Labels and Branching

Labels and Branching

Recall Labels

• Labels are used to give meaningful names to locations in memory
• Previously we saw this used to define variables:

Defines “someplace” as a label for a memory location holding a

• But we can also use labels to mark the location of instructions in our programs!

Instructions have addresses too!

• Recall that before a program is executed, it is loaded into memory
• Every instruction therefore occupies a word of memory, and has an address like any other data
• Recall also that this is a core principle of the Von Neumann architecture
• We can therefore use the address of specific instructions to jump from one location to another
• often referred to as branching

The Program Counter

• To understand labels and branching we need to understand how the CPU keeps track of which instruction to execute next
• It uses the special register, the Program Counter (PC) register

Program Counter

The Program Counter (cont)

• After each instruction is executed, the PC register is updated with the address of the next instruction to execute.
• This update is performed by a hardware counter within the ALU

Labels as branch points

To see how labels and branching work, consider this code:

MOV R0, #0

Loop:

ADD R0,R0,#1

B Loop

HALT

Labels as branch points

Consider this code:

MOV R0, #0

Loop: Label referring to the address of the next instruction

ADD R0,R0,#1

B Loop

HALT

Labels as branch points

Consider this code:

MOV R0, #0

Loop: Label referring to the address of the next instruction

ADD R0,R0,#1

B Loop A Branch instruction, telling the program counter that the HALT next instruction to execute is at address of label ‘Loop’

Labels as branch points

Consider this code:

MOV R0, #0

Loop: Label referring to the address of the next instruction

ADD R0,R0,#1

B Loop A Branch instruction, telling the program counter that the HALT next instruction to execute is at address of label ‘Loop’

Let’s load this into ARMliteand see it in action!

Unconditional Branch

B someplace

Branch to the address represented by the label someplace

Unconditional Branch

B someplace This can be any word aligned address, but is almost always given as a label, defined elsewhere in the program Branch to the address represented by the label someplace