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Emulator VirtualT Model 100

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Retro-Jack edited this page Jul 7, 2026 · 1 revision

VirtualT — Tandy TRS-80 Model 100

The Model 100 is the fourth GenX-DOS bundle compiled from source to WASM (after Emulator-atari800, Emulator-sdltrs-TRS-80 and Emulator-libretro-o2em-Odyssey2), and the first driven by VirtualT — Stephen Hurd & Ken Pettit's Model 100 / 102 / 200 emulator, BSD-2-Clause, on SourceForge since 2004.

It exists because a user, dplassgit, opened discussion #2, "Tandy 100?": "Based on the Intel 8085 CPU, it was one of the early portable/laptops. It definitely has the 8-bit feel." The reply set the bar — "I'd need 10 easily accessible software titles before continuing" — which is why the lineup ships exactly ten games.

An extraction, not a fork

VirtualT isn't a small emulator — it's a full FLTK desktop application: the machine, plus an IDE, a disassembler, a debugger, memory editors, a peripheral monitor. Forking all of that to strip it back would be dishonest about how little we kept. So the Model 100 bundle is an acknowledgement, not a fork: we compile only VirtualT's FLTK-free portable core — the 8085 CPU, memory + banking, the HD44102 LCD controllers, the keyboard scan, the ROM images, sound and serial — and never compile the ~40 FLTK GUI/IDE/debugger files at all.

What the core still calls into from that dropped layer, five small headless units supply. They live in systems/m100/src/ with a reproducible build.sh and BUILDING-WASM.md:

Unit Stands in for
frontend_sdl.c display.cpp's window + event loop — our browser front end
display_headless.c display.cpp's LCD drawing — the 240×64 framebuffer
file_headless.c file.cpp's loader (FLTK file-chooser → host filenames)
clock_headless.c clock.cpp's uPD1990AC RTC (config UI + prefs dropped)
stubs.c no-op peripherals (T200 RTC, printer, TPDD dock, remote debug)

frontend_sdl.c hooks a single gx_frame() into the core's maint() (one added call in m100emu.c); every frame it blits the LCD to an SDL2 texture, pumps keys into io.c, and emscripten_sleep()s to yield.

-sASYNCIFY and the throttle deadlock

VirtualT's emulate() is a blocking while (!gExitApp) loop, so -sASYNCIFY is non-optional — the same shape as atari800 and sdltrs. But the first builds hung after ~30,000 instructions, stone dead.

The culprit wasn't ASYNCIFY. VirtualT's throttle() calls sem_wait(&gThrottleEvent), waiting on a background timer thread that doesn't exist in single-threaded WASM — so it blocks forever. The fix is one line in gx_frame: fullspeed = 3, VirtualT's own "run unthrottled" mode, which still counts cycles but skips the blocking while/sem_wait entirely. Pacing is done instead by emscripten_sleep(16) once per frame.

The traps that aren't link errors

Two classes of bug cost real time here, both because emscripten turns C sloppiness into runtime wasm traps:

  • Signature mismatches. An implicitly-declared function is assumed int(); if the real definition is void(), the indirect call traps at runtime — it is not a link error. clang even phrases it "call to undeclared function", not "implicit declaration". Every stub in stubs.c had to match its header's signature exactly (e.g. get_model_time is void, not long; tdock_read is unsigned char(void)).
  • Data symbols stubbed as functions. setup, mem_setup, clock_serial_out are variables, not functions — stubbing them as functions gives a wasm-ld "symbol type mismatch". They're defined as zero-init variables via #include "setup.h".

The ROM is a file

The core opens the Model 100 ROM as ROMs/M100rom.bin (32 KB) — VirtualT tracks it in its own repo — so it's --embed-file'd into MEMFS rather than linked, even though m100rom.c (the ROM as a C array) is also compiled in. The early "No ROM file for m100" errors were exactly this: the ROM is loaded as a file, not from the array.

The clock — live, and Y2K-fine

The RTC was first stubbed to a no-op, and that quietly broke more than clocks: TIME$ sat at 00:00:00, which kills game timers and the ubiquitous FOR N=1 TO VAL(RIGHT$(TIME$,2)) RND-seeding idiom — so every launch dealt the same cards and drew the same maze.

clock_headless.c extracts the uPD1990AC chip (pd1990ac_chip_cmd / pd1990ac_clk_pulse) from clock.cpp, FLTK-free, and reports the real host wall-clock time so TIME$ advances. The year is separate: the chip has no year register (the M100 keeps the year in RAM), so we poke today's year into gStdRomDesc->sYear once. The 1983 ROM handles it gracefully — the menu reads Jly 07,2026 ("Jly" is the ROM's own abbreviation for July, not a bug).

PEEK collisions and the screen mirror

Several BASIC games detect collisions by PEEKing the LCD's RAM mirror — locations 32959–33279, which the ROM keeps in sync with the display. Because we run the real ROM and emulate that RAM, the mirror is maintained exactly, so PEEK-based games (Road Rally, Skydiver, Worm Hole) work. This was an open question until Road Rally's crash-detection fired correctly.

The keyboard — and Caps Lock

Only two keys differ from a PC keyboard: GRPH → Left Alt and CODE → Right Alt. Everything else (Enter, Shift, Ctrl, Tab, Esc, Backspace, arrows, F1–F8) maps 1:1. The FLTK build rebuilt io.c's keyscan[] from its own key handler; headless, pump_input calls update_keys().

The command keys io.c actually reads — PASTE (Insert), LABEL (F9), BREAK (Shift+Pause) — are momentary and wire up normally. Caps Lock does not. It's MT_CAP_LOCK, bit 0x20, and io.c reads it — but it's a toggle: desktop VirtualT does gSpecialKeys ^= MT_CAP_LOCK on key-down, whereas a naïve frontend treats it as a held key, so it flips on and instantly off ("no effect"). Porting the toggle into pump_input — flip on key-down, ignore key-up — fixed it. It's the only toggle key in VirtualT's handler; NUM (0x10) is marked "Not used" upstream.

The games — a book, not a fork

The lineup is ten type-in BASIC games, and getting them clean was its own saga.

The pre-book source was club100: its pre-tokenized .BA files load fine (Four Seasons Solitaire, ELIZA — both kept), but its ASCII listings are word-wrapped at ~40 columns with \r\r\n separators, so the emulator's load-time tokeniser corrupts them. A custom un-wrapper (rejoin any physical line that doesn't start with an ascending line number) got close but stayed flaky on bigger listings.

The fix was to change source, not tooling: David Busch's 25 Games for your TRS-80 Model 100 (TAB Books, 1984). Eight games are hand-transcribed from its scanned listings into clean single-CR-line .ba files we fully control — which load reliably, with no wrapping roulette, and the book's chapters double as the gamedocs. Games that quit to the BASIC Ok prompt are adapted to call MENU instead so you land back on the machine's own menu.

Battery backup — persistence, done authentically

Rather than a full CPU/IO save-state, the Model 100 gets what the real machine has: battery-backed RAM. save_ram shows the whole persistent state is just the 32 KB of base RAM (gBaseMemory+RAMSTART), so play.html persists exactly that to localStorage for the bare Startup Menu and restores it on load (HEAPU8.set over gx_ram_ptr(), then gx_warm_reset() = jump_to_zero(), a warm reboot that keeps RAM so the restored directory re-scans). Your files, notes and typed-in programs survive between visits — browser-contingent, and games stay ephemeral.

On a first, blank-RAM boot, empty ADRS.DO / NOTE.DO are seeded so ADDRSS and SCHEDL open instead of erroring.

Bundle layout

systems/m100/
├── play.html
├── controls.html          ← sourced from the owner's manual
├── virtualt.js
├── virtualt.wasm          ← M100 ROM embedded here
├── adrs.do, note.do       ← empty seeds for ADDRSS/SCHEDL
├── games/                 ← 10 .ba games
└── src/                   ← the 5 headless units + build.sh + LICENSE

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