Commodore 64 Emulator v0.7.9.9-alpha

Commodore 64 Emulator v0.7.9.9-alpha is released. A Commodore 64 emulator written in modern C++, built with SDL2 and designed around two priorities:Accuracy-first emulation (with a strong focus on real hardware behavior over “good enough” hacks); Debuggability (integrated monitor, break/watchpoints, and visibility into what the machine is doing). This project is actively under development; expect rough edges and incomplete features.

Commodore 64 Emulator Changelog:
This alpha release continues the emulator accuracy push from the last couple of weeks, with major work focused on VIC-II timing/graphics behavior, SID audio accuracy, chip model selection, and cleaner program/cartridge loading behavior.
Highlights
Continued VIC-II hardware accuracy improvements across raster timing, badline behavior, bus arbitration, border handling, and pixel-phase rendering.
Improved SID emulation accuracy, including better register handling, filter routing, OSC3/ENV3 reads, audio underrun behavior, and 6581/8580 model selection.
Added/expanded SID chip model selection so the emulator can switch between MOS 6581 and MOS 8580 behavior.
Fixed PRG loading flow so loading a PRG performs a cleaner machine reset path, including cold reset behavior and cartridge detach handling.
Improved emulator stability and diagnostics around display, audio, CPU, and media loading behavior.
VIC-II Accuracy Updates
This release includes another round of VIC-II accuracy work aimed at making raster effects, scrolling, badlines, and graphics modes behave closer to real hardware.
Raster, badline, and display timing
Improved badline timing behavior and diagnostics.
Separated predicted badline bus-warning behavior from sampled badline DMA-steal behavior.
Continued work on display progression timing, including first badline behavior and display sequencer state.
Improved handling of DEN, YSCROLL, display-enabled state, and badline sampling visibility.
Added/expanded diagnostics to help inspect badline state, display progression, raster timing, and internal VIC sequencing.
BA/AEC and bus arbitration
Continued cleanup and refinement of VIC-II bus ownership tracking.
Improved BA/AEC behavior around badline warning cycles and DMA steal cycles.
Added more explicit cycle/bus-owner style diagnostics to make CPU/VIC contention easier to debug.
Optimized parts of the render/bus timing path to avoid regressions in games sensitive to performance, such as Pac-Man.
Border and raster latch behavior
Improved border timing/latch behavior.
Added or preserved raster-based color latch state for save/load behavior.
Improved visibility into border open/close behavior through monitor/debug tooling.
Continued work toward more accurate mid-raster color and register behavior.
Graphics rendering path
Continued work on moving the VIC rendering path toward more cycle-aware and pixel-aware behavior.
Added pixel-aware helper paths for resolving VIC state at a specific raster/pixel position, including D016/D018 and graphics memory base helpers.
Improved groundwork for multicolor bitmap/text accuracy.
Continued investigation of multicolor bitmap artifacts seen in titles such as RoboCop 3.
Improved diagnostics to help isolate whether artifacts are coming from bitmap sampling, pipeline stepping, or final framebuffer composition.
Sprite and DMA-related groundwork
Continued sprite DMA and raster latch investigation.
Added/expanded state preservation and diagnostics around sprite priority, multicolor enable, sprite colors, and raster-specific sprite behavior.
Improved internal visibility into sprite DMA state and raster interaction.
SID Accuracy and Audio Updates
This release includes several SID-focused fixes and accuracy improvements, especially around register behavior, model-specific sound, and audio smoothness.
SID register correctness
Corrected handling of $D417 and $D418.
$D417 now correctly represents RES/FILT, including resonance and voice filter routing bits.
$D418 now correctly represents MODE/VOL, including filter mode bits, volume, and voice 3 direct-path behavior.
Improved filter routing behavior so voices are routed through the filter based on the proper SID register bits.
Implemented more accurate OSC3 and ENV3 read behavior at $D41B and $D41C.
SID model selection
Added/expanded SID chip model selection support.
Emulator can now distinguish between MOS 6581 and MOS 8580 behavior.
Added model selection plumbing through the SID/voice/audio path.
Improved groundwork for model-specific DAC behavior, tonal differences, and filter behavior.
Added UI/configuration-facing support so the selected SID model can be exposed and controlled more cleanly.
Envelope and clicking/choppiness work
Improved SID gate/release behavior so envelope release does not abruptly reset the level.
Removed hard audio cutoffs that could introduce clicks during envelope tests.
Improved handling of silent/no-waveform/test cases without forcing discontinuities into the output.
Continued work on SIDTester envelope test behavior, especially where rapid gate transitions caused clicks or choppy output.
Audio buffer and underrun behavior
Improved SID audio underrun handling.
Added/expanded underrun diagnostics.
Changed underrun behavior to avoid returning hard zeroes where possible, reducing audible discontinuities.
Improved audio generation path to reduce avoidable per-sample overhead and help prevent underrun-related choppiness.
Added better monitoring of generated/consumed samples, buffer depth, underruns, and audio health.
PRG Loading, Reset, and Cartridge Handling
This release improves the emulator’s PRG loading flow so switching from cartridge or other media into a PRG behaves more predictably.
Fixed PRG loading to use a cleaner reset path.
PRG loading now includes cold reset behavior where appropriate.
Loading a PRG now detaches the currently attached cartridge when needed.
Prevents stale cartridge mapping/state from interfering with newly loaded PRG programs.
Improved startup/load behavior when transitioning between cartridge tests, PRG files, and normal BASIC-style execution.
Reduced cases where the emulator could remain in an unexpected memory mapping state after media changes.
CPU and Bus Accuracy Fixes
This release also includes CPU-side accuracy improvements that support better compatibility with real C64 software.
Improved bus-visible dummy read behavior for indexed store operations.
Normalized dummy read addresses for unofficial indexed store opcodes such as AHX/TAS-style behavior.
Improved stack/control-flow dummy read behavior.
Improved decimal-mode ADC/SBC status handling.
Preserved more accurate pushed processor status behavior while keeping the internal B flag handling cleaner.
Continued work on IRQ/NMI timing behavior and CPU/VIC interaction.
Disk, Drive, and Media Updates
Continued work on disk/media handling paths used by the emulator.
Improved PRG overwrite behavior for D64-style workflows using scratch-then-save style logic.
Continued investigation and cleanup around 1541/1571/1581 behavior.
Improved diagnostics for IEC, drive state, and media activity.
Continued work toward more accurate ROM-controlled IEC behavior for drive emulation.
Monitor, Debugging, and Diagnostics
This release expands the emulator’s internal visibility, especially for hardware accuracy work.
Expanded VIC monitor/debug outputs for raster state, IRQs, latches, border state, badlines, and cycle-level behavior.
Expanded SID audio diagnostics for underruns, buffer depth, generated/consumed samples, and health state.
Improved logging and inspection paths for gate transitions, envelope behavior, and SID register writes.
Added more targeted diagnostics to help compare behavior against VICE and isolate hardware-timing issues.
Compatibility / Testing Notes
This release was tested and debugged against several timing-sensitive titles and tools, including:
SIDTester
Pac-Man / Ms. Pac-Man
RoboCop 2
RoboCop 3
Q*bert
Frog Master
Spy Hunter
BC’s Quest for Tires
Dig Dug
Ghostbusters
Known areas still under active development:
Multicolor bitmap timing/rendering still needs additional refinement.
RoboCop 3 still exposes remaining multicolor bitmap/raster artifacts.
Some raster IRQ effects still need tighter cycle-level behavior.
SIDTester envelope behavior is improved but still remains a key comparison target against VICE.
1571/1581 ROM-controlled IEC behavior still needs more compatibility work.
Summary
v0.7.9.9-alpha is primarily an accuracy and stability release. The biggest improvements are in VIC-II timing groundwork, SID register/audio correctness, chip model selection, and safer PRG/cartridge transition behavior. These changes move the emulator closer to real C64 hardware behavior while also improving the debug tooling needed to keep refining compatibility.

Download: Commodore 64 Emulator v0.7.9.9-alpha
Source: Here