SLAA534A June 2013 – June 2020
The ELF relocations for MSP430 are defined such that the all information needed to perform the relocation is contained in the relocation entry, the object field, and the associated symbol. The linker does not need to decode instructions, beyond unpacking the object field, to perform the relocation. This results in slightly more relocation types than the older MSP430 COFF ABI. Relocation types are not compatible between COFF and ELF.
Relocations are specified as operating on a relocatable field. Roughly speaking, the relocatable field is the bits of the program image that are affected by the relocation. The field is defined in terms of an addressable container whose address is given by the r_offset field of the relocation entry. The field's size and position within to the container, as well as the computation of the relocated value, are specified by the relocation type. The relocation operation consists of extracting the relocatable field, performing the operation, and re-inserting the resultant value back into the field.
ELF relocations can be of type Elf32_Rela or Elf32_Rel. The Rela entries contain an explicit addend which is used in the relocation calculation. Entries of type Rel use the relocatable field itself as the addend. Certain relocations are identified as Rela only. For the most part these correspond to the upper 16 bits of a 32-bit address, where the resultant value depends on carry propagation from lower bits that are not available in the field. Where Rela is specified, an implementation must honor this requirement. An implementation may choose to use Rel or Rela type relocations for other relocations.
The effects of addressing modes on relocations is briefly described in Section 4.3.