Py65 - 6502 Microprocessor Simulation in Python
Simulate 6502-based microcomputer systems in Python.
Using the Monitor
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 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 on a specific command with
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.
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
c000 will be assumed to be hexadecimal unless the default radix
is changed using the
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
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
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.
It is possible to set breakpoints to stop execution when reaching a
given address or label. Breakpoints are added using the
.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
.add_breakpoint $ff84 Breakpoint 0 added at $FF84 .delete_breakpoint 0 Breakpoint 0 removed
Breakpoint can be listed using the
.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.