Eremey Valetov
981aeabc45
- Add CHANGES.TXT with full version history (DOS-friendly format) - Add DOS/FreeDOS section to README.md - Move compiler memory safety and DOS target to Completed in roadmap - Add hal_dos.c to architecture.md module map - Add .tab-color to .gitignore
143 lines
6.6 KiB
Markdown
143 lines
6.6 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`, `hal_dos.c` | OEM*.ASM |
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## Source Layout
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```
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src/ -- core interpreter + compiler (27 files)
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include/ -- headers (18 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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## Ahead-of-Time Compiler
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`gwbasic-compile` translates tokenized `.bas` programs to C source linked
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against `libgwrt.a`:
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```
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.bas → gw_crunch() → analysis pass → C codegen → gcc → native binary
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```
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The **analysis pass** (`analysis.c`) collects variables, GOTO/GOSUB targets,
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DATA literals, and DEFINT/DEFSNG/DEFDBL/DEFSTR type defaults. When `--warn`
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is active, it also tracks variable assignment vs. use context to detect
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uninitialized variables, GOTO to nonexistent lines, and unreachable code.
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The **codegen** (`codegen.c`) walks the token stream and emits C source:
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variables become `static int16_t var_A_int`, control flow uses `goto L_100`
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labels, and expressions are compiled inline using the same precedence-climbing
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structure as `eval.c`. Optimizations include constant folding, dead code
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elimination, FOR step=1 elision, and fast-path expression emission.
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The **`--safe` mode** modifies generated C to use overflow-checked integer
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arithmetic (`gw_int_add/sub/mul` from `math_int.c`) instead of bare C
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operators, and enhanced-diagnostic array/GOSUB runtime functions that report
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variable names, subscript values, and BASIC line numbers on error.
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The **runtime library** (`gwrt.c` / `libgwrt.a`) provides initialization,
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DATA/READ support, GOSUB return-label stack, and wrappers around the existing
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interpreter modules. It includes all interpreter code except the execution
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loop, so compiled programs share the same string pool, GC, file I/O, and
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graphics implementation.
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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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