Star Wars + The Empire Strikes Back (Atari 1983 / 1985) for MiSTer FPGA

This is a fork of Videodr0me/Arcade-StarWars_MiSTer. It keeps Videodr0me's excellent Star Wars MiSTer port intact and adds The Empire Strikes Back (1985) — the sequel that ran on the same Atari color-vector cabinet — along with a bit-exact AVG drawer rewrite. One Arcade-StarWars.rbf runs both games, chosen by which .mra you launch.

Our core vs. Videodr0me's original

	Videodr0me/Arcade-StarWars_MiSTer (upstream)	This fork
Games	Star Wars (1983)	Star Wars (1983) + The Empire Strikes Back (1985)
Slapstic protection	n/a (Star Wars has no slapstic)	Atari 137412-101 alternate ("devious") banking — rtl/slapstic101.vhd, a faithful port of MAME's decapped type-101. This is the piece that makes The Empire Strikes Back actually playable.
AVG drawer	original Black Widow-heritage drawer	PROM-driven, bit-exact to MAME 0.287 (three per-VCTR math fixes, verified by a Python-vs-MAME stroke diff)
Star Wars itself	—	byte-identical to upstream (every addition is either Empire-Strikes-Back-gated via mod_esb, or a drawer-level fix)

Everything else — the dual 6809 cores, the mathbox, the audio chain (POKEY / TMS5220 / TL084 / Reticon), the DDR3 framebuffer pipeline, the OSD options, and NVRAM — is Videodr0me's. Videodr0me's original Star Wars README is preserved in full below the divider.

The detailed breakdown of what changed:

What's Videodr0me's

• Original SW MiSTer port (CPU, mathbox, AVG state machine, audio chain, NVRAM)
• vector_fb_ddram.sv triple-buffer DDR3 framebuffer + MISTER_FB display path
• Black Widow drawer heritage and the original analytic-endpoint Bresenham rewrite
• All hardware peripheral models (POKEY, TMS5220, TL084, Reticon, etc.)
• Original OSD options and DIP plumbing
• Audio filter modelling

What's in this fork

HDL math: bit-exact match to MAME 0.287 (rtl/avg/avg.vhd, rtl/avg/vector_drawer.vhd)

Three bugs in the per-VCTR math were found by running a Python model of our HDL pipeline alongside a Python port of avg_starwars_device and diffing the per-stroke endpoint trace. The fixes are:

1. SVEC opcode cycles — MAME's m_op=2 (SVEC, OP1=1) path uses cycles = 2^(8 - total_shift) instead of the VCTR 2^(15 - total_shift). A 128× difference. Without compensating, every short-vector glyph (= all small text) renders at 128× MAME's size — visible as the "3000% UI text" symptom. Fix: bump effective total_shift by 7 for SVEC.
2. scale_factor off-by-one — MAME uses m_scale ^ 0xff (range 0..255); the HDL used 256 - m_scale (range 1..256). The mismatch accumulates and is wildly wrong at m_scale=255. Fix: bitwise NOT instead of subtract.
3. High total_shift truncation — MAME's cycles formula keeps producing 8, 4, 2, 1 at total_shift=12..15. The HDL vd_scale table truncated to zero above 11, dropping ~5% of strokes (the long-normalized small-magnitude ones). Fix: pre-shift rel_x/rel_y by (total_shift - 11) for total_shift > 11, use vd_scale = 1.

Additionally the >>3 truncation on rel_x/rel_y is now applied at the drawer input (matching MAME's (m_dvx >> 3) ^ 0x200 - 0x200 sign-mapping) and the vd_scale table was widened 8× to compensate. The framebuffer pixel pitch is 2^14, calibrated against MAME's coordinate system.

The Python diff tool (sim/diff_decoders.py) verifies 100% per-VCTR exact match across all four captured attract scenes (6522 strokes total).

Frame timing — tried and reverted

The "spread vggos across one CRT frame" approach (vblank-aligned EOF + a triple-buffer same-cycle race fix) was implemented and then reverted at 534f2cb. With the math fixes in place, each vggo produces correct MAME-equivalent content on its own, so Videodr0me's per-vggo swap rate gives a clean display without needing cross-vggo accumulation. The reroute introduced a 3-4 Hz black flash that was never root-caused (sim of the buffer state machine didn't reproduce it; ground truth would need SignalTap).

Lesson: Videodr0me's triple-buffer pipeline was designed and tested for the per-vggo EOF rate. Dropping that rate ~4× exposes timing characteristics they didn't validate. Don't reroute FRAME_DONE without a sim of the consequent buffer dynamics.

Sim infrastructure (sim/)

• avg_starwars_mame.py — MAME-faithful Python AVG decoder (port of mame_avgdvg_ref.cpp avg_starwars_device) with per-vg_add_point_buf trace output
• avg_starwars_hdl.py — Python emulation of our HDL pipeline, sharing the AVG state machine but using HDL drawer math
• diff_decoders.py — per-VCTR side-by-side diff producing a CSV ordered by divergence magnitude
• render_mame_faithful.py / render_hdl_emulated.py — PNG renderers using each decoder
• tb_drawer.vhd — stroke-cap latching fix so per-stroke endpoints survive the WALK → IDLE transition

The diff workflow is what found and validated all three math bugs above. Three iterations of "diff → identify pattern → fix Python → re-diff" reduced total divergence from 939 billion to 0.

The Empire Strikes Back (rtl/slapstic101.vhd, releases/Empire Strikes Back.mra, mod_esb plumbing)

The Empire Strikes Back runs on physically identical hardware to Star Wars per MAME's esb_main_map — same AVG (same PROM CRC), same mathbox, same audio chain, same inputs — and is selectable at runtime via the MRA's <rom index="1"> byte (no rebuild to switch games). The one genuinely hard part was the Atari slapstic 137412-101 copy-protection chip:

• The slapstic's direct bank switching is simple, but The Empire Strikes Back's gameplay depends on its alternate ("devious") banking — confirmed against MAME (mame.exe esb -log over real play shows 51 alternate-banking sequences reaching banks 0/2). Attract mode uses only direct banking, which is exactly why the game would boot, render one perfect frame, then freeze ~5 frames into gameplay (a computed jump into non-ROM).
• rtl/slapstic101.vhd is a faithful port of MAME's decapped type-101 (src/mame/atari/slapstic.{cpp,h}, Aaron Giles & Frank Palazzolo). The key insight: it must see the full 16-bit address on every 6809 bus cycle — not just the $8000-$9FFF window — so it catches the out-of-range $FFFF dummy cycle that the alternate sequence pivots on. The earlier in-range-only implementation could never do this. GHDL-verified (11/11, including the negative case), and confirmed working on hardware.
• 32 KB slapstic ROM (4 banks × 8 KB) at CPU $8000-$9FFF; 64 KB Empire main ROM (banked); 32 KB Empire audio ROM. The mod_esb selector in Arcade-StarWars.sv reads ioctl_index=1 — default is Star Wars; the Empire Strikes Back MRA sets it to 1. The Star Wars path is untouched when mod_esb=0.

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⬇️ Everything below is Videodr0me's original Star Wars README, preserved as-is. It documents the upstream Star Wars core (hardware, controls, MiSTer settings, OSD options, ROMs) and applies to both games except where this fork's notes above differ.

Star Wars (Arcade, 1983) for MiSTer FPGA

An FPGA implementation of Atari's classic 1983 color vector arcade game Star Wars for the MiSTer FPGA platform.

Atari's 1983 Star Wars remains one of the most beloved arcade games ever made. With its glowing wire-frame Death Star trench, digitized voices of Obi-Wan and Darth Vader, and the iconic flight yoke controller, it was the closest thing to climbing into an X-wing cockpit — and for a generation of players, "Use the Force, Luke" still gives them chills.

Support the Project

Hey, Videor0me here! If you're having a blast with this core, consider to

Your contributions show me that there is interest in these arcade projects. With enough support, I'd love to dedicate time to tackle other complex vector games like Tempest and The Empire Strikes Back or to improve the analog vector rendering effects.

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Original Hardware

The original Star Wars arcade machine (Atari part number 136021) is built from the following major components:

Subsystem	Original Hardware	FPGA Implementation
Main + Audio CPU	Motorola MC6809E @ 1.5 MHz	Cavnex mc6809e Verilog core with AVMA/VMA wrapper
Math Processor	Custom TTL Mathbox (PROM-sequenced matrix processor, 74LS384 serial multiplier, 15-step restoring divider)	Fully modeled in mathbox.sv
Vector Generator	Atari Analog Vector Generator (AVG) with state machine PROM, 10-bit DACs, analog integrators	Digital AVG in avg.vhd driving a raster framebuffer
Sound	4× Atari C012294 POKEY + TI TMS5220 speech synthesizer	POKEY in VHDL + TMS5220 with variable rate (TMS5220C mode)
Audio Filters	TL084 quad op-amp low-pass filter + Reticon R5106 delay/reverb line	Modeled in audio_filter_tl084.sv and reticon_r5106.sv
Display	Amplifone XY color vector monitor (RGB analog)	980×700 DDRAM framebuffer, vector_fb_ddram.sv
Controls	Custom flight yoke with analog potentiometers (2-axis)	Mapped to MiSTer analog stick inputs
Non-volatile RAM	256 bytes battery-backed NOVRAM (high scores, settings)	Saved to MiSTer SD card via NVRAM system

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Controls

Star Wars uses an analog flight yoke. The yoke's X and Y axes are mapped to the primary analog axes of your MiSTer controller.

🕹️ Calibration Tip: The game auto-calibrates to your controller's range. When you first start playing, move the analog stick in a full circle through its extreme positions — this lets the game learn your stick's full range of motion. You can do this at any time, but the stage select screen is the ideal moment.

Input	Function
Analog Stick	Move crosshairs (Pitch / Yaw) — proportional, recommended
Fire (Button A)	Fire lasers — also starts the game after inserting coins
Shield (Button B)	Shield button
Aux Coin (Button Start)	Auxiliary coin input (also used to navigate Test Mode menus)
Coin L / Coin R	Insert coins (mapped to R / L by default)

Tip: The original arcade machine has no "Start" button. After inserting a coin, pressing Fire on the yoke starts the game. An analog stick is strongly recommended for the best experience.

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Recommended MiSTer Settings

720p — Best Quality (Recommended)

At 720p, the framebuffer maps perfectly the display without scaling and is also best for 4K TVs/Monitors (Aspect Ratio: Optimized or Pixel Perfect). This is the recommended setting and also enables the optional 120Hz mode for ultra-smooth vector rendering.

Append these settings to your MiSTer INI file:

[Star Wars]
video_mode=0              ; 720p 60Hz — 1:1 pixel mapping in optimized and pixel perfect modes. Also ideal for 4K displays.
vsync_adjust=2            ; Low-latency — locks HDMI output to core timing
vscale_mode=0             ; Let the core's auto aspect ratio control scaling
hdmi_limited=0            ; Full range RGB (use 1 for TVs with limited range)
hdr=1                     ; HDR output — improves contrast/luminosity
vfilter_default=          ; No filters — 720p is pixel-perfect, filtering would blur the vectors
vfilter_vertical_default= ; Override any global vertical filter
vfilter_scanlines_default=; Override any global scanline filter

With these settings, you can also enable 120Hz (720p only) in the core's OSD menu for double the refresh rate. Your display must support 720p @ 120Hz (most modern TVs and monitors do). There is no need to set 120Hz in MiSTer INI.

Tip: The empty filter lines (vfilter_default= etc.) ensure that any global filters from your [MiSTer] section are overridden. Without them, a globally set bilinear or scanline filter would still apply and blur the crisp vector lines.

1080p — With Scaler Filtering

At 1080p, the framebuffer is scaled by 1.5×. Because 1.5× scaling means some lines are 1 pixel and others are 2 pixels wide, enabling the bilinear sharp filters is necessary:

[Star Wars]
video_mode=1920,1080,60   ; 1080p output
vsync_adjust=2            ; Low-latency — locks HDMI output to core timing
vscale_mode=0             ; Let the core's auto aspect control scaling
hdmi_limited=0            ; Full range RGB (use 1 for TVs with limited range)
hdr=1                     ; HDR output — improves contrast/luminosity
vfilter_default=Upscaling - SharpBilinear/SharpBilinear_100.txt ; Smooth 1.5x vertical scaling
vfilter_vertical_default= ; No additional vertical filter
vfilter_scanlines_default=; No scanline overlay — vectors don't have scanlines

Note: The 120Hz option is automatically disabled and greyed out at resolutions above 720p.

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OSD Options

Display

Option	Description
Aspect Ratio	Optimized (recommended) auto-detects HDMI resolution and picks the cleanest scale factor. Pixel Perfect forces 1:1 pixel mapping (980×700 centered). Stretched fills the screen (not recommended).
Unbuffered Vectors	When On, bypasses buffering and uses simple double-buffer ping-pong. Fakes a vector look, best used at 120Hz.
120Hz (720p only)	Doubles the refresh rate to ~120Hz. Reduces Frame Pacing Latency and improves the look of Unbuffered Vectors.

Cabinet Audio Hardware

The original Star Wars arcade cabinet features analog audio processing that goes beyond simple DAC output. This core models Filter, Delay and Reverb stages, giving you an authentic stereo audio rendition like never before:

Option	Default	Description
TL 084 Filter	On	Models the original TI TL084 quad op-amp low-pass filter on the audio output board.
Reticon Del/Rev	On	Models the Reticon R5106 analog delay line used in the original cabinet for spatial reverb effects. Adds a subtle authentic stereo "cockpit echo" to explosions and speech.

The audio mixing stage models the original summing amplifier (TL084, 1/4 of IC 4C) with per-channel gain based on the schematic resistor values matching the original cabinet's audio balance.

Tip: For the most authentic arcade sound experience, keep both options On. Turning the TL 084 Filter off yields a more modern Hi-Fi flavour of the audio.

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Game Setup — Use Test Mode, Not DIPs

The recommended way to change game settings is NOT through the DIP switches, but through the game's built-in Test Mode menu, just like arcade operators did on the original machine:

How to Access Test Mode

1. Wait for Demo Loop and Open the MiSTer OSD (F12)
2. Go to DIP Settings → set Test Mode to On
3. Close the OSD — the game enters the game setup / diagnostic screen
4. Use the flight yoke (analog stick) to navigate the on-screen menu
5. Press Fire to select options
6. Configure difficulty, coinage, bonus shields, and other game parameters
7. The game saves your settings to NVRAM automatically
8. Set Test Mode back to Off in the OSD to return to normal gameplay

Settings changed through Test Mode are preserved in NVRAM alongside your high scores. Don't forget to save the NVRAM or use Auto Save. Auto Saves on entering the F12 Menu. The DIP switches in the OSD can configure starting shields, difficulty, coinage, and other game parameters. However, changing DIPs requires clearing NVRAM, which erases all saved high scores.

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ROMs

                                *** Attention ***

ROMs are not included. In order to use this arcade core, you need to provide the
correct ROMs.

Quick reference for folders and file placement on your MiSTer SD card:

/_Arcade/Star Wars (Rev 2).mra
/_Arcade/cores/Arcade-StarWars.rbf
/_Arcade/mame/starwars.zip

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Known Limitations

This core renders vectors as 1-pixel-wide lines on a raster framebuffer. A real Amplifone XY color vector monitor is an analog CRT where a focused electron beam traces each vector directly onto phosphor. Several visual characteristics of the original display are not (yet) reproduced:

Area	Limitation	Detail
Vector Rendering	Not cycle-exact	The AVG vector drawer does not run cycle-exact. There might be slight variance in speed compared to original hardware.
Bloom & Spot Size	Not modeled	Higher beam current increases the spot diameter, making bright vectors appear wider and softer with a visible luminous halo.
Beam Velocity	Not modeled	Perceived brightness is inversely proportional to beam velocity. This core draws all vectors at uniform brightness per Z-level regardless of length.
Phosphor Persistence	Not modeled	The P22 phosphor used in color vector CRTs has a visible afterglow decay (~1–10 ms depending on color channel). Moving objects leave fading trails.
Beam Overlap	Not modeled	Where two vectors cross or overlap on a real CRT, the phosphor is excited twice, producing additive brightness and enhanced bloom at the intersection.
Overdrive & Saturation	Not modeled	Extreme beam current events (e.g., the Death Star explosion) overdrive the CRT — the spot blooms dramatically, colors shift toward white, creating a diffuse full-screen glow.
Intensity	Limited Z-level support	The current implementation uses a simplified intensity mapping with fewer effective levels.

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Compilation

The project uses Quartus Prime Lite targeting the Cyclone V on the Terasic DE10-Nano.

1. Open Arcade-StarWars.qpf in Quartus
2. Run the full compilation flow (Analysis → Fitter → Assembler → Timing)
3. The output Arcade-StarWars.rbf is generated in output_files/

The sys/ directory contains the standard MiSTer framework. All core-specific RTL is in rtl/.

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Credits & Acknowledgments

• Original Game: Mike Hally (project lead), Greg Rivera & Norm Avellar (programming), Jed Margolin (hardware engineering), Ed Rotberg (original concept) — Atari, 1983
• Upstream: Videodr0me/Arcade-StarWars_MiSTer — the entire MiSTer port this fork builds on
• Initial FPGA Foundation: Jeroen Domburg (Black Widow MiSTer core)
• 6809 CPU Core: Greg Miller (Cavnex mc6809e)
• MiSTer Platform: Sorgelig and the MiSTer community

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License

This project is provided for educational and personal use. See individual source files for their respective licenses.