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ARM Vs GCC assembler

Gerald Murphy
Note: This was originally posted on 25th January 2013 at http://forums.arm.com

Hi,

I have a question regarding the ARM and GCC assemblers i.e. ARMASM and GAS.

MOV.W R4,#0x87C0 is translated to F248_74C0 by ARMASM.

However, GAS throws up the error "invalid constant (87c0) after fixup" for the same instruction. Why does GAS give an error for this?

I realise the operand is 16-bit but I'm using the '.syntax unified'. I thought GAS supports .W and .N suffixes. Even without the .W suffix, GAS gives the same error whereas ARMASM quietly expands the 16-bit instruction to 32-bit.

I'm using 'GCC version 4.7.2 (Sourcety CodeBench Lite 2012.09-63).

Any insight as to why GAS is giving an error is appreciated. The workaround is to use two instructions. The following 2 instructions compile OK.

MOVW  R4  ,#0xC0
MOVT R4  ,#0x87

Thanks and regards,
Ger
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  • Note: This was originally posted on 1st February 2013 at http://forums.arm.com

    Thanks for the reply Scott.

    Just one comment, the ARM V7 Thumb2 instruction set inlcudes a MOV.W encoding option (in addition to the MOVW encoding). ARMASM recognises MOV.W.

    According to the GNU assembler documentation provided by Sourcery CodeBenchLite 2012.09-63, use of the ".syntax unified" directive means that the .N and .W suffixes are recognised and honored.

    However, when using GAS assembler, the instruction "MOV.W R4,#0x87C0" gives "error: invalid constant (87c0) after fixup".  I've included the ".syntax unified" directive at the beginning of the source code.

    So the question is: does GAS support the .W suffix for MOV? It seems not but maybe I'm missing something?

    regards,
    Ger
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  • Note: This was originally posted on 1st February 2013 at http://forums.arm.com

    Thanks for the reply Scott.

    Just one comment, the ARM V7 Thumb2 instruction set inlcudes a MOV.W encoding option (in addition to the MOVW encoding). ARMASM recognises MOV.W.

    According to the GNU assembler documentation provided by Sourcery CodeBenchLite 2012.09-63, use of the ".syntax unified" directive means that the .N and .W suffixes are recognised and honored.

    However, when using GAS assembler, the instruction "MOV.W R4,#0x87C0" gives "error: invalid constant (87c0) after fixup".  I've included the ".syntax unified" directive at the beginning of the source code.

    So the question is: does GAS support the .W suffix for MOV? It seems not but maybe I'm missing something?

    regards,
    Ger
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