The Instruction Set In Cortex-M3

The Cortex-M3 supports the Thumb-2 instruction set. This is one of the most important features of the Cortex-M3 processor because it allows 32-bit instructions and 16-bit instructions to be used together for high code density and high efficiency. It is flexible and powerful yet easy to use.

In previous ARM processors, the central processing unit (CPU) had two operation states: a 32-bit ARM state and a 16-bit Thumb state. In the ARM state, the instructions are 32 bits and can execute all supported instructions with very high performance. In the Thumb state, the instructions are 16 bits, so there is a much higher instruction code density, but the Thumb state does not have all the functionality of ARM instructions and may require more instructions to complete certain types of operations.

To get the best of both worlds, many applications have mixed ARM and Thumb codes. However, the mixed-code arrangement does not always work best. There is overhead (in terms of both execution time and instruction space, see Figure 2.7) to switch between the states, and ARM and Thumb codes might need to be compiled separately in different files. This increases the complexity of software develop- ment and reduces maximum efficiency of the CPU core.

With the introduction of the Thumb-2 instruction set, it is now possible to handle all process- ing requirements in one operation state. There is no need to switch between the two. In fact, the Cortex-M3 does not support the ARM code. Even interrupts are now handled with the Thumb state. (Previously, the ARM core entered interrupt handlers in the ARM state.) Since there is no need to switch between states, the Cortex-M3 processor has a number of advantages over traditional ARM processors, such as:

• No state switching overhead, saving both execution time and instruction space

• No need to separate ARM code and Thumb code source files, making software development and

  maintenance easier

• It’s easier to get the best efficiency and performance, in turn making it easier to write software, because there is no need to worry about switching code between ARM and Thumb to try to get the best density/performance

 

Switching between ARM Code and Thumb Code in Traditional ARM Processors Such as the ARM7.

The Cortex-M3 processor has a number of interesting and powerful instructions. Here are a few examples:

• UFBX, BFI, and BFC: Bit field extract, insert, and clear instructions

• UDIV and SDIV: Unsigned and signed divide instructions

• WFE, WFI, and SEV: Wait-For-Event, Wait-For-Interrupts, and Send-Event; these allow the

  processor to enter sleep mode and to handle task synchronization on multiprocessor systems

• MSR and MRS: Move to special register from general-purpose register and move special register togeneral-purpose register; for access to the special registers

Since the Cortex-M3 processor supports the Thumb-2 instruction set only, existing program code for ARM needs to be ported to the new architecture. Most C applications simply need to be recompiled using new compilers that support the Cortex-M3. Some assembler codes need modification and porting to use the new architecture and the new unified assembler framework.

Note that not all the instructions in the Thumb-2 instruction set are implemented on the Cortex-M3. The ARMv7-M Architecture Application Level Reference Manual [Ref. 2] only requires a subset of the Thumb-2 instructions to be implemented. For example, coprocessor instructions are not supported on the Cortex-M3 (external data processing engines can be added), and Single Instruction–Multiple Data (SIMD) is not implemented on the Cortex-M3. In addition, a few Thumb instructions are not supported, such as Branch with Link and Exchange (BLX) with immediate (used to switch processor state from Thumb to ARM), a couple of change process state (CPS) instructions, and the SETEND (Set Endian) instructions, which were introduced in architecture v6. For a complete list of supported instructions, refer to Appendix A.

摘抄自：《The definitive guide to the ARM Cortex-M3》Second Edition.