Professional Embedded ARM Development

Appendix A

Terminology

When studying embedded systems, and reading through technical documentation, there are a lot of terms that, at first, make no sense. A processor might be capable of obtaining a certain amount of MIPS, or maybe it supports JTAG debugging, or even support SIMD, but what exactly does that mean? In this appendix, you will see some of the most common terms and an explanation of each.

BRANCH PREDICTION

During a branch operation, when branching to a new portion of code, there can be a performance hit while the processor gets new instructions from another portion of memory. To reduce this performance hit, branch prediction hardware is included in some ARM implementations, fetching instructions from memory according to what the branch predictor thinks will be the result. Newer branch prediction hardware can obtain 95 percent accuracy, greatly increasing branch performance. Some branch prediction hardware can obtain a 100 percent accuracy by speculatively executing both branch results, and discarding one of the two when the result is known.

CACHE

When memory became cheaper and systems started using more of it, it became clear that too much time was spent fetching data from the memory or putting data back into the memory. Analysis showed that most of the time the system would read in data from the spatial locality; in essence, if an access is made to a particular location in memory, there is a high probability that other accesses will be made to either that or neighboring ...