ARM > Introduction to ARM > Branch Instructions

by David Thomas on —

Branch Instructions

So how do we implement control structures like for and while loops? Branch instructions are used to alter control flow.

<operation>{cond} <address>

  • B – Branch
    • PC := <address>
  • BL – Branch with Link
    • R14 := address of next instruction, PC := <address>

How do we return from the subroutine which BL invoked?

MOV pc, r14

or

BX r14 (on ARMv4T or later)

Examples of Branch Instructions

Branching forward, to skip over some code:

    ...            ; some code here
    B fwd          ; jump to label 'fwd'
    ...            ; more code here
fwd

Branching backwards, creating a loop:

back
    ...            ; more code here
    B back         ; jump to label 'back'

Using BL to call a subroutine:

    ...
    ...
    BL  calc       ; call 'calc'
    ...            ; returns to here
    ...

calc               ; function body
    ADD r0, r1, r2 ; do some work here
    MOV pc, r14    ; PC = R14 to return

Remarks

Branches are PC-relative. +/-32M range (24 bits × 4 bytes).

Since ARM’s branch instructions are PC-relative the code produced is position independent — it can execute from any address in memory. Certain systems such as BREW take advantage of this to avoid the need for an MMU.

How can we perform longer branches which access the full 32-bit address space? You can set up the LR manually if needed, then load into PC:

MOV lr,pc
LDR pc,=dest