Browse Prior Art Database

ZERO DETECTION BY EXISTING ADDER

IP.com Disclosure Number: IPCOM000009546D
Original Publication Date: 1999-Sep-01
Included in the Prior Art Database: 2002-Sep-02
Document File: 2 page(s) / 83K

Publishing Venue

Motorola

Related People

Fabrice Aidan: AUTHOR [+4]

Abstract

Modem processors perform zero detection based on an arithmetic result. Usually, a dedicated flag is allocated for indicating that a result is zero. This demands adding special, large and power consum- ing logic. This proposal uses an already existing adder and performs zero detection with less cir- cuitry.

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Developments Technical 0 M MOTOROLA

ZERO DETECTION BY EXISTING ADDER

by Fabrice Aidan, Yoram Salant, Mark Elnekave and Leonid Zukerman

INTRODUCTION

  Modem processors perform zero detection based on an arithmetic result. Usually, a dedicated flag is allocated for indicating that a result is zero. This demands adding special, large and power consum- ing logic. This proposal uses an already existing adder and performs zero detection with less cir- cuitry.

PROBLEM

  Every modern processor or Digital Signal Processor (DSP) architecture comprises the com- mand CMPEQ (CoMPareEQual) or similar com- mand. CMPEQ compares arguments "A" and "B" and sets a flag if A=B (normally the "true bit" in the Status Register).

  In a usual implementation, an adder adds "-B" to "A". A dedicated zero detection logic (ZDL) ana- lyzes the result and sets the flag if the result is zero, (A equals B). The usual Zero detecting logic is large and slow.

SOLUTION

  A second adder provided to perform zero detec- tion is smaller than a dedicated ZDL and is in many cases already existent in the EXECUTION UNIT which performs the CMPEQ command. This is, for example, the case in the Address Generation Unit (AGU) of modem DSP processors. Every modem DSP features a modulo addressing mode, where a

circular buffer is defined in the memory. The point- er register decremented to the beginning of the buffer upon reaching the end of the buffer. When a pointer register is incremented by an offset, two cal- culations take place:

(1) register + offset

(2) register + offset - buffersize

  If result (1) is above the higher limit of the buffer then result (2) is used....