v-TP3-STRCAT — Session summary
==============================

Tag:    v-TP3-STRCAT
Commit: (the commit that adds this file)
Covers: v-TP3-STRCAT

This is the resume point for the session that completed "Next work" item 1c —
string concatenation `+` — in full, with the test harness extensions that the
change forced. Every claim below is pinned by fixtures in
`shell/tests/fixtures/expected.tsv` and by the gate suite.

What changed
------------

1. String concatenation — `a + b`, `writeln(a + b)`, `s := a + b`
   (`v-TP3-STRCAT`).

   TP3's `xconcat` (TPSRC2:963) builds the result in a scratch string, copies the
   left operand into it, then appends the right (`addstr`/`addch`). The compiler
   now does the same:

   - `StrConcat` allocates a fresh 256-byte global scratch STRING per `+` in
     the source (`NewScratchStr`: `@@SC` of length 256, created at `dc`; `dc +=
     256`). The scratch is a `KVar` symbol of class `TString` (size 256).
   - The result `ERes` becomes a **string variable** (kind 1), pointing at that
     scratch (`res.idx := dest`). This matches how `Assignment`, `IoCall`,
     `ParseCmp` already handle string operands; no new `ERes` kind was added.
   - For the left operand: `EmPushStrOperand (left)` → for `TString` calls
     `EmPushStrAddr`; for a CHAR (one-character literal, `char` variable, or
     `s[i]` element) it builds a two-byte `<len><char>` on the stack in a
     per-operand 2-byte scratch slot (`dc += 2`) and pushes its address. This is
     how TP3 allows `s + 'a'` to be legal while `s = 'a'` is refused.
   - Emit: `strstore(dest, left, 255)` (existing selector 17) copies left into
     the scratch. Then `strcat(dest, right, 255)` (new selector 19) appends
     right. Both share the same argument order as `strstore` (dest at `[BP+4]`,
     src at `[BP+6]`, CX = maximum).
   - Overflow: when the new length > 255, `strcat` branches to `concerr` and
     leaves through `INT 21h AH=4Ch` with `AL=16` (TP3's "string too long").
     The exit code is now asserted by the execution harness via a per-fixture
     `.exit` sidecar (default 0).

2. The type rule for string `+`.

   The hook at the **head of `BinOpEmit`** was originally:
   ```
   IF (left.cls = TString) OR (right.cls = TString) THEN
      StrConcat (...)
   END
   ```
   That sent `'a' + 'b'` down the integer path (both are `TScalar`+`chr`,
   `cls != TString`) producing `CHR(195)` — a silently wrong program, not an
   error. It now tests the same `StrOperand` predicate that `StrConcat` guards
   with:
   ```
   IF (StrOperand (left) OR StrOperand (right)) THEN
      StrConcat (...)
   END
   ```
   where `StrOperand(r) := (r.cls = TString) OR (r.cls = TChar) OR
   ((r.cls = TScalar) AND r.chr)`. This is a **forward-declared** function next
   to `StrConcat` (added at line 1916). With this hook, two CHARs, or a string
   plus a number, reach `StrConcat` and are rejected by its guard as
   `ETypeErr 56`. `t53_strcat_type` (`'a' + 'b'`) pins the refusal; it was
   **red before the hook change** (ran through integer path) and is green after.

   `StrConcat`'s error path now leaves a **well-formed non-string placeholder**
   (`res.kind = 0`, `res.cls = TScalar`, `res.imm = 0`) rather than a half-built
   kind-2/TString with an uninitialised `idx`. That is the invariant: if Err()
   was called (and abortFac set), the returned `ERes` must not claim to be a
   string, because string consumers dereference `symtab[res.idx]` for any
   `TString`.

3. Guard against using failed expression results.

   The compiler's idiom is: after `ParseExpr`, check `IF NOT OK () THEN RETURN
   END` before dereferencing any symbol from the returned `ERes`. That guard was
   missing in `IoCall`'s argument loop: when a failed concat returned (kind-2/
   TString, idx garbage) the write arm did
   `EmPushVarAddr (symtab [arg.idx].local, symtab [arg.idx].off)`. With the
   hook now routing error cases into `StrConcat`, `writeln ('a' + 'b')` became a
   compiler segfault. Adding the `OK ()` guard after `ParseExpr (arg)` in
   `IoCall` (mirroring `ParseSub`) stops consuming a failed argument and lets the
   statement loop terminate via `abortFac`. With the safe placeholder in
   `StrConcat`, nested cases such as `('a' + 'b') + s` also report the first
   error cleanly.

4. Runtime: `strcat` (selector 19).

   `Runtime.mod` gains `EmitStrCat` which appends the source string to the
   destination. The implementation preserves **AX,BX,CX,DX,SI,DI** (pushes all
   six) because the compiler may hold a string element address in `DI` across the
   call (e.g. `s := s + s[i]` in effect, or expressions that push next operands
   after a string operation). The path is:
   - check `CX` (max) <= 255
   - load `lenD` from `[BP+4]` (destination length byte), `lenS` from `[BP+6]`
   - `newLen = lenD + lenS`; if `newLen > CX` → `concerr` (exit 16)
   - else copy `lenS` chars from `src[1]..` to `dst[1 + lenD]..`, set `dst[0] =
     newLen`, return
   - `concerr`: `MOV AL,16H; MOV AH,4CH; INT 21H` (6 bytes)

   The runtime's entry table grows to 20 entries; `RT_Entry`/`RT_Build` register
   entry 19 as `strcat`. `RtProbe.mod` extends the name table to index 19.
   `check_runtime.py` and `runtime.golden` were reblessed accordingly.

5. Safety guards: HALT on full label/fixup tables.

   `Runtime.mod`'s `MaxLbl` was 64 → **96**, `MaxFix` 400 → **512**. The
   procedures `M` (add a label) and `AddFix` (add a fixup) now call `HALT` when
   full instead of silently dropping, so a blown table produces a clean abort
   rather than a subtly wrong runtime blob.

6. Boot sector output: use SI as cursor.

   `bootcom.s`'s `INT 21h AH=09h` handler kept its cursor at `0702h` (the first
   byte of the loaded program image at `0602h` in the load window), so writing a
   CRLF overwrote the start of the loaded code (turning `E8 1A` into a wild
   jump). The handler now uses `SI` as its cursor and only writes to the video
   buffer via ES/DI; this removes the code-window limit and fixes the hang seen
   during bring-up.

7. Execution harness: per-fixture exit codes.

   `tests/run_com_exec.py` now reads a `.exit` sidecar (decimal integer, default
   0) per fixture (`stem + ".exit"`) via `cases()`, and asserts
   `exit_code_of(rc) == c["exit"]` after the output comparison. The module
   docstring documents the convention. This is how `t55_strcat_overflow` asserts
   exit code 16 while still checking its partial output (`read\r\n`).

Tests
-----

New fixtures:
- `t51_strcat.pas` — all operand pairings (var+var, literal+var, var+literal,
  var+char, char+var, nested concatenation in arguments/assignment), covering
  multiple consumers (`writeln`, assignment, comparison context). Output committed
  byte-for-byte.
- `t52_strcat_nest.pas` — nested `a + b + '!'`, concatenation compared to a
  string, and a string **element** as a `+` operand. Output committed.
- `t53_strcat_type.pas` — error: `'a' + 'b'` (two CHARs). Expected `ERR 56 663`.
- `t54_strcat_strnum.pas` — error: `s + 1` (string + number). Expected `ERR 56
  463`.
- `t55_strcat_overflow.pas` — `readln` 128 characters, then `s := s + s` (length
  becomes 256, >255) → runtime exit 16. Input 128×`x`+LF in `.in`, output
  `read\r\n` in `.out`, `.exit` `16`.

`expected.tsv` rows for t51–t55 with their verdicts, code/data bytes, and error
positions (the `pos` is the 0-based offset of the lookahead token when `Err`
fires).

Gates: `run_compile_tests.sh` 56/56; `run_com_tests.sh` 48/48; `run_com_exec.py`
48/48 (t55 has exit 16 asserted); `rt_exec.py` 56/56; `check_runtime.py` PASS;
`check_8086.py` PASS; `nonvacuity.sh` 78 ok/0 failed; `run_all.sh` OVERALL ALL
PASS.

How to verify
-------------

    cd shell && make                               # tpshell rebuilt, clean
    cd shell && tests/run_compile_tests.sh         # 56 passed, 0 failed
    cd shell && tests/run_com_exec.py              # 48 passed, 0 failed
    cd shell && tests/rt_exec.py                   # 56 passed, 0 failed
    cd shell && tests/check_runtime.py             # PASS
    cd shell && tests/check_8086.py                # PASS
    cd shell && tests/nonvacuity.sh                # 78 ok, 0 failed
    cd shell && tests/run_all.sh                   # OVERALL: ALL PASS

At this tag the suite reports:

    compile matrix: 56 passed, 0 failed (of 56)
    independent .COM check: 48 checked, 0 failed
    execution: 48 passed, 0 failed (of 48) (t55 asserts exit 16 via .exit)
    exec86:    48 passed, 0 failed (of 48), cross-checked against qemu
    runtime entries: 56 passed, 0 failed (of 56) (55 cases + BP pre-flight)
    runtime: 770 bytes, code 0..738 (739), 20 entries, 65 branches checked
    check_8086: runtime 770/420, program code 48/48 (9692 bytes), clause H 11/11
    nonvacuity: 78 ok, 0 failed
    OVERALL: ALL PASS

Notes
-----

- The `-fgen-module-list ... || true` link step prints a roll-up error from
  `Compiler.mod` during the final link of `tpshell`; this is the expected pass-3
  summary and does **not** indicate a compilation failure (`Compiler.o` was
  built successfully).
- `compiletest` is rebuilt when its inputs are newer than the binary (the harness
  checks staleness). After modifying `Compiler.mod`, `run_compile_tests.sh` will
  rebuild `compiletest`.
- The commit message for the code+test changes should name: the `bootcom.s`
  AH=09h cursor fix (use SI), the `Runtime.mod` `MaxLbl`/`MaxFix` HALT guards
  and the table increases, the `BinOpEmit` hook change to use `StrOperand`
  (fixing the `'a' + 'b'` silent wrong program), the `IoCall` `OK()` guard and
  the `StrConcat` safe-placeholder fix (eliminating the compiler segfault), the
  new runtime entry `strcat` (selector 19) and register preservation, the
  `run_com_exec.py` `.exit` sidecar convention, and the new fixtures t51–t55.