ECEN 468 Advanced Logic Design

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1 ECEN 468 Advanced Logic Design Lecture 3: SystemC Concurrency ECEN 468 Lecture 3

2 Modeling of Hardware Concurrency SystemC uses simulation processes to model concurrency Works like cooperative multitasking coordinated by the simulation kernel A process or a segment of process runs and then returns control to the kernel Non-preemptive, a poorly behaved process may hold control forever Chapter , SystemC: From the Group Up ECEN 468 Lecture 3 2

3 SC_THREAD A C++ function registered as SC_THREAD(func_name) o Must be a member function of an SC_MODULE o Used only during the elaboration stage o Takes no argument and returns no values How does a thread yield control to the kernel? o Executing a return Within SC_CTOR() A thread is invoked only once, a return means permanent end, and cannot be restarted again o Call wait() ECEN 468 Lecture 3 3

4 wait() Model delays (such as those of signal propagation) in simulated time Suspends the thread temporarily Yield control to the kernel Resume the thread later wait(sc_time); // wait for specific amount of time wait(time_delay, time_unit); wait(event); ECEN 468 Lecture 3 4

5 Example of wait() #include systemc.h SC_MODULE(proc2){ void wiper(void); void blinker(void); SC_CTOR(proc2){ SC_THREAD(wiper); SC_THREAD(blinker); ; void proc2::wiper(void){ while(true){ wipe_left(); wait(99,sc_ms); wipe_right(); wait(99,sc_ms); void proc2::blinker(void){ while(true){ blink=true; wait(99,sc_ms); blink=false; wait(99,sc_ms); ECEN 468 Lecture 3 5

6 Main Steps of SytemC Run Elaboration o Instantiate modules o Instantiate connections among modules Initialization o Identify runnable and waiting processes Simulation Post-processing Runnable Running Waiting P1 P3 P4 P2 Done P5 ECEN 468 Lecture 3 6

sc_start() Initialize Evaluate Advance time

7 Simplified Simulation Engine sc_main() Simplified SystemC Simulation Kernel Elaborate Execute code, suspend wait() or exit While runnable processes exists sc_start() Initialize Evaluate Advance time Cleanup No more to do or sc_stop() Wait for time ECEN 468 Lecture 3 7

8 Simulation Termination The evaluation and advance-time loop terminates if o All processes have yielded (nothing in the runnable set) o A process executes sc_stop( ) o Internal 64-bit time variable runs out of values ECEN 468 Lecture 3 8

9 Simulation Schedule The order of process run at a specific simulated time is uncertain but repeatable Bottom line: your code should not depend on a specific order Process_A Process_B Process_C t1 t2 t3 Simulated time ECEN 468 Lecture 3 9

10 SystemC Events The occurrence of an sc_event notification and happens at a single instant in time, has no duration or value Usually there is a observer that watches the event If an event occurs and no processes are waiting to catch it, it goes unnoticed sc_event event_name; event_name.notify(void); // Immediate // Process A wait(event_b); // Process B event_b.notify(); ECEN 468 Lecture 3 10

11 Delayed Event Notification event_name.notify(sc_zero_time); // Delayed event_name.notify(sc_time); // Timed, sc_time > 0 event_name.notify(double, units); What does notify(sc_zero_time) mean? SC_ZERO_TIME is equivalent to sc_time(0, SC_SEC) Processes waiting for a delayed notification will execute only after all runnable processes in the current evaluation state have executed ECEN 468 Lecture 3 11

12 Example of Delayed Event Notification SC_MODULE(iEvent){ SC_CTOR(iEvent){ SC_THREAD(proc_A); SC_THREAD(proc_B); void proc_a(){ event_a.notify(); void proc_b(){ wait(event_a); //May miss event sc_event event_a; SC_MODULE(iEvent){ SC_CTOR(iEvent){ SC_THREAD(proc_A); SC_THREAD(proc_B); void proc_a(){ event_a.notify(sc_zero_time); void proc_b(){ wait(event_a); sc_event event_a; ECEN 468 Lecture 3 12

13 Risk of Notification without Delay SC_MODULE(iEvent){ SC_CTOR(iEvent){ SC_THREAD(proc_A); SC_THREAD(proc_B); SC_THREAD(proc_C); sc_event event_a, event_b; void proc_a(){ while(true){ event_a.notify(); wait(event_b); void proc_b(){ while(true){ event_b.notify(); wait(event_a); //Hidden forever loop void proc_c(){ while(true){ wait(1, SC_NS); ECEN 468 Lecture 3 13

14 Multiple Notifications of an Event An event may have multiple outstanding notifications Only the nearest time one takes effect sc_event ievent; ievent.notify(10, SC_NS); ievent.notify(5, SC_NS);// Only this one stays ievent.notify(15, SC_NS); ECEN 468 Lecture 3 14

15 Cancel an Event event_name.cancel(); Immediate events cannot be canceled ECEN 468 Lecture 3 15

16 Example of Event Notification sc_event action; sc_time now(sc_time_stamp()); action.notify(20, SC_MS); // Schedule action 20ms from now action.notify(1.5, SC_NS); // Reschedule for 1.5ns from now action.notify(1.5, SC_NS); // Redundant, useless action.notify(3.0, SC_NS); // Preempted by event at 1.5ns action.notify(sc_zero_time); //Reschedule to the end of now action.notify(1.0, SC_SEC); // Useless action.cancel(); action.notify(3.0, SC_US); // Schedule action 3us from now ECEN 468 Lecture 3 16

17 Catching Event wait(event); wait(event1 event2 ); wait(event1 & event2 ); wait(time, event); // Event or timeout wait(time, event1 event2 ); wait(time, event1 & event2 ); wait(); // Static sensitivity When multiple events are or ed, impossible to know which event occurred, as events have no value if (event_a) do_something(); // Syntax error! ECEN 468 Lecture 3 17

18 Example of Event Catching sc_event ack_event, bus_error; sc_time start_time(sc_time_stamp()); wait (t_max_delay, ack_event bus_error); if (sc_time_stamp() - start_time == t_max_delay) { break; ECEN 468 Lecture 3 18

SystemC. Short Introduction

SystemC. Short Introduction SystemC Short Introduction Karsten Einwich Fraunhofer IIS/EAS Dresden SystemC is a definition of C++ language constructs for the description of complex digital hardware systems on different abstraction