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Single instruction to mask off upper half word of register

Jawad Atiq
Note: This was originally posted on 3rd December 2012 at http://forums.arm.com

Hi,

I've been looking around for a single Arm instruction that takes the word stored in a general purpose register and masks off the upper half word (top 16 bits).

There are probably other ways of doing this, for example:
LDR R1 = 0xFFFF
AND R0, R0, R1

But is there a specific instruction that does this? I was looking at LDRH but this instruction does not accept a register as an operand, only constants.

Thanks!
Jawad
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  • Arm Employee Badge
    Note: This was originally posted on 3rd December 2012 at http://forums.arm.com

    One point of note would be the fact that the second operation in Sim's pre-ARMv6 code (the LSR) can often be packed into a register operand shift of another instruction, so if you wanted to extract the value and then add it to another value you would need one LSL instruciton, but the LSR can be packed in with the add operation. So it is only one cycle effective throughput.
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  • Arm Employee Badge
    Note: This was originally posted on 3rd December 2012 at http://forums.arm.com

    One point of note would be the fact that the second operation in Sim's pre-ARMv6 code (the LSR) can often be packed into a register operand shift of another instruction, so if you wanted to extract the value and then add it to another value you would need one LSL instruciton, but the LSR can be packed in with the add operation. So it is only one cycle effective throughput.
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