py65/docs/index.rst

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Py65 - 6502 Microprocessor Simulation in Python
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:Author: Mike Naberezny
:Version: |version|

.. topic:: Overview

  Py65 is a simulation of the original NMOS 6502 microprocessor from MOS
  Technology, written in Python.

Using the Monitor
=================

Introduction
------------

Py65 includes a program called Py65Mon that functions as a machine language
monitor. This kind of program is sometimes also called a debugger. Py65Mon
provides a command line with many convenient commands for interacting with the
simulated 6502-based system.

The monitor is started using the ``py65mon`` command::

  $ py65mon

  Py65 Monitor

  <6502: A=00, X=00, Y=00, Flags=20, SP=ff, PC=0000>
  .

Once the monitor has started, it will display a register dump and the
dot prompt.  You can then enter commands for the monitor at this prompt.

Py65Mon uses commands that are very similar to those used by the monitor
included with the `VICE emulator <http://viceteam.org>`_ for Commodore 
computers.  You can get a list of available commands with ``help`` or 
help on a specific command with ``help command``.

Number Systems
--------------

When working with Py65Mon, you will frequently need to enter numbers, addresses,
and ranges of addresses.  Almost all Py65 command support entering numbers in 
binary, decimal, and hexadecimal.

Numbers can be entered with a prefix to specify the radix, e.g. ``$c000`` 
instructs Py65Mon that the number ``c000`` is hexadecimal.  The following
prefixes are supported:

- ``$c000``: The dollar sign indicates hexadecimal.
- ``+828``: The plus sign indicates decimal.
- ``%0101``: The percent sign indicates binary.

Numbers can also be entered without a prefix.  Most of the time, working in
hexadecimal will be the most convenient so this is the default radix.  The
number ``c000`` will be assumed to be hexadecimal unless the default radix
is changed using the ``radix`` command.

Address Ranges
--------------

Some commands accept a range of memory addresses::

  .disassemble ff80:ff84
  $ff80  d8        CLD
  $ff81  a2 ff     LDX #$ff
  $ff83  9a        TXS
  $ff84  a0 1c     LDY #$1c

The syntax for a range is ``start:end``.  Each of the two parts may have
a prefix to indicate the radix, or no prefix to use the default radix.  

Sometimes it is useful to have the starting and ending address in a range
be the same, such as when you want to inspect a single byte of memory.  In
this case, you can enter ``ff80:ff80`` or simply ``ff80``.

Assigning Labels
----------------

Large assembly language programs may have hundreds of procedures. It is
difficult to remember the memory address of each procedure and the addresses
may change if the program is reassembled. 

You can add a label for any memory address using the ``add_label`` command.
This label can then be used anywhere the address could be used::

  .add_label ff80 start

  .disassemble start
  $ff80  d8        CLD

When using labels, you can also specify an offset (plus or minus)::

  .disassemble start:start+4
  $ff80  d8        CLD
  $ff81  a2 ff     LDX #$ff
  $ff83  9a        TXS
  $ff84  a0 1c     LDY #$1c

Offsets are interpreted like any other numbers.  In the example above,
``start+4`` implies that the offset (``4``) uses the default radix.  This
could also be written as ``start+$04`` for explicit hexadecimal.

Command Reference
=================

.. describe:: add_label <address> <label>

  Assign a label to an address::

    .add_label f000 start

  Once defined, the label may be used in place of the address in other
  commands.  If a label already exists at the address, it will be silently
  overwritten.

.. describe:: assemble <address> <statement>
  
  Assemble a statement at the address::

    .assemble c000 lda $a0,x
    $c000  b5 a0     LDA $a0,X
  
  The assembler supports all legal NMOS 6502 opcodes and addressing modes.  If
  you have defined labels with ``add_label``, you may use those labels in the
  address and the operand.

.. describe:: cd <path>

  Change the current working directory to the path specified::
  
    .cd /path/to/my/files
    /path/to/my/files
    
  After changing the directory, the new working directory will be displayed.
  The default working directory is the directory from which the monitor was
  started.

.. describe:: cycles

  Display the number of cycles that the microprocessor has run
  since it was last reset::
  
    .cycles
    12  

.. describe:: delete_label <label>

  Delete a label that was previously defined with ``add_label``::
  
    .delete_label foo
  
  If the label does not exist, the command will fail silently.

.. describe:: disassemble <address_range>

  Disassembles a range of memory.  All legal NMOS 6502 opcodes and addressing
  modes are supported::

    .disassemble ff80:ff84
    $ff80  d8        CLD
    $ff81  a2 ff     LDX #$ff
    $ff83  9a        TXS
    $ff84  a0 1c     LDY #$1c

  If labels have been defined, they will be substituted for
  addresses in the operands.

.. describe:: fill <address_range> <byte> [<byte> <byte> ...]

  Fill a range of memory using one or more bytes from the list::

    .fill c000:c003 aa bb
    Wrote +4 bytes from $c000 to $c003

    .mem c000:c003
    c000:  aa  bb  aa  bb

  If the range is larger than the number of bytes in the list, the list
  will repeat as shown above.

.. describe:: goto <address>

  Set the program counter to an address and resume execution::
  
    .goto c000

.. describe:: help [<command>]

  Display help for all commands or a single command.  If no command is
  given, a list of commands will be displayed::
  
    .help
  
  If a command is given, help for that comand is displayed::
  
    .help disassemble
    disassemble <address_range>
    Disassemble instructions in the address range.

.. describe:: load <filename> <address>

  Load a binary file into memory starting at the address specified::
  
    .load hello.bin c000
    Wrote +29 bytes from $c000 to $c01c

  The file will be loaded relative to the current working directory.  You
  may also specify an absolute path.  If the filename contains spaces, use
  quotes around it::

    .load "say hello.bin" c000
    Wrote +29 bytes from $c000 to $c01c

  .. note:: 
  
    Unlike the VICE monitor, Py65Mon's ``load`` command does not expect
    the first two bytes to be a Commodore-style load address.  It will start
    reading the file at byte 0, not byte 2.

.. describe:: mem <address_range>

  Display the contents of memory an address range::

    .mem ff80:ffa0
    ff80:  d8  a2  ff  9a  a0  1c  b9  bb  ff  99  04  02  88  d0  f7  b9  d8  ff
    ff92:  f0  06  20  a6  e0  c8  d0  f5  20  a3  e0  90  fb  29  df

  The contents will be wrapped to the terminal width specified by the 
  ``width`` command.

.. describe:: pwd

  Display the current working directory::
  
    .pwd
    /home/mnaberez

.. describe:: quit

  Quit the monitor::
  
    .quit

.. describe:: radix [<H|D|O|B>]
  
  Display or set the default radix that is assumed for numbers that have no prefix.  
  If no argument is given, the default radix is displayed::
  
    .radix
    Default radix is Hexadecimal
  
  If an argument is given, the default radix will be changed::
  
    .radix d
    Default radix is Decimal
  
  The default radix may be changed to Hexadecimal, Decimal, Octal, or Binary.  

.. describe:: registers [<name=value>, <name=value>, ...>]
  
  Display or change the registers of the microprocessor.  If no arguments are
  given, the registers are displayed::

    .registers

    <6502: A=00, X=00, Y=00, Flags=20, SP=ff, PC=0000>

  Registers can changed giving ``name=value``, separated by commas if
  multiple registers are to be changed::
  
    .registers a=02, x=04

    <6502: A=02, X=04, Y=00, Flags=20, SP=ff, PC=0000>

.. describe:: reset

  Reset the microprocessor to its default state.  All memory will
  also be cleared::
  
    .reset

.. describe:: return

  Continue execution and return to the monitor just before the next 
  RTS or RTI is executed::

    .return  

.. describe:: show_labels
  
  Display labels that have been defined with ``add_label``::

    .show_labels
    ffd2: charout

.. describe:: step
  
  Execute a single instruction at the program counter.  After the instruction
  executes, the next instruction is disassembled and printed::

    <6502: A=00, X=00, Y=00, Flags=34, SP=fc, PC=0000>
    .registers pc=c000

    <6502: A=00, X=00, Y=00, Flags=34, SP=fc, PC=c000>
    .step
    $c002  a9 42     LDA #$42

    <6502: A=00, X=00, Y=00, Flags=34, SP=fc, PC=c000>
    .
    
  In the example above, the instruction at ``$C000`` executes and the monitor
  prompt returns.  

  .. note:: 

    After the instruction executes, the disassembly of the **next** instruction
    is printed.  This allows you to see what will be executed on the next step.

.. describe:: tilde

  Display a number in the supported number systems::
  
    .~ c000
    +49152
    $c000
    140000
    1100000000000000

  The number will be displayed in this order: decimal, hexadecimal, octal, 
  and then binary.

.. describe:: version

  Display version information::

    .version

    Py65 Monitor

.. describe:: width [<columns>]

  Display or set the terminal width.  The width is used to wrap the output
  of some commands like ``mem``.  With no argument, the current width is
  displayed::
  
    .width
    Terminal width is 78

  If a column count is given, the width will be changed::

    .width 130
    Terminal width is 130

  The number of columns is always specified as a decimal number.