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commit a9388ee62cd0553aa62f7305956b37d677d23c97
parent 4ff3671b42f011862ab7a8c6b8ddf66780a2054b
Author: Mattias Andrée <>
Date:   Sat, 14 May 2016 17:01:55 +0200

On bit-truncation

Signed-off-by: Mattias Andrée <>

doc/bit-operations.tex | 25++++++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/doc/bit-operations.tex b/doc/bit-operations.tex @@ -88,7 +88,30 @@ whenever possible. One such wrapper could be \section{Truncation} \label{sec:Truncation} -TODO % ztrunc +In \secref{sec:Shift} we have seen how bit-shift +operations can be used to multiply or divide by a +power of two. There is also a bit-truncation +operation: {\tt ztrunc}, which is used to keep +only the lowest bits, or equivalently, calculate +the remainder of a division by a power of two. + +\begin{alltt} + void ztrunc(z_t r, z_t a, size_t b); +\end{alltt} + +\noindent +is consistent with {\tt zmod}; like {\tt zlsh} and +{\tt zrsh}, {\tt a}'s sign is preserved into {\tt r} +assuming the result is non-zero. + +{\tt ztrunc(r, a, b)} stores only the lowest {\tt b} +bits in {\tt a} into {\tt r}, or equivalently, +calculates $r \gets a \mod 2^b$. For example, if + +$a = 100011000_2$ then + +$r = \phantom{10001}1000_2$ after calling +{\tt ztrunc(r, a, 4)}. \newpage