Fast conversion two's complement encoded shift value for a barrel shifter

Abstract

In accordance with the present invention, a method and apparatus are provided for controlling an N-bit barrel shifter to shift the bits of an input word by a shift value. The method includes the steps of performing a ones' complement of an m-bit binary representation of a shift value to generate an input when the shift direction takes on a first direction and passing the binary representation of the shift value as the input when the shift direction takes on a second direction. Decoding the input into 2.sup.m control signals and generating a plurality of groups of control signals from the 2.sup.m control signals. Selecting one of the plurality of groups of control signals as the control activation for the barrel shifter. The apparatus provides bits of a binary representation of the shift value are passed through a plurality of exclusive OR gates and provided as inputs to a first decoder. The first decoder decodes the inputs into a plurality of bits used as control signals. The control signals are grouped in a plurality of groups. Some of the groups of control signals may be supplemented with additional control signals such as zero filling. Based on a select input a multiplexer selects which one of the plurality of groups of control signals is passed to control the barrel shifter. A second decoder decodes the shift direction and sign of the shift value to provide a select input to the multiplexer.