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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

  • 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

System Emulators

The directory

contains a number of header-only 8-bit computer emulators which can be embedded into applications that provide keyboard input, and render the emulator's generated video- and audio-output.

Note that accuracy of the system emulators varies quite a lot, and is mainly defined by what games, demos and tests have been used for testing and improving the emulation.

The following system emulators are provided:

KC85/2, /3 and /4

An East German Z80-based computer with 320x256 color graphics, beeper sound a powerful expansion slot system and (for its time) innovative operating system. The KC85/2 family was designed and built by VEB Mikroelektronik Mühlhausen between 1984 and 1989.

Z9001 (aka KC85/1), KC87

Another East German 8-bitter created by Robotron Dresden. This was a more conventional, less innovative design compared to the KC85/2, both in hardware and software. The Z9001 family only had 40x20 ASCII pseudo-graphics display with optional color support.


This was the most simple and cheapest Z80-based computer built in Eastern Germany that still resembled a 'proper' computer which could be attached to a TV. It was also the only Eastern German computer that was available as assemble-youself-kit for the general public.

ZX Spectrum 48k and 128

The Sinclair ZX Spectrum is supported as the original 48k model and the improved 128 model with a proper sound chip (the AY-3-8912, which was also used in the Amstrad CPC).

CPC 464, 6128 and KC Compact




Acorn Atom


Chip Emulators

Z80 CPU (chips/z80.h)

The Zilog Z80 CPU.

  • tick-callback with CPU-pin bitmask, called with machine-cycle or single-tick granularity
  • fast switch-case instruction decoder generated via python script
  • up to 7 wait states can be injected per machine cycle by setting WAIT pins in the CPU tick callback
  • all undocumented instructions supported
  • internal WZ register and undocumented XF and YF flags supported
  • support for interrupt-priority handling (daisy chain) with help from the tick callback
  • runs the ZEXDOC and ZEXALL tests
  • runs the CPU test of the FUSE ZX Spectrum emulator, with the following exceptions:
    • the state of the XF/YF undocumented flags is ignored for indirect BIT test instructions, FUSE doesn't agree here with ZEXALL and I think ZEXALL is right (the state of the XF/YF flags depends on the current state of the internal WZ register)
    • FUSE assumes that the PC after a HALT instruction has been incremented, while the chips Z80 emulator doesn't incrmenent the PC, this shouldn't make any difference though
  • properly handles sequences of DD/FD prefix bytes
  • flexible trap callback for hooking in debuggers and "native code" handlers
    • interrupt mode 0
    • refresh cycle in second half of opcode fetch machine cycle
    • bus request/acknowledge (BUSRQ/BUSAK pins)

Z80 PIO (chips/z80pio.h)

The Zilog Z80 Parallel Input/Output controller.

  • programmed via Z80-compatible chip-pin bitmask
  • two callbacks for passive port A/B input/output
  • write-function for active port A/B input (may trigger interrupt)
  • can act as interrupt controller in a Z80 interrupt-daisy-chain
    • bidirectional mode

Z80 CTC (chips/z80ctc.h)

The Zilog Z80 Counter/Timer Channels.

  • programmed via Z80-compatible chip-pin bitmask
  • emulates the CLK/TRG and ZC/TO input/output pins
  • can act as interrupt controller in a Z80 interrupt-daisy-chain

MOS 6502 CPU (chips/m6502.h)

The MOS Technology 6502 CPU.

  • single tick-callback with CPU-pin bitmask, called with tick-granularity
  • fast switch-case instruction decoder generated via python script
  • emulates all(?) quirks (like redundant and 'junk' read/write cycles, variable cycle counts in some addressing modes, page boundary wrap-around in indirect jump, etc...), mostly verified via
  • emulates the known and useful 'documented-undocumented' opcodes (like LAX, SAX, DCP, ...)
  • decimal mode implemented, can be disabled
  • same powerful trap callback as the Z80 emulator
  • test coverage:
    • NESTEST: completely working (this runs through all documented, and most 'common' undocumented instructions but doesn't test decimal mode)
    • Wolfgang Lorenz C64 Test Suite: all CPU opcode tests working, including the unintended and unstable instructions, instruction timings, branchwrap etc. Only two known issues remaining: the irq and nmi tests (also see the C64 notes in

AY-3-8910 (chips/ay38910.h)

The General Instrument AY-3-8910 sound generator chip and its low-cost variants AY-3-8912 and AY-3-8913 (the 3 variants only differ in the number of provided I/O ports, the 8910 has 2 ports, the 8912 has 1, and the 8913 has none.

  • programmable via chip-pin bitmask
  • separate tick function called from CPU tick callback, returns true when a new audio sample is ready

MC6845 (chips/mc6845.h)

Motorola 6845 video address generator and variants.

  • programmable via chip-pin bitmask
  • tick function which generates hsync, vsync, display-enable, 13-bit memory-address and 5-bit row-address, returned as pin-mask
    • interlace mode
    • the cursor pin
    • the light-pen functionality
  • NOTE: emulation quality is "ok" for most Amstrad graphics demos, but more improvements are needed

MC6847 (chips/mc6847.h)

Motorola 6847 video display generator.

  • programmable via chip-pin bitmask
  • tick function which directly generates a RGBA8 framebuffer
  • memory-fetch callback called from the tick function, this returns a complete pin-mask and can be used to set the mode-select input pins (this is used in the Acorn Atom for instance, which directly connects data bus pins to MC6847 mode-select pins)

i8255 PPI (chips/i8255.h)

Intel 8255 Programmable Peripheral Interface

  • programmable via chip-pin mask
    • Mode 1 (strobed input/output)
    • Mode 2 (bi-directional bus)
    • interrupt generation

MOS 6522 VIA (chips/m6522.h)

MOS Technology 6522 Versatile Interface Adapter.

Currently this just contains the minimal required functionality to make some games on the Acorn Atom work (basically just timers, and even those or likely not correct).

MOS 6526 CIA (chips/m6526.h)

MOS Technology 6526 Complex Interface Adapter

(Work In Progress)

  • Wolfgang Lorenz C64 Test Suite Status:
    • cia1pb6, cia1pb7, cia2pb6, cia2pb7: OK
    • cia1ta, cia1tb, cia2ta, cia2tb: FAIL (these are marked as "under construction" in the test-suite readme though)
    • cia1tab: OK
    • cia1tb123, cia2tb123: OK
    • cntdef, cnto2: OK (but note that CNT pin is not emulated, it's always high)
    • flipos: OK
    • icr01: OK
    • imr: OK
    • loadth: OK
    • oneshot: OK
    • time-of-day features
    • serial port
    • PC pin
    • CNT pin is always high

MOS 6569 VIC-II for PAL-B (chips/m6569.h)

MOS Technology 6569 Video Interface Chip VIC-II (FIXME: needs more info)

MOS 6581 SID (chips/m6581.h)

The C64 sound chip (FIXME: needs more info)

AM40010 Amstrad CPC Gate Array Emulation (chips/am40010.h)

This emulated the Amstrad CPC gate array chip, and also integrates the PAL chip for bankswitching in the 6128.

UPD765 Floppy Disc Controller (chips/upd765.h)

This is a basic emulation of the UPD765 floppy controller. Currently only features required by the Amstrad CPC are implemented.

Helper Code

Memory (chips/mem.h)

Helper code for 8-bit home computer memory systems.

  • map 16-bit address ranges to host memory with 1-KByte page granularity
  • memory pages can be mapped as RAM, ROM or RAM-behind-ROM via separate read/write pointers
  • up to 4 'mapping layers' to simplify bank-switching or memory expansion systems

Keyboard Matrix (chips/kbd.h)

Helper code for keyboard matrix typically found in 8-bit home computers.

  • map host system key- or ASCII-codes to keyboard matrix locations
  • up to 12x12 matrix size
  • up to 4 configurable modifier keys (for shift keys)
  • internal pressed-key-buffer for simultaneously pressed host system keys
  • configurable 'sticky count' to lengthen short host-system key presses to give the emulated system enough time to scan the keyboard matrix state

Sound Beeper (chips/beeper.h)

A square-wave beeper found in many simple home computers.

  • toggle beeper state on/off from CPU tick callback
  • tick function returns true when new audio-sample is ready

Clock (chips/clk.h)

Helper function to convert a clock frequency in Hz to a number of ticks, and to keep track of the 'left over' ticks from one frame to the next.

Floppy Disc Drive (chips/fdd.h)

A basic floppy disc drive emulator, currently only basic functionality as needed by the Amstrad CPC emulation.

Amstrad CPC Disk Image Loader (chips/fdd_cpc.h)

This is an extension-header for fdd.h which adds support to read an Amstrad CPC .DSK disk image file and "insert" it into the Floppy Disc Drive (fdd.h).

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