Revision 1 of 1, by Kai Burghardt on 8-6-2022. The page as it is now.

Label

The label keyword is used for declaration of labels (markers for unconditional jumps using goto keyword) used further in the unit/program. It is a reserved word.

Pascal

To declare a label (not defining it) label starts a section with (comma separated) identifiers:

pascal

program labelDemo(input, output, stderr);

{$goto on}
// familiarize the compiler with symbols
// that are meant as 'goto' targets
label
    // the symbol 'done' becomes of "type" label
    done;

You then define the label, that means associating a symbol with a statement and effectively in the compiled code with an address, by writing the previously declared label in a statement block followed by a colon:

pascal

var
    n: qword;
    x, sum, arithmeticMean: real;
begin
    n := 0;
    sum := 0;
    while true do
    begin
        readLn(x);
        if x < 0 then
        begin
            // leave loop and continue at 'done'
            goto done;
        end;
        sum := sum + x;
        inc(n);
    end;
done:
    arithmeticMean := sum / n;
    writeLn('arithemetic mean = ', arithmeticMean);
end.

Pascal imposes further restrictions on labels: Labels have to be associated with statements: Putting a label right before an end is not allowed. To bypass that, you can insert an empty instruction ; right after the label definition.

In addition to normal identfiers, labels may be non-negative integers.

Assembler

A label section is also required for jump targets in asm-blocks, unless the @-address-operator is used.

pascal

program sumExample(input, output, stderr);

{ iteratively calculates the sum over first n integers }
function iterativeSumFirstNIntegers(const n: longword): qword;
{$ifdef CPUX86_64} // ============= optimized implementation
// assembler modifier appended to routine declaration
assembler;
// you have to familiarize the compiler with symbols
// which are meant to be jump targets
{$goto on}
label
    isfni_iterate;
{$asmMode intel}
asm// rax := 0// ecx is used as counter by loop instruction// ecx := n// rax := rax + ecx// dec(ecx)// if ecx <> 0 then goto isfni_iterate// the @result macro represents the functions return value// result := rax// note, a list of modified registers (here ['rax', 'ecx'])//       is ignored for pure assembler routines
    xor rax, rax // rax := 0
    // ecx is used as counter by loop instruction
    mov ecx, n   // ecx := n
isfni_iterate:
    add rax, qword(ecx) // rax := rax + ecx
    loop isfni_iterate  // dec(ecx)
    // if ecx <> 0 then goto isfni_iterate
    
    // the @result macro represents the functions return value
    mov @result, rax // result := rax
// note, a list of modified registers (here ['rax', 'ecx'])
//       is ignored for pure assembler routines
END;
{$else} // ========================== default implementation
var
    i: longword;
    x: qword;
begin
    x := 0; // mov rax, 0
    for i := n downto 1 do // mov ecx, n
    begin
        x := x + i; // add rax, ecx
    end; // loop isfni_iterate
    iterativeSumFirstNIntegers := x; // mov @result, rax
end;
{$endif}

// M A I N =================================================
var
    n: longword;
begin
    readLn(n);
    writeLn(iterativeSumFirstNIntegers(n));
end.

Of course in a production program, you would use an algorithm applying the formula sum := (n * (n + 1)) div 2 (“Gaussian sum formula”).

Label

Pascal

Assembler

See also