Introduction to ARM

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>

<operation>

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

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 use this.

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

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