move fpcommon to separate subdir

[ieee754fpu.git] / src / ieee754 / fpcommon / prenormalise.py

diff --git a/src/ieee754/fpcommon/prenormalise.py b/src/ieee754/fpcommon/prenormalise.py
new file mode 100644 (file)
index 0000000..0b3a65c
--- /dev/null
+++ b/src/ieee754/fpcommon/prenormalise.py
@@ -0,0 +1,83 @@
+# IEEE Floating Point Adder (Single Precision)
+# Copyright (C) Jonathan P Dawson 2013
+# 2013-12-12
+
+from nmigen import Module, Signal, Cat,
+from nmigen.lib.coding import PriorityEncoder
+from nmigen.cli import main, verilog
+from math import log
+
+from fpbase import Overflow, FPNumBase
+from fpbase import MultiShiftRMerge
+
+from fpbase import FPState
+
+
+class FPNormaliseModSingle:
+
+    def __init__(self, width):
+        self.width = width
+        self.in_z = self.ispec()
+        self.out_z = self.ospec()
+
+    def ispec(self):
+        return FPNumBase(self.width, False)
+
+    def ospec(self):
+        return FPNumBase(self.width, False)
+
+    def setup(self, m, i):
+        """ links module to inputs and outputs
+        """
+        m.submodules.normalise = self
+        m.d.comb += self.i.eq(i)
+
+    def elaborate(self, platform):
+        m = Module()
+
+        mwid = self.out_z.m_width+2
+        pe = PriorityEncoder(mwid)
+        m.submodules.norm_pe = pe
+
+        m.submodules.norm1_out_z = self.out_z
+        m.submodules.norm1_in_z = self.in_z
+
+        in_z = FPNumBase(self.width, False)
+        in_of = Overflow()
+        m.submodules.norm1_insel_z = in_z
+        m.submodules.norm1_insel_overflow = in_of
+
+        espec = (len(in_z.e), True)
+        ediff_n126 = Signal(espec, reset_less=True)
+        msr = MultiShiftRMerge(mwid, espec)
+        m.submodules.multishift_r = msr
+
+        m.d.comb += in_z.eq(self.in_z)
+        m.d.comb += in_of.eq(self.in_of)
+        # initialise out from in (overridden below)
+        m.d.comb += self.out_z.eq(in_z)
+        m.d.comb += self.out_of.eq(in_of)
+        # normalisation decrease condition
+        decrease = Signal(reset_less=True)
+        m.d.comb += decrease.eq(in_z.m_msbzero)
+        # decrease exponent
+        with m.If(decrease):
+            # *sigh* not entirely obvious: count leading zeros (clz)
+            # with a PriorityEncoder: to find from the MSB
+            # we reverse the order of the bits.
+            temp_m = Signal(mwid, reset_less=True)
+            temp_s = Signal(mwid+1, reset_less=True)
+            clz = Signal((len(in_z.e), True), reset_less=True)
+            m.d.comb += [
+                # cat round and guard bits back into the mantissa
+                temp_m.eq(Cat(in_of.round_bit, in_of.guard, in_z.m)),
+                pe.i.eq(temp_m[::-1]),          # inverted
+                clz.eq(pe.o),                   # count zeros from MSB down
+                temp_s.eq(temp_m << clz),       # shift mantissa UP
+                self.out_z.e.eq(in_z.e - clz),  # DECREASE exponent
+                self.out_z.m.eq(temp_s[2:]),    # exclude bits 0&1
+            ]
+
+        return m
+
+