From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Tue, 23 Jul 2019 07:52:14 +0000 (+0100)
Subject: update explanatory comments
X-Git-Tag: ls180-24jan2020~761
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=c85a883eed32e199ead347e24c40ef8ea87dc81e;p=ieee754fpu.git

update explanatory comments
---

diff --git a/src/ieee754/fpmul/mul1.py b/src/ieee754/fpmul/mul1.py
index 841efcdf..6d2f9ff4 100644
--- a/src/ieee754/fpmul/mul1.py
+++ b/src/ieee754/fpmul/mul1.py
@@ -30,14 +30,16 @@ class FPMulStage1Mod(FPState, Elaboratable):
         """ links module to inputs and outputs
         """
         m.submodules.mul1 = self
-        #m.submodules.mul1_out_overflow = self.o.of
-
         m.d.comb += self.i.eq(i)
 
     def elaborate(self, platform):
         m = Module()
         m.d.comb += self.o.z.eq(self.i.z)
         with m.If(~self.i.out_do_z):
+            # results are in the range 0.25 to 0.999999999999
+            # sometimes the MSB will be zero, (0.5 * 0.5 = 0.25 which
+            # in binary is 0b010000) so to compensate for that we have
+            # to shift the mantissa up (and reduce the exponent by 1)
             p = Signal(len(self.i.product), reset_less=True)
             with m.If(self.i.product[-1]):
                 m.d.comb += p.eq(self.i.product)
@@ -45,13 +47,16 @@ class FPMulStage1Mod(FPState, Elaboratable):
                 # get 1 bit of extra accuracy if the mantissa top bit is zero
                 m.d.comb += p.eq(self.i.product<<1)
                 m.d.comb += self.o.z.e.eq(self.i.z.e-1)
+
+            # top bits are mantissa, then guard and round, and the rest of
+            # the product is sticky
             mw = self.o.z.m_width
             m.d.comb += [
-                self.o.z.m.eq(p[mw+2:]),
-                self.o.of.m0.eq(p[mw+2]),
-                self.o.of.guard.eq(p[mw+1]),
-                self.o.of.round_bit.eq(p[mw]),
-                self.o.of.sticky.eq(p[0:mw].bool())
+                self.o.z.m.eq(p[mw+2:]),            # mantissa
+                self.o.of.m0.eq(p[mw+2]),           # copy of LSB
+                self.o.of.guard.eq(p[mw+1]),        # guard
+                self.o.of.round_bit.eq(p[mw]),      # round
+                self.o.of.sticky.eq(p[0:mw].bool()) # sticky
             ]
 
         m.d.comb += self.o.out_do_z.eq(self.i.out_do_z)