Py65 - 6502 Microprocessor Simulation in Python

Author:Mike Naberezny
Version:0.24

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 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.

Breakpoints

It is possible to set breakpoints to stop execution when reaching a given address or label. Breakpoints are added using the add_breakpoint command:

.disassemble start:start+4
$ff80  d8        CLD
$ff81  a2 ff     LDX #$ff
$ff83  9a        TXS
$ff84  a0 1c     LDY #$1c
.add_breakpoint $ff84
Breakpoint 0 added at $FF84
.goto $ff80
Breakpoint 0 reached.
       PC  AC XR YR SP NV-BDIZC
6502: ff84 00 ff 00 ff 10110000

Note that a number is assigned to each breakpoint, similar to how VICE operates. Deleting a breakpoint can be done via the delete_breakpoint command using the breakpoint identifier given by add_breakpoint:

.add_breakpoint $ff84
Breakpoint 0 added at $FF84
.delete_breakpoint 0
Breakpoint 0 removed

Breakpoint can be listed using the list_breakpoint command:

.add_breakpoint $1234
Breakpoint 0 added at $1234
.add_breakpoint $5678
Breakpoint 1 added at $5678
.add_breakpoint $9ABC
Breakpoint 2 added at $9ABC
.show_breakpoints
Breakpoint 0 : $1234
Breakpoint 1 : $5678
Breakpoint 2 : $9ABC

Keep in mind that breakpoint identifiers are not recycled throughout a session, this means that if you add three breakpoints (#0, #1, #2) and then delete breakpoint #1, the next breakpoint you add will be breakpoint #3, not #1. Also, invoking reset clears breakpoints too, not just labels.

Command Reference

add_breakpoint <address|label>

Sets a breakpoint on execution at the given address or at the address represented by the given label:

.add_breakpoint $1234
.add_label f000 start
.add_breakpoint start

Breakpoints get a numeric identifier to be used with delete_breakpoint, the list of identifiers can be retrieved with show_breakpoints.

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.

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.

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.

cycles

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

.cycles
12
delete_breakpoint <breakpoint_id>

Removes the breakpoint associated with the given identifier:

.add_breakpoint $1234
Breakpoint 0 added at $1234
.add_label f000 start
.add_breakpoint start
Breakpoint 1 added at $F000
.delete_breakpoint 0
Breakpoint 0 removed

The list of identifiers added with add_breakpoint can be retrieved with show_breakpoints.

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.

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.

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.

goto <address>

Set the program counter to an address and resume execution:

.goto c000
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.
show_breakpoints

Lists all the breakpoints that have been set so far:

.add_breakpoint $1234
Breakpoint 0 added at $1234
.add_breakpoint $5678
Breakpoint 1 added at $5678
.add_breakpoint $9ABC
Breakpoint 2 added at $9ABC
.show_breakpoints
Breakpoint 0 : $1234
Breakpoint 1 : $5678
Breakpoint 2 : $9ABC
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
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.

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.

pwd

Display the current working directory:

.pwd
/home/mnaberez
quit

Quit the monitor:

.quit
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.

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
reset

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

.reset
return

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

.return
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.

show_labels

Display labels that have been defined with add_label:

.show_labels
ffd2: charout
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.

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.

version

Display version information:

.version

Py65 Monitor
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.