This is an example of code that declares and uses a local label legally:
Label1: CMP r1, #0 ; Compare r1 to zero.
BCS $1 ; If carry is set, branch to $1;
ADDS r0, r0, #1 ; else increment to r0
MOVCS pc, lr ; and return.
$1: LDR r2, [r5], #4 ; Load indirect of r5 into r2
; with write back.
.newblock ; Undefine $1 so it can be used
; again.
ADDS r1, r1, r2 ; Add r2 to r1.
BPL $1 ; If the negative bit isn't set,
; branch to $1;
MVNS r1, r1 ; else negate r1.
$1: MOV pc, lr ; Return.
The following code uses a local label illegally:
BCS $1 ; If carry is set, branch to $1;
ADDS r0, r0, #1 ; else increment to r0
MOVCS pc, lr ; and return.
$1: LDR r2, [r5], #4 ; Load indirect of r5 into r2
; with write-back.
ADDS r1, r1, r2 ; Add r2 to r1.
BPL $1 ; If the negative bit isn't set,
; branch to $1;
MVNS r1, r1 ; else negate r1.
$1: MOV pc, lr ; Return.
The $1 label is not undefined before being reused by the second branch instruction. Therefore, $1 is redefined, which is illegal.
Local labels are especially useful in macros. If a macro contains a normal label and is called more than once, the assembler issues a multiple-definition error. If you use a local label and .newblock within a macro, however, the local label is used and reset each time the macro is expanded.
Up to ten local labels of the $n form can be in effect at one time. Local labels of the form name? are not limited. After you undefine a local label, you can define it and use it again. Local labels do not appear in the object code symbol table.
For more information about using labels in macros see Section 6.6.