Soft-Core Processor Design - CiteSeer

Chapter 2
Background
Soft-core processors are one aspect of the trend in architecture generally known as
reconfigurable computing. Recent developments in FPGA technology made FPGAs a suitable
target for processor implementations. FPGAs can be reprogrammed, so the processor parameters
can be changed during the lifetime of the product, if the need arises. However, an FPGA
implementation of a soft-core processor will typically provide lower performance than the
corresponding ASIC design. In this chapter we give an overview of the related work, and put soft-
core processors in the context of reconfigurable computing. We also review the FPGA technology
and CAD flow for FPGAs.
2.1. Related work
The concept of reconfigurable computing has been in existence since the early 1960s [4].
Reconfigurable computing systems use some form of programmable hardware to accelerate
algorithm execution. Computation intensive parts of the algorithm are mapped to the
programmable hardware, while the code that cannot be efficiently mapped is usually executed on
a general-purpose processor. Depending on the proximity of programmable hardware to the
general-purpose processor, a system is said to be closely or loosely coupled. In a closely coupled
system, reconfigurable resources allow customization of the processor’s functional units. On the
other end, reconfigurable hardware in a loosely coupled system can be a standalone network unit.
Reconfigurable systems are usually categorized between these two extremes. There has been a lot
of research in the area of reconfigurable computing. An extensive survey of reconfigurable
systems can be found in [4].
In reconfigurable systems, performance critical parts of the application are implemented in
hardware. Since various systems tend to use common algorithms, many of the developed
components can be reused. Reusable components come in the form of intellectual property (IP)
cores. An IP core is a standard block of logic or data that can be used to build a larger or more
complex system. IP cores are divided into three categories, depending on the level of
optimization, and flexibility of reuse: soft cores, firm cores, and hard cores [5]. A soft core is
usually a synthesizable HDL specification that can be retargeted to various semiconductor
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