ECEN 468
Advanced Logic Design
Lecture 6:
SystemC Channels and Signals
ECEN 468 Lecture 6
Communication Between Processes
v  Events
v  Tracing the event notification and catching can be complex
v  wait(request1 | request2)
v  Sometimes the source event is not clear
v  For the gas station example, the attendant is not clear which
requests gas fill
v  Additional check is required to find the event source
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SystemC Channels
v  Simpler mechanisms for communications
v  base class sc_prim_channel
v  others
o  sc_mutex
o  sc_semaphore
o  sc_fifo<T>
v  Chapter 8, “SystemC: from the ground up”
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sc_mutex
v  mutex: mutually exclusive text
v  Program object that lets multiple threads share a
common resource without colliding
v  A process cannot access a locked mutex
sc_mutex NAME;
NAME.lock(); // lock the mutex, blocking
NAME.unlock();
int NAME.trylock();
// One may try to lock a mutex
// that has been locked by another process
// returns if lock success, non-blocking
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Examples of sc_mutex
SC_MODULE(car){
sc_mutex drivers_seat;
public:
void drive(void);
…
};
void car::drive(void){
drivers_seat.lock();
…
drivers_seat.unlock();
…
}
SC_MODULE(bus){
sc_mutex bus_access;
…
void write(int addr, int data)
{
bus_access.lock();
…
bus.access.unlock();
}
};
void grab_bus(){
if (bus_access.trylock()==0){
… // access bus
bus_access.unlock(); }
}
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sc_semaphore
v  Model shared resource that has multiple copies
v  Like parking lot spots
v  Specify the amount when creating semaphore
sc_semaphore NAME;
NAME.wait();//lock 1 semaphore, wait if not available
// different from wait() for event
NAME.post(); //Free one locked semaphore
int NAME.trywait(); // non-blocking
int NAME.get_value(); //Returns available semaphores
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Examples of sc_semaphore
SC_MODULE(gas_station){
sc_semaphore
pump(12);
void customer1(){
…
pump.wait();
}
};
SC_MODULE(multiport_RAM){
sc_semaphore read_ports(3);
sc_semaphore write_ports(2);
…
void read(int addr, int& data){
read_ports.wait();
…
read_ports.post();
}
void write(int addr, const int&
data){
write_ports.wait();
…
write_ports.post();
}
};
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sc_fifo<T>
v  Needs to specify data type
v  Default depth is 16
sc_fifo<TYPENAME> NAME(size);
NAME.write(value); //Blocking, wait if full
NAME.read(); // Blocking, wait if empty
bool NAME.nb_read(T&); // Non-blocking
int NAME.num_available(); //Num of available data
int NAME.num_free(); // Num of free slots
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Examples of sc_fifo
SC_MODULE(kahn_ex){
sc_fifo<double> a,b,y;
…
};
kahn_ex::kahn_ex():a(24),b(24),y(48){ //Constructor
…}
void kahn_ex::stim_thread(){ …
a.write(double(rand()/1000));
… }
void kahn_ex:addsub_thread(){…
y.write(kA*a.read() + kB*b.read());
…
}
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SystemC Signals
v  Chapter 9, “SystemC: from the Ground Up”
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An Example
Reg1
Data
D
Q
Reg2
Q1
D
Q
Reg3
Q2
D
Q
Reg4
Q3
D
Q
Q4
Clk
Q4=Q3;
Q3=Q2;
Q2=Q1;
Q1=Data;
v  The order of assignment at a specific simulated
time is indefinite
v  An order can be enforced by notify() and wait(),
but awkward
v  Or implement each register as a two-deep FIFO,
which is clumsy
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Completed Simulation Engine
SystemC Simulation Kernel
sc_main()
Elaborate
sc_start()
.notify()
Initialize
Evaluate
.notify(SC_ZERO_TIME)
Cleanup
δ cycle
Update
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Advance
time
.notify(t)
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Signal Channels
v  Signal channels use the update phase as data synchronization
v  Every signal channel has two storage locations: the current
value and the new value
v  When a process writes to a signal channel, stores into the new
value
v  The new value is copied to the current during the update phase
v  If check immediately after writing, see the old value
New
Current
s1=v2
s1
v0
s2=v3
s2
v1
Update()
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sc_signal<T>
sc_signal<TYPENAME> signame;
signame.write(value);
signame.read(); // Returns current value, not new
wait(signame.value_changed_event());
if(signame.event() == true) …
//if occurred in the immediately previous δ-cycle
event( ) can be employed to tell which signal triggered an event
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Example of sc_signal
int cnt;
string msg;
sc_signal<int> cnt_sig;
sc_signal<string> msg_sig;
cnt=11 cnt_sig=0
msg:’Who’ msg_sig:‘’
cnt=20 cnt_sig=10
msg:’Who’ msg_sig:‘Hi’
cnt_sig.write(10); msg_sig.write(“Hi”);
cnt = 11; msg = “Who”;
cout << “cnt=” << cnt << “ cnt_sig=” <<
<< “msg:” << msg << “ msg_sig:” <<
wait(SC_ZERO_TIME);
cnt_sig.write(cnt); cnt = 20;
cout << “cnt=” << cnt << “ cnt_sig=” <<
<< “msg:” << msg << “ msg_sig:” <<
ECEN 468 Lecture 6
cnt_sig << endl
msg_sig << endl;
cnt_sig << endl
msg_sig << endl;
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Alternative Assignment to sc_signal
sc_signal<TYPENAME> signame;
signame.write(value);
signame.read(); // Returns current value, not new
signame = value; // implicit .write() dangerous
variableA = signame; // implicit .read() mild danger
v  Only a single process may write to a given signal during a
specific delta-cycle
v  This restriction avoids risk of race condition
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Resolved Signal Channels
sc_signal_resolved signame;
sc_signal_rv<WIDTH> signame;
// For vectors
v  Allow multiple writers
A\B
‘0’
‘1’
‘X’
‘Z”
‘0’
‘0’
‘X’
‘X’
‘0’
‘1’
‘X’
‘1’
‘X’
‘1’
‘X’
‘X’
‘X’
‘X’
‘X’
‘Z’
‘0’
‘1’
‘X’
‘Z’
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Template Specialization
v Some template specialization has additional behaviors not
available for the general case
v sc_signal<bool> and sc_signal<sc_logic> have functions
posedge_event() and negedge_event(), which are not available
to general sc_signal<T>
v bool and sc_bv<WIDTH> support two value logic:
o  ‘0’ and ‘1’
v sc_logic and sc_lv<WIDTH> support four value logic:
o  SC_LOGIC_0 (‘0’), SC_LOGIC_1 (‘1’), SC_LOGIC_Z (‘Z’), SC_LOGIC_X (‘X’)
v For sc_logic, a posedge_event occurs on any transition to
SC_LOGIC_1, including from SC_LOGIC_X and
SC_LOGIC_Z
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