9.4.5 Opcodes

as implements all the standard ARM opcodes. It also implements several pseudo opcodes, including several synthetic load instructions.

NOP

            nop

This pseudo op will always evaluate to a legal ARM instruction that does nothing. Currently it will evaluate to MOV r0, r0.

LDR

            ldr <register> , = <expression>

If expression evaluates to a numeric constant then a MOV or MVN instruction will be used in place of the LDR instruction, if the constant can be generated by either of these instructions. Otherwise the constant will be placed into the nearest literal pool (if it not already there) and a PC relative LDR instruction will be generated.

ADR

            adr <register> <label>

This instruction will load the address of label into the indicated register. The instruction will evaluate to a PC relative ADD or SUB instruction depending upon where the label is located. If the label is out of range, or if it is not defined in the same file (and section) as the ADR instruction, then an error will be generated. This instruction will not make use of the literal pool.

If label is a thumb function symbol, and thumb interworking has been enabled via the -mthumb-interwork option then the bottom bit of the value stored into register will be set. This allows the following sequence to work as expected:

            adr     r0, thumb_function
            blx     r0

ADRL

            adrl <register> <label>

This instruction will load the address of label into the indicated register. The instruction will evaluate to one or two PC relative ADD or SUB instructions depending upon where the label is located. If a second instruction is not needed a NOP instruction will be generated in its place, so that this instruction is always 8 bytes long.

If the label is out of range, or if it is not defined in the same file (and section) as the ADRL instruction, then an error will be generated. This instruction will not make use of the literal pool.

If label is a thumb function symbol, and thumb interworking has been enabled via the -mthumb-interwork option then the bottom bit of the value stored into register will be set.

For information on the ARM or Thumb instruction sets, see ARM Software Development Toolkit Reference Manual, Advanced RISC Machines Ltd.