Variable length instruction decoder

Abstract

The present invention is a novel and improved method and circuit for digital signal processing. One aspect of the invention calls for the use of a variable length instruction set. A portion of the variable length instructions may be stored in adjacent locations within memory space with the beginning and ending of instructions occurring across memory word boundaries. Furthermore, additional aspects of the invention are realized by having instructions contain variable numbers of instruction fragments. Each instruction fragment causes a particular operation, or operations, to be performed allowing multiple operations during each clock cycle. Thus, multiple operations are performed during each clock cycle, reducing the total number of clock cycles necessary to perform a task. The exemplary DSP includes a set of three data buses over which data may be exchanged with a register bank and three data memories. The use of more than two data buses, and especially three data buses, realizes another aspect of the invention, which is significantly reduced bus contention. One embodiment of the invention calls for the data buses to include one wide bus and two narrow buses. The wide bus is coupled to a wide data memory and the two narrow buses are coupled to two narrow data memories. Another aspect of the invention is realized by the use of a register bank that has registers accessible by at least two processing units. This allows multiple operations to be performed on a particular set of data by the multiple processing units, without reading and writing the data to and from a memory. The processing units in the exemplary embodiment of the invention include an arithmetic logic (ALU) and a multiply-accumulate (MAC) unit. When combined with the use of the multiple bus architecture, highly parallel instructions, or both, an additional aspect of the invention is realized where highly pipelined, multi-operation, processing is performed.