ARM VERSION 1.2 Access to the barrel shifter, Differences between Thumb and ARM instruction sets

Writing ARM and Thumb Assembly Language

Refer to Chapter 5 Thumb Instruction Reference for a complete list of the Thumb data processing instructions that can access the high registers.

Access to the barrel shifter

In Thumb state you can use the barrel shifter only in a separate operation, using an LSL, LSR, ASR, or ROR instruction.

2.2.9Differences between Thumb and ARM instruction sets

The general differences between the Thumb instruction set and the ARM instruction set are dealt with under the following headings:

•Branch instructions

•Data processing instructions

•Single register load and store instructions on page 2-11

•Multiple register load and store instructions on page 2-11.

There are no Thumb coprocessor instructions, no Thumb semaphore instructions, and no Thumb instructions to access the CPSR or SPSR.

Branch instructions

These instructions are used to:

•branch backwards to form loops

•branch forward in conditional structures

•branch to subroutines

•change the processor from Thumb state to ARM state.

Program-relative branches, particularly conditional branches, are more limited in range than in ARM code, and branches to subroutines can only be unconditional.

Data processing instructions

These operate on the general-purpose registers. In many cases, the result of the operation must be put in one of the operand registers, not in a third register. There are fewer data processing operations available than in ARM state. They have limited access to registers r8 to r15.

The ALU status flags in the CPSR are always updated by these instructions except when MOV or ADD instructions access registers r8 to r15. Thumb data processing instructions that access registers r8 to r15 cannot update the flags.

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