From aab5818c35905f7f28d437298693409a6fb21a28 Mon Sep 17 00:00:00 2001
From: Eric Kooistra <kooistra@astron.nl>
Date: Tue, 26 Oct 2021 16:57:15 +0200
Subject: [PATCH] Added comments to RESIZE_NUM(), no functional change.
---
libraries/base/common/src/vhdl/common_pkg.vhd | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/libraries/base/common/src/vhdl/common_pkg.vhd b/libraries/base/common/src/vhdl/common_pkg.vhd
index 55a7a6a844..4b7c0f4b71 100644
--- a/libraries/base/common/src/vhdl/common_pkg.vhd
+++ b/libraries/base/common/src/vhdl/common_pkg.vhd
@@ -388,7 +388,10 @@ PACKAGE common_pkg IS
-- appropriate approach is to ignore the MSbit sign and just keep the LS part. For too large values this
-- means that the result gets wrapped, but that is fine for default behaviour, because that is also what
-- happens for RESIZE of UNSIGNED. Therefor this is what the RESIZE_NUM for SIGNED and the RESIZE_SVEC do
- -- and better not use RESIZE for SIGNED anymore.
+ -- and better not use RESIZE for SIGNED anymore. When w keeps or increases the data width then the values
+ -- do not change (of course). When w reduces the data width then:
+ -- * RESIZE() wraps between -, 0 for negative and 0, + for positive, so it keeps the sign and w-1 LSbits
+ -- * RESIZE_NUM() removes MSbits, so it wraps from + to - and from - to +, and it keeps the w LSbits
FUNCTION RESIZE_NUM( u : UNSIGNED; w : NATURAL) RETURN UNSIGNED; -- left extend with '0' or keep LS part (same as RESIZE for UNSIGNED)
FUNCTION RESIZE_NUM( s : SIGNED; w : NATURAL) RETURN SIGNED; -- extend sign bit or keep LS part
FUNCTION RESIZE_UVEC(sl : STD_LOGIC; w : NATURAL) RETURN STD_LOGIC_VECTOR; -- left extend with '0' into slv
@@ -1975,8 +1978,8 @@ PACKAGE BODY common_pkg IS
FUNCTION RESIZE_NUM(u : UNSIGNED; w : NATURAL) RETURN UNSIGNED IS
BEGIN
- -- left extend with '0' or keep LS part (same as RESIZE for UNSIGNED)
- RETURN RESIZE(u, w);
+ -- left extend with '0' or remove MSbits and keep LS part (= u[w-1:0])
+ RETURN RESIZE(u, w); -- same as RESIZE for UNSIGNED
END;
FUNCTION RESIZE_NUM(s : SIGNED; w : NATURAL) RETURN SIGNED IS
@@ -1985,7 +1988,12 @@ PACKAGE BODY common_pkg IS
IF w>s'LENGTH THEN
RETURN RESIZE(s, w); -- extend sign bit
ELSE
- RETURN SIGNED(RESIZE(UNSIGNED(s), w)); -- keep LSbits (= vec[w-1:0])
+ -- RESIZE() wraps between -, 0 for negative and 0, + for positive, so it keeps the sign and w-1 LSbits
+ -- RESIZE_NUM() removes MSbits, so it wraps from + to - and from - to +, and it keeps the w LSbits
+
+ -- remove MSbits and keep LS part (= s[w-1:0])
+ -- use RESIZE(UNSIGNED()) rather than s[w-1:0] to be independent of RANGE of s
+ RETURN SIGNED(RESIZE(UNSIGNED(s), w));
END IF;
END;
--
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