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8-bit chip and system emulators in standalone C headers

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A toolbox of 8-bit chip-emulators, helper code and complete embeddable system emulators in dependency-free C headers (a subset of C99 that compiles on gcc, clang and cl.exe).

Tests and example code is in a separate repo:

The example emulators, compiled to WebAssembly:

For schematics, manuals and research material, see:

What's New

  • 16-Dec-2021: An entirely new 'cycle-stepped' Z80 emulator, along with various improvements mainly in the Amstrad CPC emulation. Accompanying blog post upcoming. A snapshot of the old emulator is under the git tag pre-cycle-stepped-z80 both in the chip and chips-test repo. PS: blog post is here:

  • 14-May-2020: A small breaking change in kbd.h: the function

    now takes a new argument
    uint32_t frame_time_us
    which is the current frame time (duration) in microseconds. This is necessary to make the sticky-key handling frame rate independent. The
    initialization parameter in
    remains unchanged. This is the number of 60Hz frames a key press should remain sticky.
  • 20-Jan-2020: The i8255 and MC6847 chips emulations have been changed to a 'tick-only API', continuing the 'API streamlining' that started with the 6522 VIA chip. The Atom system emulation has been updated accordingly, and the minimal necessary changes to the CPC emulation have been added (but only as a quick hack, the other CPC support chips haven't had their API updated yet).

  • 15-Jan-2020: The CIA (m6526.h), VIC-II (m6569.h), SID (m6581.h) chip emulators have merged their *iorq() functions for reading and writing chip registers into the regular *tick() functions, and the C64 emulation in systems/c64.h has been updated accordingly (this API change started with the chips in the vic20.h and will continue for the other chip emulators).

  • 03-Jan-2020: Another VIA- and VIC-20 related update:

    • The VIA (m6522.h) API has been simplified: the separate m6522iorq() function to read and write chip registers has been merged into the m6522tick() function, and all IO callbacks have been removed. Instead of handling the VIA IO ports through callbacks, the port input pins are now set before calling m6522tick(), and the port outputs are inspected in the pin mask returned by m6522tick(). Similar, if chip registers should be read or written, the chip-select and RW pin must be set on the input pin mask for m6522_tick(). This 'API streamlining' makes writing system tick functions more straightforward. System schematics now translate more directly into code which sets and inspects pin bit masks, instead of handling the address decoding, register reads/write, and IO through completely different code (such as IO callbacks).
    • The same API change (merging the iorq function into the tick function has been implemented for the VIC emulation (m6561.h). In the future the other chip emulations will follow too. See the
      function in the
      header for a code example of how the new VIA and VIC APIs are used in a system's tick function.
    • The VIA emulation is now more feature complete and accurate, but not yet complete:
      • the entire shift-register functionality is not implemented
      • timers and interrupts are not cycle accurate in some situations
    • The VIC-20 emulation is now good enough to support TAP-file loading through c1530.h datassette emulation (this depends mostly on somewhat accurate VIA timers and interrupts). Also, the first modern demo-scene demos are now running in the VIC-20 emulation, although with glitches here and there.
  • 24-Dec-2019: A small "inbetween merge" because the feature branch I was working on became too unfocused: The plan was to add 1541 floppy support to the C64 emulation, but I soon realized that the VIA emulation would essentially need to be rewritten for this. To help getting the VIA emulation right, I started with a VIC-20 system emulation. Here's what's new:

    • a new VIC-20 system emulation, still quite WIP
    • started to rewrite the 6522 VIA emulation from scratch, it's still very WIP, but works "at least as good" as the previous implementation
    • a new VIC-I PAL (MOS 6561) emulation (used by the VIC-20)
    • moved the datasette emulation out of c64.h into its own header (c1530.h), for attaching peripheral devices the c64.h emulation now emulates the interface ports (IEC and CASPORT) instead, which the peripheral device emulations "connect to"
    • started with a 1541 floppy drive emulation in the header c1541.h, not functional yet
    • various optimizations in kbd.h with the goal to make the frequently called keyboard matrix scanning functions so cheap that they can be called in each emulated tick
    • various other minor cleanups and optimizations in m6526.h (CIA) and m6569.h (VIC-II)

    I will experiment next with more radical changes to the VIA emulation, idea is to merge the currently separate m6522iorq() function (which handles reads and writes to the VIA registers) into the regular m6522tick() function. If those experiments are successful, other chips will use the same model in the future, starting with the IO/timer chips (m6526, i8255, z80pio and z80ctc).

  • 13-Dec-2019: The new 'cycle-stepped' 6502/6510 emulator has been merged to master. The new emulator has a slightly different programming model, please see the updated header documentation and this blog post. I have created a git tag which preserves the previous emulator. All the 6502-based system emulators on the Tiny Emulators page have been updated with the new cycle-stepped 6502.

  • 14-Oct-2019: Improvements to the 6502 and C64 emulation:

    • All tests of the Wolfgang Lorenz test suite are passing now, except:
      • irq and nmi: the timing for interrupt requests is slightly off, this is most likely because the 6502 emulation currently doesn't delay interrupt handling to the end of the next instruction under some circumstances
      • cia1ta, cia1tb, cia2ta, cia2tb: these are marked as "under construction" in the test suite's readme, so I assume it's normal that they are failing(?)
    • With the remaining tests all passing, this means:
      • all unintended and unstable 6502 instructions are now supported as tested by the Wolfgang Lorenz test suite
      • all instruction clock cycles are now correct (previously two unintended NOP instructions were one clock tick off because they used the wrong addressing mode)
    • Some code cleanup and (very minor) optimizations in the 6569 (VIC-II) emulation, and improved the raster interrupt timing which was slightly off.
  • 05-Aug-2019:

    • The Z80 and 6502 CPU emulators are now each in a single header instead of being split into a manually written "outer header" which includes another code-generated header with the instruction decoder. No functional changes (I tried a variation of the Z80 emulator which goes back to separate byte registers in a struct instead of merging the registers into 64-bit integers, this saved a couple KBytes code size in WASM but was about 10% slower so I discarded that experiment)
  • 31-Dec-2018:

    • A complete set of debugging UI headers using Dear ImGui has been added, each chip emulator has a window which visualizes the pin- and internal-state, and there are helper windows which implement a memory editor, memory "heatmap" (visualize read/write/execute operations), disassembler and CPU step debugger. Finally there are 'integration headers' which implement an entire UI for an emulated system. Note that the implementation part of the UI headers needs to be compiled as C++, the 'public API' of the headers are callable from C though.
    • The CPU emulators (z80.h and m6502.h) have new trap handling. Instead of predefined "slots", a trap evaluation callback is now installed, which is called at the end of each CPU instruction. This is used extensively by the new debugging UIs to keep track of CPU operations and breakpoint support.
    • The Amstrad CPC emulation has gained floppy disc loading support, and the video system precision has been improved (many modern graphics demos at least work now instead of having completely broken rendering, but there's still more to be done).
    • Loading local files via drag'n'drop has been improved in the WebAssembly version, all emulators can now properly detect and load all supported file formats via drag'n'drop.
  • 23-Jul-2018: all chip emulators with callbacks now have an extra

    argument in the callbacks which is provided in the init function, this makes the chip emulators a bit more flexible when more than one emulator of the same type is used in a program

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