git.haldean.org x6502 / video
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Tree @video (Download .tar.gz)

+-------------------------------------------------------------+
|                                                             |
|                           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 (Atari,
    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 sample_programs/echo.s.

Reading the source

    x6502 was written to be easy to understand and read. A
    good place to start is `cpu.h', which defines a few
    constants used throughout the code (mostly around CPU
    flags) as well as the `cpu' struct, which is used pretty
    much everywhere.
    
    `emu.c' is where the interesting stuff happens; this is
    the main loop of the emulator where opcodes are decoded
    as dispatched. It also handles interrupts and calls out
    to I/O handlers.

    The code for actual opcode interpretation 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.

    The opcode handlers all use convenience functions
    defined in `functions.h', most of which are for the
    various addressing modes of the 6502 or for dealing with
    CPU flags.

    `io.c' is where the I/O bus lives; this is where we
    check to see if the emulated character device has been
    written to and where we raise an interrupt if we've
    gotten input from stdin.

    `generate_debug_names.py' reads the `opcodes.h' header
    and generates `debug-names.h', which contains a mapping
    from opcode to a string representation of that opcode.
    It's only used when dumping CPU state, either because
    the DEBUG flag was set at compile time or because a
    DEBUG instruction was hit in the binary.

    The rest of the files are pretty boring; `main.c' is
    pretty much only responsible for loading bytecode into
    memory and parsing command line arguments and `debug.c' is
    used to provide the `dump_cpu' function, which is a
    fascinating function consisting of almost nothing but
    printfs.

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

THANKS:
    - voltagex on Github for sending me a patch to improve
      the sample_programs readme.
    - anatoly on HN for suggesting I add a bit on source
      code structure to the README.