• State Accepted Answer
  • +1 person also asked this people also asked this
  • Locked Locked
  • Replies 3 replies
  • Subscribers 350 subscribers
  • Views 4911 views
  • Users 0 members are here
2025 survey

We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.

Options
Related
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Cortex-M0 Thumb-2 instruction: Is this instruction valid?

Héctor

STM     r0!, {}

I have looked at Thumb2 instruction set web but I can't find the behaviour of STM command if the reglist is empty.

Thanks in advance.

  • 0

    Looking at the spec it says for encoding T2

    if n == 15 || BitCount(registers) < 2 then UNPREDICTABLE;

    So the effect if you have even only one register in the list never mind zero is not defined. I must admit to be a bit surprised when I saw this, for ARM it is undefined only for zero registers, same for encoding T1.

  • 0 in reply to daith

    Just noticed you are talking about the M0 which has the ARMv6-M instruction set not ARMv7-M. It excludes most of the 32 bit Thumb-2 instructions including the T2 form of LDMIA.

  • +1

    Hi hector.bravo,

    it is not valid and any compiler probably cannot deal with the instruction. As Mr. daith says, STMDB/LDMDB register list requires more than one register. However, a compiler can deal with the register list which has one register. In the case, STM would be replaced with STR.

    stmdb r0!,{r1} =>  str.w   r1, [r0, #-4]!

    I guess some disassembler might show "stm r0!,{}" for the code "0xc000".

    Best regards,
    Yasuhiko Koumoto.