Very Long Instruction Word (VLIW) architecture in P-DSPs (programmable DSP) increases the number of instructions that are processed per cycle. It is a concatenation of several short instructions and requires multiple execution units running in parallel, to carry out the instructions in a single cycle. A language compiler or pre-processor separates program instructions into basic operations and places them into VLWI processor which then disassembles and transfers each operation to an appropriate execution unit.

VLIW P-DSPs have a number of processing units (data paths) i.e. they have a number of ALUs, MAC units, shifters, etc. The VLIW is accessed from memory and is used to specify the operands and operations to be performed by each of the data paths.

As shown in figure, the multiple functional units share a common multiported register file for fetching the operands and storing the results. Parallel random access by the functional units to the register file is facilitated by the read/write cross bar. Execution of the operations in the functional units is carried out concurrently with the load/ store operation of data between a RAM and the register file.

The performance gains that can be achieved with VLIW architecture depends on the degree of parallelism in the algorithm selected for a DSP application and the number of functional units. The throughput will be higher only if the algorithm involves execution of independent operations. For example, in convolution by using eight functional units, the time required can be reduced by a factor of 8 compared to the case where a single functional unit is used.

However, it may not always be possible to have independent stream of data for processing. The number of functional units is also limited by the hardware cost for the multi-ported register file and cross bar switch.

Advantages of VLIW architecture

• Increased performance.
• Potentially scalable i.e. more execution units can be added and so more instructions can be packed into the VLIW instruction.

Disadvantages of VLIW architecture

• New programmer needed.
• Program must keep track of Instruction scheduling.
• Increased memory use.
• High power consumption.