Three input arithmetic logic unit with shifter

Abstract

A data processing apparatus includes a three input arithmetic logic unit (230) that generates a combination of the three inputs that is selected by a function signal. Data registers (200) store the three data inputs and the arithmetic logic unit output. The second input signal comes from a controllable shifter (235). The shift amount is a default shift amount stored in a special data register, a predetermined set of bits of data recalled from a data register or zero. A one's constant source (236) is connected to the shifter (235) to supply a multibit digital signal of '1'. This permits generating a second input signal of the form 2.sup.N, with N being the shift amount. The output of the shifter (235) may be stored independently of the arithmetic logic unit (230) result. The third input signal comes from a multiplexer (233) that selects between an instruction specified immediate field, data recalled from a data register or a mask input from a mask generator (239). One preferred form of the mask has a number of right justified 1's corresponding to a mask input signal. This mask input signal may be the default shift amount or a predetermined number of the least significant bits of a third input signal as selected by a multiplexer. A second preferred form of the mask is selected one of the left most 1, the right most 1, the left most bit change or the right most bit change of a predetermined set of the least significant bits of data recalled from a data register. In the preferred embodiment of this invention, the three input arithmetic logic unit (230) is embodied in a data processor circuits as a part of a multiprocessor integrated circuit (100) used in image processing.