Branch Instructions

Question 1

how do we implement control structures like for and while loops?

Answer 1

Branch instructions are used to alter control flow.

Question 2

• B – Branch
  
  • ?
• BL – Branch with Link
  
  • ?

Answer 2

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

Question 3

How do we return from the subroutine which BL invoked?

Answer 3

MOV pc, r14

or

BX r14 (on ARMv4T or later)

Question 4

What does the following code do?

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

Answer 4

Branching forward, to skip over some code:

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

Question 5

What does this code do?

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

Answer 5

Branching backwards, creating a loop:

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

Question 6

What does this code do?

... ... 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

Answer 6

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

Question 7

Are Branches PC-relative?

Answer 7

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

Question 8

ARM’s branch instructions are PC-______ the code produced is position ___________ — it can execute from any address in memory. Certain systems such as BREW use this.

Answer 8

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.

Question 9

How can we perform longer branches which access the full 32-bit address space?

Answer 9

You can set up the LR manually if needed, then load into PC:

MOV lr,pc LDR pc,=dest