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R0003-string-engine.md

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R0003 — String engine
Status ✅ Implemented (DWORD-wide copy under $CPU 80386, in-place append paths); free-lists and the two-digit formatter are ⬜ planned
Stage Runtime + emitter
Source EmitRepMovsbWidened, Runtime/DosRuntime — rt_strcatlit, rt_strcatvar, rt_strcatn
Gate --optimize; DWORD widening needs $CPU 80386
Verified by tests/diff/DIFF40.BAS (a 386 string storm), DIFF23.BAS (block moves)
Related O0009, O0024, C0001
Split into O0241, O0242

What it is

This page is the string subsystem's roadmap entry — the engine as a whole, and what it still lacks. The individual implemented widenings and in-place paths each have their own entry:

Implemented Entry
DWORD-wide string copy O0241
DWORD block copy for TYPE/LSET O0242
in-place append (literal / variable / chain) O0208, O0209, O0210
single-allocation multi-concat O0024

Sample

$CPU 80386
DIM a$, b$
a$ = STRING$(4000, "x")
b$ = a$ + a$

Without the optimizer

    mov     cx, 0FA0h
    rep     movsb            ; 4 000 byte moves

With the optimizer

    mov     cx, 03E8h
    rep     movsd            ; 1 000 dword moves + a <=3-byte tail

The copied bytes are identical — only the transfer width changes.

Equivalent BASIC

Unchanged.

What is still planned

  • Heap free-lists to avoid compaction storms: today a freed block is reclaimed by compaction, which is O(heap) at exactly the moment a program is allocating hardest.
  • Two-digit-table number formatting, which halves the divide-by-ten loop for PRINT-heavy code and pairs with O0056's reciprocal division.
  • Named access to the string-manager ABI from inline asm — see R0004.