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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.