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Instruction and directive relocations
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| Home > Writing ARM Assembly Language > Instruction and directive relocations | |||
A relocation is a directive embedded in the object file that enables source code to refer to a label whose target address is unknown or cannot be calculated at assembly time. The assembler will emit a relocation in the object file, and the linker resolves this to the address where the target is placed.
The assembler relocates the data directives DCB, DCW, DCWU, DCD,
and DCDU if their syntax contains an external symbol,
that is a symbol declared using IMPORT or EXTERN.
This causes the bottom 8, 16, or 32 bits of the address to be used
at link-time.
The REQUIRE directive emits a relocation to signal
to the linker that the target label must be present if the current
section is present.
The assembler is permitted to emit a relocation for these instructions:
LDR(PC-relative)All ARM and Thumb instructions, except the Thumb doubleword instruction, can be relocated.
PLD,PLDW, andPLIAll ARM and Thumb instructions can be relocated.
B,BL, andBLXAll ARM and Thumb instructions can be relocated.
CBZandCBNZAll Thumb instructions can be relocated but this is discouraged because of the limited branch range of these instructions.
LDCandLDC2Only ARM instructions can be relocated.
VLDROnly ARM instructions can be relocated.
The assembler emits a relocation for the instructions listed above if the label used meets any of the following requirements, as appropriate for the instruction type:
the label is
WEAKthe label is not in the same
AREAthe label is external to the object (
IMPORTorEXTERN).
For B, BL, and BX instructions,
the assembler emits relocation also if:
the label is a function
the label is function exported using
EXPORTorGLOBAL.
Note
The RELOC directive can be used to control the
relocation at a finer level, but this needs knowledge of the ABI.
ExampleIMPORT sym ; sym is an external symbol DCW sym ; Because DCW only outputs 16 bits, only the lower 16 bits ; of the address of sym are inserted at link-time.
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| Non-Confidential | ID062912 |
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