This is my attempt at a cycle-accurate 6502.
My ultimate goal is to either build an NES emulator or a Commodor 64 emulator.
The 6502 and its variants were integrated into all sorts of different systems, such as the Apple II, the Atari 2600, the NES, and the Commodore 64. Often, these systems used memory or register banking to extend number of addressable peripherals beyond what could be reached with the 16 address pins.
Accordingly, you'll notice the cpu_tick() and cpu_step() functions receive a pointer to a bus. While the CPU code uses bus_peek() and bus_poke() to read and write data, it doesn't assume any specific bus implementation. You can provide a bus implementation that handles memory banking or capture register reads/wrtes to forward to other components.
You'll find example bus implementations inside:
example/main.ctest/6502_functional_test.ctest/test.c
These are simple examples, but it should give you an idea of how more complex buses could be constructed.
I use a VS Code dev container to work on this project.
If you have Docker, VS Code, and the Remote Container Extension, you shouldn't need to install anything else on your machine. Open this project in the dev container and have fun!
If you have...
makegcc- a typical *nix environment
...you should be good to go. You will need dasm in order to assemble the example program.
make test will build and run all the tests.
There are a few unit tests for the bus and cpu inside test/bus_test.c and cpu/cpu_test.c. However, the most meaningful test comes from test/6502_functional_test.c. This test verifies the functionality of all legal opcodes and address modes. It runs the functional test program that can be found in this repo. Thank you @Klaus2m5!
The code in this repo is meant to be integrated into other code. It doesn't produce a final artifact.
You can choose to build a shared library, use git submodules, or copy the code directly.
The Example section describes a small, example computer provided in this repo.
Compy is a small, example computer that utilizes this code. It's implementation can be found in example/main.c.
On startup, it will load a 64K image and set the PC to the address found in the reset vector (0xFFFC). Compy will run the program until it detects that the CPU is in a branch-to-self loop, at which point it will print out the contents of address 0x0000.
The example program at example/program.asm computes 1 + 2. This example program uses the dasm assembler.
make example will build Compy, assemble the example program, and run it.