The Scratchpad Memory Management Unit for Microblaze: Implementation, Testing, and Case Study

This report proposes
the scratchpad memory management unit (SMMU) to act as a perfect
data cache for a known subset of the data used by a program. This enables
the execution time for each load or store operation in the program
to be precisely determined. The SMMU must be explicitly controlled
by the program, which commands the addition and removal of objects
from the SMMU and its associated scratchpad memory.

This report explains why the SMMU
enables memories with high access latency to be used efficiently
within hard real-time embedded systems. It describes the SMMU in abstract form,
explaining how it solves the memory latency and pointer aliasing
problems and how programs could use it.
Then, it describes the implementation of a version of the SMMU for
the Microblaze processor, enabling SMMU experiments within real
embedded systems. Lastly, it conducts a case study involving a large
C program to understand the costs and benefits of the SMMU for real
code. A memory profiler is used to detect objects that can be
usefully allocated within the scratchpad by identifying their base
pointers.

The implementation for Microblaze is found to be practical
in terms of clock frequency and logic area. The case study shows that
the SMMU capabilities cannot be applied to all variables, but the
performance of the SMMU within the functions being considered
is within a factor of two of the performance of a perfect data cache.