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Program Counter update

Maya Maya
Note: This was originally posted on 15th June 2009 at http://forums.arm.com

In the Architectural Reference Manual for ARMv7 the PC update is described such that it gets update with adding 8 to the current PC while executing an ARM instruction and adding 4 for Thumb instruction. ARM instructions are 32-bit, hence I'd expect the PC to be updated by adding 4 to it rather than 8. Does anyone knows what is the reason behind this? Am I missing something or is it a mistake?

Thanks
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  • Arm Employee Badge
    Note: This was originally posted on 16th June 2009 at http://forums.arm.com

    Any Thumb instruction (whether 16-bit or 32-bit) will always see PC + 4; the hardware stitches up the difference.

    This is one of those areas where modern processor microarchitecture doesn't really match the original design assumptions, but you need some strict definition of what PC is at any point in time to be able to write software =)
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  • Arm Employee Badge
    Note: This was originally posted on 16th June 2009 at http://forums.arm.com

    Any Thumb instruction (whether 16-bit or 32-bit) will always see PC + 4; the hardware stitches up the difference.

    This is one of those areas where modern processor microarchitecture doesn't really match the original design assumptions, but you need some strict definition of what PC is at any point in time to be able to write software =)
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