Throughout the book, we’ve been using two different instruction sets, ARM and Thumb-2, only mentioning 16-bit Thumb here and there. Recall that the Cortex-M4 executes only Thumb-2 instructions, while the ARM7TDMI executes ARM and 16-bit Thumb instructions. Keeping in mind that a processor’s microarchitecture and a processor’s instruction set are two different things, Table 17.1 shows how the ARM processor architectures have evolved over the years, along with the instruction sets. They often get developed at the same time, but it is possible for a given microarchitecture to be modified only slightly to support additional instructions, adding more control logic and a bit more datapath, adding registers, etc. Consider the ARM9TDMI which supports the version 4T instruction set and the ARM9E, loosely the same microarchitecture, which supports version 5TE instructions. So when we discuss a processor like the Cortex-A15, we think of pipeline depth, memory management units, cache sizes, and the like, but at the end of the day we’re really interested in what instructions the machine supports. Historically for most ARM cores, two instruction sets were supported at the same time-ARM and Thumb-where the processor could switch between them as needed. In 2003, ARM (the company) introduced something called Thumb-2, and well, the water was muddied somewhat, so it’s worth a look back to see why there are now effectively three different instruction sets for ARM processors and in particular, which processors support any given instruction set.