ICEMU is a transistor-level IC emulator, heavily influenced by the perfect6502 and visual6502 projects.
I got it in my head that I would write an NES emulator, and after several weeks of research this turned into an interest in writing a half-cycle-accurate MOS 6502 emulator. It's a popular chip for emulation, and this has been done before (by both projects credited below).
As written, ICEMU is a general-purpose emulator for any digital IC, provided the transistor netlist is known. The ICEMU runtime implements a simple language for writing test cases and manipulating memory in a simulated environment.
In addition to transistor-level emulation, ICEMU also includes native emulation of higher-level sub-components like logic gates and flip-flops. The ICEMU compiler (written in Node.js) searches the netlist for known transistor sub-graphs and replaces them with these sub-components, the aim being to improve performance by reducing the number of emulated components, while preserving identical behavior.
$ make runtime
$ make tests
I'm writing this after several years, so my memory is fuzzy; what follows is merely a general guide.
The icemu.h header provides the interface via which the low-level emulator (icemu_t) for any device is constructed from its netlist (icemu_layout_t). At this level of emulation, the only possible actions are writing a bit to a node, reading a bit from a node, and synchronizing circuit to account for any writes.
Each emulated device must define an adapter that is returned by an externally linked function matching the adapter_func declaration in runtime.h. The emulated device is compiled into a shared object that can then be loaded dynamically by the runtime application with the .device command.
The layout of the mos6502 device directory is as follows:
- layout.h — Device layout, generated by netlist compiler.
- mos6502.c — Device-specific emulator using icemu.h with named setters and getters for pins on the device.
- memory.c — Board memory emulation.
- controller.c — High-level emulation of things like clock steps and reset sequence.
- adapter.c — Adapter for ICEMU runtime, tying together memory, controller, and pin interfaces.
The ICEMU runtime (./runtime) uses the adapter to run *.ice scripts, which can be used to measure performance or implement regression tests. Examples for MOS 6502 can be found in mos6502/tests.
The ICEMU compiler (bin/compile) uses a netlist of transistors and voltage loads defined in JSON to generate a chip layout. The circuits property allows known sub-graphs of transistors to be reduced to predefined components for faster emulation.
ICEMU is inspired by and derived from perfect6502, written by Michael Steil, which in turn was derived from visual6502, written by Greg James, Brian Silverman, and Barry Silverman.
See original licenses for derived code in the MOS 6502 layout and the core transistor emulation logic.
Identifying potential subcomponents for reducing the MOS 6502 is basically impossible without a schematic on hand. I made extensive use this block diagram from an article by Michael Steil and the transistor-level schematic constructed from a die photo by Beregnyei Balazs found here.
