add comments, remove one of the Muxes

diff --git a/src/ieee754/fsgnj/fsgnj.py b/src/ieee754/fsgnj/fsgnj.py
index dbdb23d1933b6c4f0390bb2956f1e82264824937..d6de4d4b04f1539dc22ab30f82c54149d35d81c3 100644 (file)
--- a/src/ieee754/fsgnj/fsgnj.py
+++ b/src/ieee754/fsgnj/fsgnj.py
@@ -32,9 +32,10 @@ class FSGNJPipeMod(PipeModBase):
     def elaborate(self, platform):
         m = Module()
 
-        width = self.pspec.width
+        # useful clarity variables
         comb = m.d.comb
-
+        width = self.pspec.width
+        opcode = self.i.ctx.op
         z1 = self.o.z
 
         # Decode the input operands into sign, exponent, and mantissa
@@ -45,14 +46,13 @@ class FSGNJPipeMod(PipeModBase):
         comb += [a1.v.eq(self.i.a),
                  b1.v.eq(self.i.b)]
 
-        opcode = self.i.ctx.op
-
-
-        # Calculate the sign bit
-        sign = Signal(reset_less=True)
+        # Calculate the sign bit, with a chain of muxes.  has to be done
+        # this way due to (planned) use of PartitionedSignal.  decreases
+        # readability slightly, but hey.
 
         # Handle opcodes 0b00 and 0b01, copying or inverting the sign bit of B
-        sign = Mux(opcode[0], ~b1.s, b1.s)
+        sign = opcode[0] ^ b1.s # op[0]=0, sign unmodified, op[0]=1 inverts.
+
         # Handle opcodes 0b10 and 0b11, XORing sign bits of a and b together.
         # opcode 0b11 is not defined in the RISCV spec; it is handled
         # here as equivalent to opcode 0b10 (i.e. a1.s XOR b1.s)
@@ -60,7 +60,6 @@ class FSGNJPipeMod(PipeModBase):
         # same as opcode 0b00 (1 less Mux).
         sign = Mux(opcode[1], b1.s ^ a1.s, sign)
 
-
         # Create the floating point number from the sign bit
         # calculated earlier and the exponent and mantissa of operand a
         comb += z1.eq(a1.fp.create2(sign, a1.e, a1.m))