112 lines
5.1 KiB
Markdown
112 lines
5.1 KiB
Markdown
# Architecture
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## Pipeline
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```
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Source text → Tokenizer (CRUNCH) → Token stream
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↓
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Expression evaluator (FRMEVL)
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↓
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Statement dispatcher (NEWSTT)
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↓
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TUI screen buffer (interactive)
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↓
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HAL (platform I/O)
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```
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The interpreter mirrors the original GW-BASIC's internal pipeline, which — like
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most Microsoft interpreters of the era — is a tight loop around three core
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routines. CRUNCH tokenizes source lines into a compact byte stream. NEWSTT
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dispatches each statement. FRMEVL evaluates expressions. All platform I/O
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goes through a HAL vtable (`hal_ops_t`), so the core interpreter has no idea
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whether it's talking to an ANSI terminal or a teletype from 1975.
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In interactive mode, the TUI layer swaps in its own HAL function pointers and
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redirects all output through a dynamically allocated screen buffer, displayed
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via ANSI escape sequences. In piped mode the TUI stays out of the way and the
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HAL writes straight to stdout.
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## Module Map
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| Module | Source | Original Assembly |
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|--------|--------|--------------------|
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| Tokenizer (CRUNCH/LIST) | `tokenizer.c` | GWMAIN.ASM |
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| Expression evaluator | `eval.c` | GWEVAL.ASM |
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| Execution loop + control flow | `interp.c` | BINTRP.ASM |
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| TUI screen editor | `tui.c` | — |
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| Graphics engine | `graphics.c` | — |
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| Token/keyword tables | `tokens.c`, `tokens.h` | IBMRES.ASM |
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| Error handling | `error.c` | GWDATA.ASM |
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| Integer arithmetic | `math_int.c` | MATH1.ASM |
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| Float ops + MBF conversion | `math_float.c` | MATH2.ASM |
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| Transcendentals | `math_transcend.c` | MATH1.ASM |
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| String functions | `strings.c` | BISTRS.ASM |
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| PRINT / LPRINT | `print.c` | BINTRP.ASM |
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| PRINT USING | `print_using.c` | BIPRTU.ASM |
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| Variables + arrays | `vars.c`, `arrays.c` | GWMAIN.ASM |
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| File I/O + random access | `fileio.c` | BIPTRG.ASM |
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| Program I/O (SAVE/LOAD) | `program_io.c` | BIMISC.ASM |
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| INPUT/LINE INPUT | `input.c` | BINTRP.ASM |
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| Sound engine | `sound.c` | — |
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| Virtual memory (PEEK/POKE) | `virmem.c` | — |
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| Hardware I/O ports | `portio.c` | — |
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| String space pool + GC | `strpool.c` | GWEVAL.ASM (GETSPA/GARBAG) |
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| AOT compiler analysis | `analysis.c` | — |
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| AOT compiler codegen | `codegen.c` | — |
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| Compiled program runtime | `gwrt.c` | — |
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| Platform abstraction | `hal_posix.c` | OEM*.ASM |
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## Source Layout
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```
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src/ — core interpreter (23 files)
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include/ — headers (15 files)
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platform/ — HAL backends (1 file)
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gwbasickernel/ — Jupyter notebook kernel (Python, 6 files)
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tests/ — 72 automated test programs, 4 classic interactive programs, compat harness
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docs/ — Sphinx documentation
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```
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## TUI Architecture
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The TUI (`tui.c`) implements the classic GW-BASIC full-screen editor:
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- **Screen buffer** — `tui_cell_t *screen` is dynamically allocated at
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`rows × cols` (default 25×80, or full terminal size with `--full`).
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Each cell stores a character and color attribute, accessed via `TUI_CELL(r, c)`.
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- **HAL interception** — `tui_init()` swaps HAL function pointers so all
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existing PRINT/LIST/error output automatically goes through the screen buffer.
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No changes needed to `print.c`, `error.c`, or most of `interp.c`.
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- **Line editor** — `tui_read_line()` implements the defining GW-BASIC UX:
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free cursor movement with arrow keys, and pressing Enter on any screen line
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re-enters that line's content as BASIC input.
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- **Function keys** — F1-F10 with default GW-BASIC bindings, configurable via
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the `KEY n, "string"` statement. `KEY ON` shows the bar on the bottom row.
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- **Break handling** — SIGINT sets a flag checked each statement in the run loop.
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## Design Decisions
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### Relation to Original Assembly
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Microsoft [released the original GW-BASIC source](https://github.com/microsoft/GW-BASIC)
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in 2020 — 43,771 lines of 8088 assembly spread across 43 `.ASM` files, complete
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with Greg Whitten's comments and Neil Konzen's transcendental math routines
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(which are, frankly, impressive for 16-bit fixed-point). This reimplementation
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uses that assembly as a reference, not as input to a transliterator — the
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algorithms are reimplemented in idiomatic C with modern data structures.
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### Key Differences from the Original
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- **IEEE 754 floating point** — MBF (Microsoft Binary Format) conversion is used
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at the binary save/load boundary and for file I/O (CVI/CVS/CVD, MKI$/MKS$/MKD$),
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matching the original's on-disk format
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- **Dynamic memory allocation** — `malloc`/`free` instead of a 64KB segment layout
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- **String space pool** — 32KB contiguous pool with compacting GC at statement
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boundaries, matching the original's GETSPA/GARBAG approach
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- **`setjmp`/`longjmp`** — for error recovery, matching the original's stack reset
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behavior
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- **ANSI terminal** — TUI uses ANSI escape sequences and alternate screen buffer
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instead of direct CGA memory access
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- **Dynamic screen buffer** — allocated at runtime based on terminal size, rather
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than hardcoded to 25×80
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