x6502: a simple 6502 CPU emulator

x6502 is an emulator for the 6502 class of processors. It currently supports the full instruction set of the 6502 (plus a few extensions) and has an extremely rudimentary simulated I/O bus. It should be able to run arbitrary x6502 bytecode with "correct" results, although most binaries for common 6502 systems (Amiga, C64, Apple II, etc) won't function as expected, since they expect I/O devices to be mapped into memory where there are currently none.

Building and running x6502

To build x6502, just run make in the project root. You will need clang and Python installed. To use gcc, change the CC var in the Makefile. No libraries beyond POSIX libc are used. This will produce the x6502 binary.

x6502 takes the compiled 6502 object file as an argument, and runs it until it encounters an EXT instruction (EXT instructions are an extension to 6502 bytecode, see below). You can use any 6502 assembler to compile to 6502 bytecode; xa is one that is bundled with Debian-based distros. Note that, by default, x6502 loads code in at address 0x00; you therefore need to either tell your assembler that that's the base address for the text section of your binary or override the default load address using the -b flag of x6502.

If you want to compile a version of x6502 that dumps machine state after every instruction, run make debug instead of make. This will also disable compiler optimizations.

Extensions to the 6502 instruction set

x6502 recognizes two instructions that are not in the original 6502 instruction set. These are:

• DEBUG (0xFC) prints debugging information about the current state of the emulator
• EXT (0xFF) stops the emulator and exits

Both of these are defined as macros in stdlib/stdio.s. To disable these extensions, compile with -DDISABLE_EXTENSIONS (right now, this can be done by adding that flag to the Makefile).

I/O memory map:

There are only two I/O devices right now: a character input device and a character output device. Convenience constants and macros for all I/O devices are defined in stdlib/stdio.s for use in user programs. Add stdlib/ to your include path and then add #include <stdio.s> to your program to use these constants.

The character output device is mapped to FF00. Any character written to FF00 is immediately echoed to the terminal.

The character input device is mapped to FF01. When a character is available on standard in, an interrupt is raised and FF01 is set to the character that was received. Note that one character is delivered per interrupt; if the user types abc, they will get three interrupts, one after the other.

A commented example of how to use the I/O capabilities of x6502 is provided in test-programs/echo.s.

Source code formatting

The code for opcode handling is a little strange; there are lots of "header" files in the opcode_handlers directory that are not really header files at all. These files all contain code for handling opcode parsing and interpretation; with over 150 opcodes, having all of the code to handle these in one file would be excessive and difficult to navigate, and dispatching out to functions to handle each opcode carries unnecessary overhead in what should be the tightest loop in the project. Thus, each of these header files is #included in emu.c in the middle of a switch statement, and gets access to the local scope within the main_loop function. It's weird but it gets the job done, and is the least bad of all considered options.

TODO:

• support buffered input, where the program can read more than one input character at once.
• graphics? definitely more I/O.