A symbol or label is an identifier that starts with a letter and is followed
by letters and digits. Depending on some features enabled (see
other characters may be present. Use of identifiers consisting of a single
character will not work in all cases, because some of these identifiers are
reserved keywords (for example "A" is not a valid identifier for a label,
because it is the keyword for the accumulator).
The assembler allows you to use symbols instead of naked values to make the source more readable. There are a lot of different ways to define and use symbols and labels, giving a lot of flexibility.
Numeric constants are defined using the equal sign or the label assignment operator. After doing
two = 2
may use the symbol "two" in every place where a number is expected, and it is evaluated to the value 2 in this context. The label assignment operator causes the same, but causes the symbol to be marked as a label, which may cause a different handling in the debugger:
io := $d000
The right side can of course be an expression:
four = two * two
A label is defined by writing the name of the label at the start of the line (before any instruction mnemonic, macro or pseudo directive), followed by a colon. This will declare a symbol with the given name and the value of the current program counter.
.PROC directive, it is possible to
create regions of code where the names of labels and symbols are local to this
region. They are not known outside of this region and cannot be accessed from
there. Such regions may be nested like PROCEDUREs in Pascal.
See the description of the
directive for more information.
Cheap local labels are defined like standard labels, but the name of the
label must begin with a special symbol (usually '@', but this can be
changed by the
Cheap local labels are visible only between two non cheap labels. As soon as a
standard symbol is encountered (this may also be a local symbol if inside a
region defined with the
.PROC directive), the
cheap local symbol goes out of scope.
You may use cheap local labels as an easy way to reuse common label names like "Loop". Here is an example:
Clear: lda #$00 ; Global label ldy #$20 @Loop: sta Mem,y ; Local label dey bne @Loop ; Ok rts Sub: ... ; New global label bne @Loop ; ERROR: Unknown identifier!
If you really want to write messy code, there are also unnamed labels. These labels do not have a name (you guessed that already, didn't you?). A colon is used to mark the absence of the name.
Unnamed labels may be accessed by using the colon plus several minus or plus characters as a label designator. Using the '-' characters will create a back reference (use the n'th label backwards), using '+' will create a forward reference (use the n'th label in forward direction). An example will help to understand this:
: lda (ptr1),y ; #1 cmp (ptr2),y bne :+ ; -> #2 tax beq :+++ ; -> #4 iny bne :- ; -> #1 inc ptr1+1 inc ptr2+1 bne :- ; -> #1 : bcs :+ ; #2 -> #3 ldx #$FF rts : ldx #$01 ; #3 : rts ; #4
As you can see from the example, unnamed labels will make even short sections of code hard to understand, because you have to count labels to find branch targets (this is the reason why I for my part do prefer the "cheap" local labels). Nevertheless, unnamed labels are convenient in some situations, so it's your decision.
While there are drawbacks with this approach, it may be handy in some
.DEFINE, it is
possible to define symbols or constants that may be used elsewhere. Since
the macro facility works on a very low level, there is no scoping. On the
other side, you may also define string constants this way (this is not
possible with the other symbol types).
.DEFINE two 2 .DEFINE version "SOS V2.3" four = two * two ; Ok .byte version ; Ok .PROC ; Start local scope two = 3 ; Will give "2 = 3" - invalid! .ENDPROC
.DEBUGINFO is enabled (or
-g is given on the command line), global, local and
cheap local labels are written to the object file and will be available in the
symbol file via the linker. Unnamed labels are not written to the object file,
because they don't have a name which would allow to access them.