5.8 · Assembler labels

A label table turns names into instruction positions.

An assembler can scan controller text, count only executable instructions, and replace symbolic branch targets with positions in the assembled sequence.

Guiding question

How can a controller use readable labels before the machine needs numeric positions?

Distinguish label symbols from instruction lists
Count instruction positions without counting labels
Build a table from each label to its next instruction
Resolve branch and goto targets while removing labels

extract-labels carries the position of the next instruction. Encountering a symbol records that position without incrementing it; encountering an instruction increments the position. start therefore names 0, loop names 1, and done names 5.

assemble skips label symbols and asks resolve to replace only branch and goto targets. Ordinary instructions remain unchanged. The returned sequence is assembled controller data with numeric targets; this lesson does not execute that sequence or prove a complete machine assembler.

Lispex · SICP sourceScheme-compatible SICP syntax executed by the Lispex SICP profile.

(begin
  (define (extract-labels controller position)
    (cond ((null? controller) '())
          ((symbol? (car controller))
           (cons (cons (car controller) position)
                 (extract-labels (cdr controller) position)))
          (else
           (extract-labels (cdr controller) (+ position 1)))))
  (define controller
    '(start
       (assign n 3)
      loop
       (test zero?)
       (branch done)
       (assign n sub1)
       (goto loop)
      done
       (halt)))
  (extract-labels controller 0))

Lispex learning runtimeLispex SICP profile 1.0.0

Loading Lispex SICP runtime

Lispex · SICP source524 / 1,048,576 UTF-8 bytes

Examples

Result—

Output: —
Value: —
Diagnostic: —

Visible execution0 / 0 trace events

This browser result is not a Lispex Vouch record or authority.wasm —

Expected observation

The first program returns ((start . 0) (loop . 1) (done . 5)). The second returns ((assign n 3) (test zero?) (branch 5) (assign n sub1) (goto 1) (halt)).

Trace focus

Watch position advance only for instruction lists, then locate assoc lookups for done and loop before branch and goto receive 5 and 1. The resulting list is assembled data, not an executed machine run.

Try it yourself

Change the program before you read the hint.

Insert a label named check immediately before (test zero?). Predict its table entry and whether any existing instruction position changes.

Show one hint

A label names the next instruction but does not occupy an instruction position.