sim: support for signed numbers

examples/basic_sim.py

@@ -5,7 +5,7 @@ from migen.sim.icarus import Runner
 class Counter:
 	def __init__(self):
 		self.ce = Signal()
-		self.count = Signal(BV(4))
+		self.count = Signal(BV(37, True), reset=-5)
 	
 	def do_simulation(self, s, cycle):
 		if cycle % 2:
@@ -22,6 +22,6 @@ class Counter:
 def main():
 	dut = Counter()
 	sim = Simulator(dut.get_fragment(), Runner(), TopLevel("my.vcd"))
-	sim.run(10)
+	sim.run(20)
 
 main()

migen/sim/generic.py

@@ -111,12 +111,17 @@ class Simulator:
 		self.ipc.send(MessageRead(name))
 		reply = self.ipc.recv()
 		assert(isinstance(reply, MessageReadReply))
-		# TODO: negative numbers + cleanup LSBs
-		return reply.value
+		nbits = signal.bv.width
+		value = reply.value & (2**nbits - 1)
+		if signal.bv.signed and (value & 2**(nbits - 1)):
+			value -= 2**nbits
+		return value
 	
 	def wr(self, signal, value):
 		name = self.top_level.top_name + "." \
 		  + self.top_level.dut_name + "." \
 		  + self.namespace.get_name(signal)
-		# TODO: negative numbers
+		if value < 0:
+			value += 2**signal.bv.width
+		assert(value >= 0 and value < 2**signal.bv.width)
 		self.ipc.send(MessageWrite(name, value))