7. Branch instructions¶
These cause execution to jump to a target location usually specified by a label (see the label
assembler directive). Conditional branches and the it and ite instructions test
the Application Program Status Register (APSR) N (negative), Z (zero), C (carry) and V
(overflow) flags to determine whether the branch should be executed.
Most of the exposed assembler instructions (including move operations) set the flags but there are explicit comparison instructions to enable values to be tested.
Further detail on the meaning of the condition flags is provided in the section describing comparison functions.
7.1. Document conventions¶
Notation: Rm denotes ARM registers R0-R15. LABEL denotes a label defined with the
label() assembler directive. <condition> indicates one of the following condition
specifiers:
eq Equal to (result was zero)
ne Not equal
cs Carry set
cc Carry clear
mi Minus (negative)
pl Plus (positive)
vs Overflow set
vc Overflow clear
hi > (unsigned comparison)
ls <= (unsigned comparison)
ge >= (signed comparison)
lt < (signed comparison)
gt > (signed comparison)
le <= (signed comparison)
7.2. Branch to label¶
b(LABEL) Unconditional branch
beq(LABEL) branch if equal
bne(LABEL) branch if not equal
bge(LABEL) branch if greater than or equal
bgt(LABEL) branch if greater than
blt(LABEL) branch if less than (<) (signed)
ble(LABEL) branch if less than or equal to (<=) (signed)
bcs(LABEL) branch if carry flag is set
bcc(LABEL) branch if carry flag is clear
bmi(LABEL) branch if negative
bpl(LABEL) branch if positive
bvs(LABEL) branch if overflow flag set
bvc(LABEL) branch if overflow flag is clear
bhi(LABEL) branch if higher (unsigned)
bls(LABEL) branch if lower or equal (unsigned)
7.3. Long branches¶
The code produced by the branch instructions listed above uses a fixed bit width to specify the branch destination, which is PC relative. Consequently in long programs where the branch instruction is remote from its destination the assembler will produce a “branch not in range” error. This can be overcome with the “wide” variants such as
beq_w(LABEL) long branch if equal
Wide branches use 4 bytes to encode the instruction (compared with 2 bytes for standard branch instructions).
7.4. Subroutines (functions)¶
When entering a subroutine the processor stores the return address in register r14, also known as the link register (lr). Return to the instruction after the subroutine call is performed by updating the program counter (r15 or pc) from the link register, This process is handled by the following instructions.
bl(LABEL)
Transfer execution to the instruction after LABEL storing the return address in
the link register (r14).
bx(Rm) Branch to address specified by Rm.
Typically bx(lr) is issued to return from a subroutine. For nested subroutines the
link register of outer scopes must be saved (usually on the stack) before performing
inner subroutine calls.