---
title: User scripts
---

## Introduction

pyOCD has support for customization through what are called user scripts. These are Python scripts
that are loaded by pyOCD before it connects to the target. A user script can define functions that
are called as hooks at certain points in the connection lifetime, and can extend, modify, or
completely override the default behaviour.

If a file named `pyocd_user.py` or `.pyocd_user.py` is placed in the project directory, pyOCD will
automatically detect it and load it as a user script. If you prefer another name, you can set the
`user_script` session option, for example in a [config file]({% link _docs/configuration.md %}), or
by passing the `--script=<path>` command line argument. If a relative path is set either with the
option or command line, it will be searched for in the project directory.


## Examples

This example user script shows how to add a new memory region.

```py
# This example applies to the Nordic nRF52 devices.

def will_connect(board):
    # Create the new ROM region for the FICR.
    ficr = RomRegion(
                name="ficr",
                start=0x10000000,
                length=0x460
                )

    # Add the FICR region to the memory map.
    target.memory_map.add_region(ficr)
```

This example shows how to override the flash algorithm for an external flash memory.

```py
# This example applies to the NXP i.MX RT10x0 devices.

# Unlike the previous example, the board argument is excluded here.
def will_connect():
    # Look up the external flash memory region.
    extFlash = target.memory_map.get_first_matching_region(name="flexspi")

    # Set the path to an .FLM flash algorithm.
    extFlash.flm = "MIMXRT105x_QuadSPI_4KB_SEC.FLM"
```

This example demonstrates setting the DBGMCU_CR register after connecting for STM32 devices.

```py
# This example applies to the ST STM32L0x1 devicess.

DBG_CR = 0x40015804

def did_connect():
    # Set STANDBY, STOP, and SLEEP bits all to 1.
    target.write32(DBG_CR, 0x7)
```

Another common use for a script is to initialize external memory such as SDRAM.


## User-defined commands

New commands accessible from the commander subcommand or gdbserver monitor commands can be easily created in a user
script.

User defined commands are created by using the `@command()` decorator on a function. The name of the new command can
either be the same as the name of the decorated function, or can be set explicitly with a `name` (or first positional)
argument to the decorator. For instance, either `@command('mycmd')` or `@command(name='anothercmd')`. Note that the
decorator requires parentheses (it must be called as a function) even if there are no parameters.

Parameters for the new command are automatically determined using introspection and type annotations. Arguments for
parameters of these types are converted to the appropriate type before the function is called as a command.

Supported parameter types:
- `int`
- `float`
- `str`
- Variable arguments, e.g. `*args`.

Keyword parameters are not allowed.

An `int` parameter is converted using the same method as for other pyOCD commands. Hexadecimal and binary numbers are
allowed, digits can be separated by underscores, and so on. For variable arguments, type annotations are ignored and the
tuple passed to the function will contain strings as entered in the command invocation.

The decorated function remains accessible as a regular function in the user script namespace, and is therefore callable
from other functions within the user script. This is true even if the function definition is not compatible with the
command decorator, for instance if it has invalid parameter types.

Help for the new command can be specified by passing a `help` argument to the `@command` decorator.

Example:

```py
@command(help="Decode and print the first few vectors")
def vectable(base: int):
    vecs = target.read_memory_block32(base, 4)
    print(f"Initial SP:     {vecs[0]:#010x}")
    print(f"ResetHandler:   {vecs[1]:#010x}")
    print(f"NMI:            {vecs[2]:#010x}")
    print(f"HardFault:      {vecs[3]:#010x}")
```


## Script globals

A number of useful symbols are made available in the global namespace of user scripts. These include
both target related objects, as well as parts of the pyOCD Python API.

The usual Python builtins are available.

### Objects and functions

| Symbol | Description |
|--------|-------------|
| `aps` | Dictionary of CoreSight Access Port (AP) objects. The keys are the APSEL value. |
| `board` | The `Board` object. |
| `command` | Decorator for defining new commands. See [user-defined commands](#user_defined_commands) for details. |
| `debug` | Log a debug message. |
| `dp` | The CoreSight Debug Port (DP) object. |
| `error` | Output an error log. |
| `info` | Output an info-level log message. |
| `LOG` | `Logger` object for the user script. |
| `options` | The session options dictionary. |
| `probe` | The connected debug probe object. |
| `session` | The session object, which is the root of the connection object graph. |
| `target` | The `CoreSightTarget` or subclass instance representing the MCU. |
| `warning` | Log a warning. |

### Modules and classes

| Symbol | Description |
|--------|-------------|
| `BreakpointType` | Enumeration of breakpoint types. |
| `DeviceRegion` | Device-type memory region class. |
| `Error` | The base class for all pyOCD exceptions. |
| `Event` | Enumeration of notification event types. |
| `exceptions` | Module containing the exception classes. |
| `FileProgrammer` | Utility class to program files to target flash. |
| `FlashEraser` | Utility class to erase target flash. |
| `FlashLoader` | Utility class to program raw binary data to target memory. Deprecated, use `MemoryLoader` instead. |
| `FlashRegion` | Flash memory region. |
| `HaltReason` | Enumeration of halt reasons. |
| `MemoryLoader` | Utility class to program raw binary data to target memory. |
| `MemoryMap` | Class representing the device's memory map. |
| `MemoryType` | Memory region type enumeration. |
| `pyocd` | The root pyOCD module. |
| `RamRegion` | RAM memory region. |
| `ResetType` | Reset type enumeration. |
| `RomRegion` | ROM memory region. |
| `RunType` | Enumeration of types of run operations (step or run). |
| `SecurityState` | Enumeration of core security states. |
| `State` | Enumeration of target state. |
| `Target` | Base target class. |
| `TransferError` | Exception class for all transfer errors. |
| `TransferFaultError` | Exception subclass of `TransferError` for memory transfer faults. |
| `VectorCatch` | Namespace class containing bit mask constants for vector catch options. |
| `WatchpointType` | Enumeration of watchpoint types. |


## Delegate functions

This section documents all functions that user scripts can provide to modify pyOCD's behaviour. Some are simply
notifications, while others allow for overriding of default behaviour. Collectively, these are called delegate functions.

All parameters of user script delegate functions are optional. Parameters can be declared in any order, and
those that are not needed can be excluded. In fact, most parameters are not necessary because the same objects
are available as [script globals](#script_globals), for instance `session` and `target`.

Those parameters that are present must have names matching the specification below, and there must not be
additional unspecified, required parameters (those without a default value). Extra optional parameters are
allowed but will never be passed any value other than the default, unless you call the function yourself from
within the script.

_Note:_ Delegate functions override CMSIS-Pack debug sequences. See the [debug sequence documentation]({% link _docs/open_cmsis_pack_support.md %}#debug-access-sequences) for more details.


### will_connect

Pre-init notification for the board.
```
will_connect(board: Board) -> None
```

**Parameters** \
*board* - A `Board` instance that is about to be initialized. \
**Result** \
Ignored.

### did_connect

Post-initialization notification for the board.
```
did_connect(board: Board) -> None
```

**Parameters** \
*board* - A `Board` instance. \
**Result** \
Ignored.

### will_init_target

Hook to review and modify init call sequence prior to execution.
```
will_init_target(target: SoCTarget, init_sequence: CallSequence) -> None
```

**Parameters** \
*target* - An `SoCTarget` object about to be initialized. \
*init_sequence* - The `CallSequence` that will be invoked. Because call sequences are
  mutable, this parameter can be modified before return to change the init calls. \
**Result** \
Ignored.

### did_init_target

Post-initialization notification.
```
did_init_target(target: SoCTarget) -> None
```

**Parameters** \
*target* - An `SoCTarget` object. \
**Result** \
Ignored.

### unlock_device

Hook to perform any required unlock sequence.
```
unlock_device(target: SoCTarget) -> None
```

**Parameters** \
*target* - An `SoCTarget` object. \
**Result** \
Ignored.

Called after the DP is initialised but prior to discovery. This hook delegate can be used to unlock debug
access to the target. It can also be used to perform other pre-discovery actions.

Note that because this hoook is called prior to discovery, APs and cores are not yet created. This means
that any register accesses must be performed through the DP's methods. (However, it's possible to create
a temporary instance of 'AccessPort' or one of its subclasses, such as `MEM_AP`.)

### will_start_debug_core

Notification hook for before core debug is enabled.

```
will_start_debug_core(core: CoreTarget) -> None
```

**Parameters** \
*core* - A `CoreTarget` object about to be initialized. \
**Result** \
Ignored.

This hook is called during connection, prior to any register accesses being performed on the
indicated core (aside from the CoreSight peripheral ID registers that were read to identify
the core's presence during discovery).

### start_debug_core

Hook to enable debug for the given core.
```
start_debug_core(core: CoreTarget) -> Optional[bool]
```

**Parameters** \
*core* - A `CoreTarget` object about to be initialized. \
**Result** \
*True* Do not perform the normal procedure to start core debug. \
*False/None* Continue with normal behaviour.

### did_start_debug_core

Notification hook that core debug has been enabled.
```
did_start_debug_core(core: CoreTarget) -> None
```

**Parameters** \
*core* - A `CoreTarget` object. \
**Result** \
Ignored.

This hook method is called once a debug has been enabled for a core, and it has been fully
identified.

### will_stop_debug_core

Pre core disconnect notification hook for the core.
```
will_stop_debug_core(core: CoreTarget) -> None
```

**Parameters** \
*core* - A `CoreTarget` object. \
**Result** \
Ignored.

### stop_debug_core

Core debug disable hook.
```
stop_debug_core(core: CoreTarget) -> Optional[bool]
```

**Parameters** \
*core* - A `CoreTarget` object. \
**Result** \
*True* Do not perform the normal procedure to disable core debug. \
*False/None* Continue with normal behaviour.

This delegate is only called if resuming the core on disconnect, e.g. the `resume_on_disconnect` session
option is True. Therefore, the delegate should ensure that the core has properly resumed execution if it
returns True.

### did_stop_debug_core

Post core disconnect notification hook for the core.
```
did_stop_debug_core(core: CoreTarget) -> None
```

**Parameters** \
*core* - A `CoreTarget` object. \
**Result** \
Ignored.

### will_disconnect

Pre-disconnect notification.
```
will_disconnect(target: SoCTarget, resume: bool) -> None
```

**Parameters** \
*target* - An `SoCTarget` object. \
*resume* - The value of the `disconnect_on_resume` option. \
**Result** \
Ignored.

### did_disconnect

Post-disconnect notification.
```
did_disconnect(target: SoCTarget, resume: bool) -> None
```

**Parameters** \
*target* - An `SoCTarget` object. \
*resume* - The value of the `disconnect_on_resume` option. \
**Result** \
Ignored.

### will_reset

```
will_reset(core: CoreTarget, reset_type: Target.ResetType) -> Optional[bool]
```
Pre-reset hook.

**Parameters** \
*core* - A `CoreTarget` instance. \
*reset_type* - One of the `Target.ResetType` enumerations. \
**Result** \
*True* The hook performed the reset.  \
*False/None* Caller should perform the normal reset procedure.

### did_reset

Post-reset notification.
```
did_reset(core: CoreTarget, reset_type: Target.ResetType) -> None
```

**Parameters** \
*core* - A `CoreTarget` instance. \
*reset_type* - One of the `Target.ResetType` enumerations. \
**Result** \
Ignored.

### set_reset_catch

Hook to prepare target for halting on reset.
```
set_reset_catch(core: CoreTarget, reset_type: Target.ResetType) -> Optional[bool]
```

**Parameters** \
*core* - A `CoreTarget` instance. \
*reset_type* - One of the `Target.ResetType` enumerations. \
**Result** \
*True* This hook handled setting up reset catch, caller should do nothing. \
*False/None* Perform the default reset catch set using vector catch.

### clear_reset_catch

Hook to clean up target after a reset and halt.
```
clear_reset_catch(core: CoreTarget, reset_type: Target.ResetType) -> None
```

**Parameters** \
*core* - A `CoreTarget` instance. \
*reset_type* - One of the `Target.ResetType` enumerations. \
**Result** \
Ignored.

### mass_erase

Hook to override mass erase.
```
mass_erase(target: SoCTarget) -> Optional[bool]
```

**Parameters** \
*target* - An `SoCTarget` object. \
**Result** \
*True* Indicate that mass erase was performed by the hook. \
*False/None* Mass erase was not overridden and the caller should proceed with the standard mass erase procedure.

### trace_start

Notification to prepare for tracing the target.
```
trace_start(target: SoCTarget, mode: int) -> None
```

**Parameters** \
*target* - A `CoreSightTarget` object. \
*mode* - The trace mode. Currently always 0 to indicate SWO. \
*Result* - Ignored.

### trace_stop

Notification to clean up after tracing the target.
```
trace_stop(target: SoCTarget, mode: int) -> None
```

**Parameters** \
*target* - A `CoreSightTarget` object. \
*mode* - The trace mode. Currently always 0 to indicate SWO. \
**Result** \
Ignored.