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Definition
The best way to understand the requirements is to examine typical DSP algorithms
and identify how their compositional requirements have influenced the architectures
of DSP processor. Let us consider one of the most common processing tasks the
finite impulse response filter. For each tap
of the filter a data sample is multiplied by a filter coefficient with result
added to a running sum for all of the taps .Hence the main component of the FIR
filter is dot product: multiply and add .These options are not unique to the FIR
filter algorithm; in fact multiplication is one of the most common operation performed
in signal processing -convolution, IIR filtering and Fourier transform also involve
heavy use of multiply -accumulate operation. Originally, microprocessors implemented
multiplication by a series of shift and add operation, each of which consumes
one or more clock cycle .First a DSP processor requires a hardware which can multiply
in one single cycle. Most of the DSP algorithm require a multiply and accumulate
unit (MAC). In comparison to other type of
computing tasks, DSP application typically have very high computational requirements
since they often must execute DSP algorithms in real time on lengthy segments
,therefore parallel operation of several independent execution units is a must
-for example in addition to MAC unit an ALU and shifter is also required .
Executing a MAC in every clock cycle requires more than just single cycle MAC
unit. It also requires the ability to fetch the MAC instruction, a data sample,
and a filter coefficient from a memory in a single cycle. Hence good DSP performance
requires high memory band width-higher than that of general microprocessors, which
had one single bus connection to memory and could only make one access per cycle.
The most common approach was to use two or more separate banks of memory, each
of which was accessed by its own bus and could be written or read in a single
cycle. This means programs are stored in a memory and data in another .With this
arrangement, the processor could fetch and a data operand in parallel in every
cycle .since many DSP algorithms consume two data operands per instruction a further
optimization commonly used is to include small bank of RAM near the processor
core that is used as an instruction cache. When a small group of instruction is
executed repeatedly, the cache is loaded with those instructions, freeing the
instruction bus to be used for data fetches instead of instruction fetches -thus
enabling the processor to execute a MAC in a single cycleHigh memory bandwidth
requirements are often further supported by dedicated hard ware for calculating
memory address. These memory calculating units operate in parallel with DSP processors
main execution units, enabling it to access data in new location in the memory
without pausing to calculate the new address.
Memory
accesses in DSP algorithm tend to exhibit very predictable pattern: for example
For sample in FIR filter , the filter coefficient are accessed sequentially from
start to finish , then accessed start over from beginning of the coefficient vector
when processing the next input sample .This is in the contrast of other computing
tasks ,such as data base processing where accesses to memory are less predictable
.DSP processor address generation units take advantage of this predictability
of supporting specialize addressing modes that enable the processor to efficiently
access data in the patterns commonly found in DSP algorithms .
The most common
of these modes is register indirect addressing with post increment , which is
used to automatically increment the address pointer for the algorithms where repetitive
computations are performed on a series of data stored sequentially in the memory
.Without this feature , the programmer would need to spend instruction explicitly
incrementing the address pointer .
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