ARM VERSION 1.2 - Manual (Page 170)

ARM Instruction Reference
4-60 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
4.6.3 BLX
Branch with Link, and optionally exchange instruction set. This instruction has two
alternative forms:
• an unconditional branch with link to a program-relative address
• a conditional branch with link to an absolute address held in a register.
Syntax
BLX{cond} Rm
BLX label
where:
cond is an optional condition code (see Conditional execution on page 4-4).
Rm is an ARM register containing the address to branch to.
Bit 0 of Rm is not used as part of the address.
If bit 0 of Rm is set, the instruction sets the T flag in the CPSR, and the
code at the destination is interpreted as Thumb code.
If bit 0 of Rm is clear, bit 1 must not be set.
label is a program-relative expression. See Register-relative and
program-relative expressions on page 3-23 for more information.
Note
BLX label cannot be conditional. BLX label always causes a change to Thumb state.
Usage
The BLX instruction:
• copies the address of the next instruction into r14 (lr, the link register)
• causes a branch to label , or to the address held in Rm
• changes instruction set to Thumb if either:
— bit 0 of Rm is set
— the BLX label form is used.
The machine-level BLX label instruction cannot branch to an address outside
±32Mb of
the current instruction. When necessary, the ARM linker adds code to allow longer
branches (see The ARM linker chapter in ADS Linker and Utilities Guide). The added
code is called a veneer.