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

.. topic:: Overview

  Simulate 6502-based microcomputer systems 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

         PC  AC XR YR SP NV-BDIZC
  6502: 0000 00 00 00 ff 00110000
  .

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 single statement at an address::

    .assemble c000 lda $a0,x
    $c000  b5 a0     LDA $a0,X

  If no statement is given, interactive assembly mode will start::

    .assemble c000
    $c000

  Enter a statement and it will be assembled at the current address.  The
  address will then be incremented and another statement may be entered.
  Press Enter or Return without entering a statement to exit interactive
  assembly mode.

  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>

  Disassemble a range of memory::

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

  The disassembly will use the instruction set of the selected MPU.  For
  example, the extra instructions of the 65C02 will only be displayed if
  a 65C02 MPU is selected.  On an NMOS 6502, those instructions would be
  disassembled as ``???``.

  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 data at byte 0, not byte 2.

  If the filename is a URL, it will be retrieved::

    .load https://github.com/mnaberez/py65/raw/0.11/examples/ehbasic.bin 0000
    Wrote +65536 bytes from $0000 to $ffff

.. 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:: mpu [<mpu_name>]

  Display or set the current microprocessor.  If no argument is given, the
  current microprocessor will be displayed::

    .mpu
    Current MPU is 6502
    Available MPUs: 6502, 65C02, 65Org16

  If an argument is given, the microprocessor will be changed::

    .mpu 65C02
    Reset with new MPU 65C02

  The default microprocessor is ``6502``, the original NMOS 6502 from
  MOS Technology.

.. 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

           PC  AC XR YR SP NV-BDIZC
    6502: 0000 00 00 00 ff 00110000

  Registers can changed giving ``name=value``, separated by commas if
  multiple registers are to be changed::

    .registers a=02, x=04

           PC  AC XR YR SP NV-BDIZC
    6502: 0000 02 04 00 ff 00110000

.. 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:: save <filename> <start_address> <end_address>

  Save the specified memory range to disk as a binary file::

    .save hello.bin c000 c01c
    Wrote +29 bytes from $c000 to $c01c

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

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

  .. note::

    Unlike the VICE monitor, Py65Mon's ``save`` command does not write
    the first two bytes as a Commodore-style load address.  It will start
    writing the data at byte 0, not byte 2.

.. 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::

           PC  AC XR YR SP NV-BDIZC
    6502: 0000 00 00 00 ff 00110000
    .registers pc=c000

           PC  AC XR YR SP NV-BDIZC
    6502: c000 00 00 00 ff 00110000
    .step
    $c002  a9 42     LDA #$42

           PC  AC XR YR SP NV-BDIZC
    6502: c002 00 00 00 ff 00110000
    .

  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.