Add more/better comments to fsignj.py

[ieee754fpu.git] / src / ieee754 / fsgnj / fsgnj.py

diff --git a/src/ieee754/fsgnj/fsgnj.py b/src/ieee754/fsgnj/fsgnj.py
index 31501743a159124216ccb29200b381977dda24e2..be6060bdde8323d1b85cb04c60af02119d27da89 100644 (file)
--- a/src/ieee754/fsgnj/fsgnj.py
+++ b/src/ieee754/fsgnj/fsgnj.py
@@ -36,8 +36,8 @@ class FSGNJPipeMod(PipeModBase):
         comb = m.d.comb
 
         z1 = self.o.z
-        a = self.i.a
-        b = self.i.b
+
+        # Decode the input operands into sign, exponent, and mantissa
         a1 = FPNumBaseRecord(width, False)
         b1 = FPNumBaseRecord(width, False)
         m.submodules.sc_decode_a = a1 = FPNumDecode(None, a1)
@@ -47,12 +47,22 @@ class FSGNJPipeMod(PipeModBase):
 
         opcode = self.i.ctx.op
 
-        sign = Signal()
 
+        # Calculate the sign bit
+        sign = Signal(reset_less=True)
+
+        # Handle opcodes 0b00 and 0b01, copying or inverting the sign bit of B
         sign = Mux(opcode[0], ~b1.s, b1.s)
-        sign = Mux(opcode[1], sign ^ a1.s, sign)
+        # Handle opcodes 0b10 and 0b11, XORing the 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)
+        # because this requires slightly less logic than making it the
+        # 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))
 
         # copy the context (muxid, operator)