Differences between ARM instructions and Thumb instructions

       Thumb instructions can be seen as a subset of ARM instructions in compressed form. They are proposed to address the problem of code density1], and have a code density of 16 bits. Thumb is not a complete architecture, and you cannot expect the handler to only execute Thumb instructions without supporting the ARM instruction set. Therefore, Thumb instructions only need to support general functions, and when necessary, they can use the complete ARM instruction set, for example: all exceptions automatically enter the ARM state.

       When writing Thumb instructions, you must first use the pseudo-instruction CODE16 declaration, and in the ARM instruction, you must use the BX instruction to jump to the Thumb instruction to switch the processor state. When writing ARM instructions, you can use the pseudo-instruction CODE32 declaration.

1]. Code density: the number of instructions contained in a unit of storage space. For example

              ARM instructions are 32 bits, while Thumb instructions are 16 bits. If 32 ARM instructions can be placed in 1K of storage space, 64 Thumb instructions can be placed. Therefore, when storing Thunb instructions, the code density is high.

The difference between Thumb instruction set and ARM instruction set

       The Thumb instruction set does not have coprocessor instructions, semaphore instructions, instructions for accessing CPSR or SPSR, multiplication and addition instructions, 64-bit multiplication instructions, etc., and the second operand of the instruction is restricted; except for the conditional execution function of jump instruction B, all other instructions are unconditionally executed; most Thumb data processing instructions use a 2-address format. The differences between the Thumb instruction set and the ARM instruction set are generally as follows:

Ø Jump instruction

Program relative transfers, especially conditional jumps, are more restricted in scope than jumps under ARM code. Jumping to a subroutine is an unconditional transfer.

Ø Data processing instructions

Data processing instructions operate on general registers, and in most cases the result of the operation must be placed in one of the operand registers rather than a third register.

Data processing operations are less frequent than in ARM state, and access to registers R8-R15 is subject to certain restrictions.

(Except for MOV and ADD instructions accessing registers R8-R15, other data processing instructions always update the ALU status flags in the CPSR)

Thumb data processing instructions that access registers R8-R15 cannot update the ALU status flags in the CPSR.

Ø Single register load and store instructions

In Thumb state, single register load and store instructions can only access registers R0-R7.

Ø Bulk register load and store instructions

The LDM and STM instructions can load or store any subset of the registers in the range R0-R7.