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2.2. Thumb-2 instruction set

Thumb-2 is an enhancement to the 16-bit Thumb instruction set. It adds 32-bit instructions that can be freely intermixed with 16-bit instructions in a program. The additional 32-bit instructions enable Thumb-2 to cover the functionality of the ARM instruction set. The 32-bit instructions enable Thumb-2 to combine the code density of earlier versions of Thumb, with performance of the ARM instruction.

The most important difference between the Thumb-2 instruction set and the ARM instruction set is that most 32-bit Thumb instructions are unconditional, whereas most ARM instructions can be conditional. Thumb-2 introduces a conditional execution instruction, IT, that is a logical if-then-else function that you can apply to following instructions to make them conditional.

Thumb-2 instructions are accessible as were Thumb instructions when the processor is in Thumb state, that is, the T bit in the CPSR is 1 and the J bit in the CPSR is 0.

In addition to the 32-bit Thumb instructions, there are several 16-bit Thumb instructions and a few 32-bit ARM instructions, introduced as part of the Thumb-2 architecture.

The main enhancements are:

  • 32-bit instructions added to the Thumb instruction set to:

    • provide support for exception handling in Thumb state

    • provide access to coprocessors

    • include Digital Signal Processing (DSP) and media instructions

    • improve performance in cases where a single 16-bit instruction restricts functions available to the compiler.

  • addition of a 16-bit IT instruction that enables one to four following Thumb instructions, the IT block, to be conditional

  • addition of a 16-bit Compare with Zero and Branch (CZB) instruction to improve code density by replacing two-instruction sequence with a single instruction.

The 32-bit ARM Thumb-2 instructions are added in the space occupied by the Thumb BL and BLX instructions. Figure 2.1 shows the 32-bit ARM Thumb-2 instruction format.

Figure 2.1. 32-bit ARM Thumb-2 instruction format

Figure 2.1. 32-bit ARM Thumb-2 instruction format

The first halfword (hw1) determines the instruction length and functionality. If the processor decodes the instruction as 32-bit long, then the processor fetches the second halfword (hw2) of the instruction from the instruction address plus two.

The availability of both 16-bit Thumb and 32-bit instructions in the Thumb-2 instruction sets, gives you the flexibility to emphasize performance or code size on a subroutine level, according to the requirements of their applications. For example, you can code critical loops for applications such as fast interrupts and DSP algorithms using the 32-bit media instructions in Thumb-2 and use the smaller 16-bit classic Thumb instructions for the rest of the application. This is for code density and does not require any mode change.

See the ARM Architecture Reference Manual for information on the ARM, Thumb, and Thumb-2 instruction set.