SystemC 11 Delta Cycle
Delta Cycle
A delta cycle can be thought of as a very small step of time within the simulation, which does not increase the user-visible time. A delta cycle is comprised of separate evaluate and update phases, and multiple delta cycles may occur at a particular simulated time. When a signal assignment occurs, other processes do not see the newly assigned value until the next delta cycle.
When to use delta cycle
-
notify(SC_ZERO_TIME)causes the event to be notified in the evaluate phase of the next delta cycle, this is called a “delta notification”. - A (direct or indirect) call to
request_update()causes theupdate()method to be called in the update phase of the current delta cycle.
Note: Multiple wait(SC_ZERO_TIME) proceeds the simulated time by multiple delta cycles.
Code
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// Learn with Examples, 2020, MIT license
#include <systemc>
using namespace sc_core;
SC_MODULE(DELTA) {
int x = 1, y = 1; // defines two member variables
SC_CTOR(DELTA) {
SC_THREAD(add_x); // x += 2
SC_THREAD(multiply_x); // x *= 3
SC_THREAD(add_y); // y += 2
SC_THREAD(multiply_y); // y *= 3
}
void add_x() { // x += 2 happens first
std::cout << "add_x: " << x << " + 2" << " = ";
x += 2;
std::cout << x << std::endl;
}
void multiply_x() { // x *= 3 happens after a delta cycle
wait(SC_ZERO_TIME);
std::cout << "multiply_x: " << x << " * 3" << " = ";
x *= 3;
std::cout << x << std::endl;
}
void add_y() { // y += 2 happens after a delta cycle
wait(SC_ZERO_TIME);
std::cout << "add_y: " << y << " + 2" << " = ";
y += 2;
std::cout << y << std::endl;
}
void multiply_y() { // y *=3 happens first
std::cout << "multiply_y: " << y << " * 3" << " = ";
y *= 3;
std::cout << y << std::endl;
}
};
int sc_main(int, char*[]) {
DELTA delta("delta");
sc_start();
return 0;
}
// Result:
//because of the delta_cycle, x and y goes through different orders of arithmetic calculation, achieving different results.
//x += 2 happens first
add_x: 1 + 2 = 3
//y *= 3 happens first
multiply_y: 1 * 3 = 3
//(y*3) + 2 = 5
add_y: 3 + 2 = 5
//(x+2) * 3 = 9
multiply_x: 3 * 3 = 9