Soft-Core Processor Design - CiteSeer

Chapter 5
Performance
This chapter investigates the performance of the UT Nios implementation described in the
previous chapter. First, the methodology used to measure the performance is described. Next, the
effects of various architectural parameters on performance are investigated. Finally, a
performance comparison of the UT Nios and 32-bit Altera Nios is presented. Although the
emphasis is on the performance, the FPGA area requirements are also taken into consideration.
5.1. Methodology
5.1.1. UT Nios Benchmark Set
To assess the performance of a computer architecture, a set of benchmark programs has to be
defined. The benchmark set should be representative of the programs that are commonly run on
the architecture. Hence, to define a good benchmark set, it is necessary to determine the
characteristics of programs that are used in Nios-based systems. We refer to the benchmark set
defined in this chapter as the UT Nios benchmark set.
The Nios architecture is intended for use in FPGA-based systems. FPGAs are typically built
into systems for two reasons: speeding up the performance of critical parts of an application, and
system reconfigurability. Performance critical parts of the application are implemented in logic
because such implementation does not incur overheads associated with the instruction fetching
and decoding in a software implementation. For instance, DSP functions can be implemented
efficiently in the DSP blocks of Stratix FPGA chips, while an equivalent implementation in
software running on a general-purpose microprocessor incurs the overheads that limit the
performance. Auxiliary functions, performing simple operations, are more likely to be
implemented in software using a general-purpose microprocessor like Nios. They include
functions such as simple data conversions or header processing in network applications. Using a
soft-core processor, instead of mapping the whole application into hardware, results in better
logic utilization [55]. Performance is still an issue, because the soft-core processor has to be able
to handle the data at the same rate as the rest of the system.
47