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LA instruction in assembler


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janishyathi bonam

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Joined: 05 May 2020
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PostPosted: Tue May 05, 2020 8:17 am
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Hi!
I'm trying to LA R1,5000 and it says invalid displacement. Is there a limit to the value of the second operand? How can I achieve this?
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steve-myers

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Joined: 30 Nov 2013
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PostPosted: Tue May 05, 2020 9:52 am
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Yes, there is a limit: 4095. There are at least three ways to achieve your goal.
  • Two LA instructions, for example
    Code:
             LA    R1,2500
             LA    R1,2500(,R1)

    Dinosaurs like me would use this solution. The actual immediate values can be anything that adds to 5000; they do not have be 2500.
  • LHI instruction
    Code:
             LHI   R1,5000

    This solution uses just 4 bytes.
  • LGFI instruction
    Code:
             LGFI  1,5000

    This is hidden in Principles of Operation, but the sign of the immediate value is extended to bits 0 through 31 of the register. This solution uses 6 bytes.
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janishyathi bonam

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PostPosted: Tue May 05, 2020 9:56 am
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Thank you!!
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Ravi GA

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PostPosted: Tue May 05, 2020 10:47 am
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janishyathi bonam wrote:
Hi!
I'm trying to LA R1,5000 and it says invalid displacement. Is there a limit to the value of the second operand? How can I achieve this?


LAY R1,5000 is one more way to load values greater than 4095.
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steve-myers

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PostPosted: Tue May 05, 2020 11:43 am
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Ravi GA wrote:
... LAY R1,5000 is one more way to load values greater than 4095.
Yes, indeed. I had forgotten about LAY. I would suggest you read Principles of Operation very carefully before using it.
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