36

Chapter 6. Symbols

definition of a specific local label for a forward reference. It is also worth noting that the first 10 local

labels (

0:

. . .

9:

) are implemented in a slightly more efficient manner than the others.

Here is an example:

1:

branch 1f

2:

branch 1b

1:

branch 2f

2:

branch 1b

Which is the equivalent of:

label_1:

branch label_3

label_2:

branch label_1

label_3:

branch label_4

label_4:

branch label_3

Local symbol names are only a notational device. They are immediately transformed into more con 

ventional symbol names before the assembler uses them. The symbol names stored in the symbol

table, appearing in error messages and optionally emitted to the object file. The names are constructed

using these parts:

L

All local labels begin with

L

. Normally both

as

and

ld

forget symbols that start with

L

. These

labels are used for symbols you are never intended to see. If you use the

 L

option then

as

retains

these symbols in the object file. If you also instruct

ld

to retain these symbols, you may use them

in debugging.

number

This is the number that was used in the local label definition. So if the label is written

55:

then

the number is

55

.

C B

This unusual character is included so you do not accidentally invent a symbol of the same name.

The character has ASCII value of

\002

(control B).

ordinal number

This is a serial number to keep the labels distinct. The first definition of

0:

gets the number

1

.

The 15th definition of

0:

gets the number

15

, and so on. Likewise the first definition of

1:

gets

the number

1

and its 15th defintion gets

15

as well.

So for example, the first

1:

is named

L1C B1

, the 44th

3:

is named

L3C B44

.

6.3.2. Dollar Local Labels

as

also supports an even more local form of local labels called dollar labels. These labels go out of

scope (ie they become undefined) as soon as a non local label is defined. Thus they remain valid for

only a small region of the input source code. Normal local labels, by contrast, remain in scope for the

entire file, or until they are redefined by another occurrence of the same local label.

Dollar labels are defined in exactly the same way as ordinary local labels, except that instead of being

terminated by a colon, they are terminated by a dollar sign. eg

55$

.