ARMASM: B, BL, BX, BLX, and BXJ

Branch, Branch with Link, Branch and exchange instruction set, Branch with Link and exchange instruction set, Branch and change to Jazelle state.

Syntax

op1{cond}{.W} label

op2{cond} Rm

where:

op1

is one of:

B: Branch.
BL: Branch with link.
BLX: Branch with link, and exchange instruction set.

op2

is one of:

BX: Branch and exchange instruction set.
BLX: Branch with link, and exchange instruction set.
BXJ: Branch, and change to Jazelle execution.

cond

is an optional condition code (see Conditional execution). cond is not available on all forms of this instruction, see Instruction availability and branch ranges.

.W

is an optional instruction width specifier to force the use of a 32-bit B instruction in Thumb-2. See B in Thumb-2 for details.

label

is a program-relative expression. See Register-relative and program-relative expressions for more information.

Rm

is a register containing an address to branch to.

Operation

All these instructions cause a branch to label, or to the address contained in Rm. In addition:

The BL and BLX instructions copy the address of the next instruction into lr (r14, the link register).
The BX and BLX instructions can change the processor state from ARM to Thumb, or from Thumb to ARM.

BLX label always changes the state.

BX Rm and BLX Rm derive the target state from bit[0] of Rm:
- if bit[0] of Rm is 0, the processor changes to, or remains in, ARM state
- if bit[0] of Rm is 1, the processor changes to, or remains in, Thumb state.
The BXJ instruction changes the processor state to Jazelle.

Instruction availability and branch ranges

Table 4.7 shows the instructions that are available in ARM and Thumb state. Instructions that are not shown in this table are not available. Notes in brackets show the first architecture version where the instruction is available.

Table 4.7. Branch instruction availability and range

Instruction	ARM		16-bit Thumb		32-bit Thumb
`B label`	±32MB	(All)	±2KB	(All T)	±16MB^[1]	(All T2)
`B{cond} label`	±32MB	(All)	252 to +258	(All T)	±1MB^a	(All T2)
`BL label`	±32MB	(All)	±4MB ^[2]	(All T)	±16MB	(All T2)
`BL{cond} label`	±32MB	(All)	-		-	-
`BX Rm`	Available	(4T, 5)	Available	(All T)	Use 16-bit	(All T2)
`BX{cond} Rm`	Available	(4T, 5)	-		-	-
`BLX label`	±32MB	(5)	±4MB ^[2]	(5T)	±16MB	(All T2 except ARMv7-M)
`BLX Rm`	Available	(5)	Available	(5T)	Use 16-bit	(All T2)
`BLX{cond} Rm`	Available	(5)	-		-	-
`BXJ Rm`	Available	(5J, 6)	-		Available	(All T2 except ARMv7-M)
`BXJ{cond} Rm`	Available	(5J, 6)	-		-	-
^[1]Use `.W` to instruct the assembler to use this 32-bit instruction. ^[2]This is an instruction pair.

Extending branch ranges

Machine-level B and BL instructions have restricted ranges from the address of the current instruction. However, you can use these instructions even if label is out of range. Often you do not know where the linker places label. When necessary, the linker adds code to enable longer branches. See Chapter 3 Using the Basic Linker Functionality in the Linker User Guide. The added code is called a veneer.

B in Thumb-2

You can use the .W width specifier to force B to generate a 32-bit instruction in Thumb-2 code.

B.W always generates a 32-bit instruction, even if the target could be reached using a 16-bit instruction.

For forward references, B without .W always generates a 16-bit instruction in Thumb code, even if that results in failure for a target that could be reached using a 32-bit Thumb instruction.

BX, BLX, and BXJ in Thumb-2EE

These instructions can be used as branches in Thumb-2EE code, but cannot be used to change state. You cannot use the op{cond} label form of these instructions in Thumb-2EE. In the register form, bit[0] of Rm must be 1, and execution continues at the target address in ThumbEE state.

Condition flags

These instructions do not change the flags.

Architectures

See Instruction availability and branch ranges for details of availability of these instructions in each architecture.

Examples

    B       loopA
    BLE     ng+8
    BL      subC
    BLLT    rtX
    BEQ     {pc}+4  ; #0x8004

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