From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Sat, 15 Feb 2020 11:55:06 +0000 (+0000)
Subject: comment why shift mask exists
X-Git-Tag: ls180-24jan2020~167
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=8570823645b26d03edc089d9fd18baac78efa872;p=ieee754fpu.git

comment why shift mask exists
---

diff --git a/src/ieee754/part_shift/part_shift_dynamic.py b/src/ieee754/part_shift/part_shift_dynamic.py
index 95f329ed..b298271f 100644
--- a/src/ieee754/part_shift/part_shift_dynamic.py
+++ b/src/ieee754/part_shift/part_shift_dynamic.py
@@ -55,6 +55,9 @@ class PartitionedDynamicShift(Elaboratable):
         min_bits = math.ceil(math.log2(intervals[0][1] - intervals[0][0]))
         max_bits = math.ceil(math.log2(width))
 
+        # shifts are normally done as (e.g. for 32 bit) result = a & (b&0b11111)
+        # truncating the b input.  however here of course the size of the
+        # partition varies dynamically.
         shifter_masks = []
         for i in range(len(b_intervals)):
             mask = Signal(b_intervals[i].shape(), name="shift_mask%d" % i)