Blue Fox

CHAPTER 8Control Flow

Instructions are executed sequentially. But what happens when a program uses a condition statement or calls a subroutine? The first section of this chapter discusses how branch instructions alter the flow of execution. As you will learn in the second part of this chapter, these instructions are primarily used to invoke functions and subroutines, which will be discussed in more detail.

Branch Instructions

Branch instructions change the flow of execution by updating the program counter to the target address specified in the branch instruction. In assembly, this target address can be specified either as a label or as a register holding an address. At the binary level, labels are encoded into instructions as an immediate offset that is added to the PC when the instruction is executed. Together, these branch instructions can be used to encode conditional logic, loops, and calls to subroutines.

Table 8.1 contains an overview of branch instructions that branch to a specified label.

Table 8.1: Immediate Branches

STATE	INSTRUCTION	SYNTAX
AArch64	Unconditional branch	B <label>
Conditional branch	B.<cond> <label>
Branch with link	BL <label>
Compare and branch if nonzero	CBNZ Wt|Xt, <label>
Compare and branch if zero	CBZ Wt|Xt, <label>
Test bit and branch nonzero	TBNZ Wt|Xt, #imm, <label>
Test bit and branch zero	TBZ Wt|Xt, #imm, <label>
AArch32	Unconditional branch	B <label>
Branch unconditionally	B<cond> <label>
Branch ...