 1.
 *  A net has a list of driving and reading models.
 *  A model has a unique instance name, a delay, a form and a list of nets it contacts.
 *  It also may have some internal state, held in the last field.
 *)
datatype value_t = V_n of int | V_z | V_x;

datatype m_t = M_AND | M_OR | M_INV | M_XOR | M_DFF | M_BUFIF | M_TLATCH | M_CLOCK;

datatype internal_state_t =
  IS_NONE
| IS_DFF of value_t ref
| IS_CLOCK of int ref
;

datatype
   net_t = NET of value_t ref * string * int * model_t list ref * model_t list ref

and
  model_t = MODEL of string * int * m_t * net_t list * internal_state_t
;

This form of model requires the leaf components (gates) to be built in to the simulator with hard-coded behaviour.

Another form of model, derived originally from Simula in the 1960s, uses user-coded leaf models to be implemented using an imperative language that is interpreted by the simulator.

Verilog RTL supports both built-in gate models and modelling with behavioural threads. par SystemC has no built-in models.