Motorola Corporation

Keith Duy Dang

Donald C Anderson

A low power high speed multiply/accumulator ( ) utilizes a modified Booth's recoder ( ) to identify situations to power down the partial product array ( ). The modified Booth's recoder ( ) is responsive to a NOP signal ( ) and a add/subtract signal ( ) that result from instruction decode. The partial product array ( ) can be partially or fully shut-down to conserve power in response to the recoder ( ) detecting certain operands and NOP instructions. It also allows implementation a multiply-and-subtract instruction. The output of the partial product array ( ) is registered in a high order product register ( ) and a low order product register ( ). The low order product register ( ) accumulates partial products for multiply-and-accumulate and multiply-and-subtract instructions. The carry bit of the low order product register ( ) is added ( ) to the high order product register ( ) to generate the high order result ( ), while the low order result ( ) are derived from the low order product register ( ).

Low power pipelined multiply/accumulator with modified booth's recoder

An arithmetic logic unit (30) for a digital signal processor (DSP) contains circuitry for preshifting (46, 48) and prerounding (54) the 2's-complement fractional input operands (32, 34) before they are used by a carry look-ahead adder (56). The preshifting (46, 48) provides for efficient divide-by-2 and divide-by-4 functionality and reduces early overflow. Concurrent preshifting (46, 48) and prerounding (54) improve the critical path timing in the carry look-ahead adder (56).

Arithmetic logic unit having preshift and preround circuits

A high-speed barrel shifter (20) includes a shifter array (25) having a matrix of transistors (40) located at intersections of rows and columns of the matrix (40). The rows and columns alternately function as source and destination terminals. A fill portion (48) fills either a predetermined value or a data-dependent value such as a sign bit into vacated bit positions along rows in a bottom left portion (42). Thus the barrel shifter (20) can perform a data-dependent fill instruction within the shifter array (25) and avoids extra clock cycles associated with post-array processing. In one embodiment, an isolation portion (44, 45) separates a top right portion (41) of the matrix (40) from the bottom left portion (42) along a diagonal (43). The isolation portion (44, 45) isolates transistors in the bottom left portion (42), which are associated with rotates and fills, from transistors in the top right portion (41), which are associated with shifts, according to the direction of the shift.

High-speed barrel shifter

A priority encoder (12) has a most significant bit circuitry (18), a first less significant bit circuitry (20) and a second less significant bit circuitry (22). The priority encoder detects a leading one within a plurality of data bits. Each data bit is associated with a different one of a plurality of input signals. The most significant bit circuitry is coupled to a first one of the input signals and generates a first output signal and a parallel blocking signal. Both of the first output signal and the parallel blocking signal are representative in a first logic state of a leading one associated with the first signal. The first less significant bit cell is coupled to a second one of the input signals and to the parallel blocking signal. The first less significant bit cell generates a second output signal and a less significant carry signal. Both the second output signal and the less significant carry signal are representative in a first logic state of a leading one associated with the second input signal. The second less significant bit cell is coupled to a third one of the input signals, to the parallel blocking signal and to the less significant carry signal. The second less significant bit cell generates a third output signal representative in a first logic state of a leading one associated with the third input signal.

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Pflugerville, TX, United States of America

Keith Duy Dang