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+-------------------------------------------------------------+
|                                                             |
|                           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.

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.