### 4.6 The WAM debugger

In some cases it is interesting to have access to the WAM data structures. This sub-debugger allows the user to inspect/modify the contents of any stack or register of the WAM. The WAM debugger is invoked using the built-in predicate `wam_debug/0` (section 4.3.1) or the `W` debugger command (section 4.5). The following table presents the specific commands of the WAM debugger:

Command | Description
`write` `A` [`N`] | write `N` terms starting at the address `A` using `write/1` (section 7.14.6)
`data` `A` [`N`] | display `N` words starting at the address `A`
`modify` `A` [`N`] | display and modify `N` words starting at the address `A`
`where` `A` | display the real address corresponding to `A`
`what` `RA` | display what corresponds to the real address `RA`
`deref` `A` | display the dereferenced word starting at the address `A`
`envir` [`SA`] | display the contents of the environment located at `SA` (or the current one)
`backtrack` [`SA`] | display the contents of the choice-point located at `SA` (or the current one)
`backtrack all` | display all choice-points
`quit` | quit the WAM debugger
`help` | display a summary of available commands

In the above table the following conventions apply:

- elements between [ and ] are optional.
- `N` is an optional integer (defaults to 1).
- `A` is a WAM address, its syntax is: `BANK_NAME` [ `[ N ]` ], i.e. a bank name possibly followed by an index (defaults to 0). `BANK_NAME` is either:
  - `reg`: WAM general register (stack pointers, continuation, ...).
  - `x`: WAM X register (temporary variables, i.e. arguments).
  - `y`: WAM Y register (permanent variables).
  - `ab`: WAM X register saved in the current choice-point.
  - `STACK_NAME`: WAM stack (`STACK_NAME` in `local`, `global`, `trail`, `cstr`).
- `SA` is a WAM stack address, i.e. `STACK_NAME` [ `[ N ]` ] (special case of WAM addresses).
- `RA` is a real address, its syntax is the syntax of C integers (in particular the notation `0x…` is recognized).

It is possible to only use the first letters of a commands and bank names when there is no ambiguity. Also the square brackets `[` `]` enclosing the index of a bank name can be omitted. For instance the following command (showing the contents of 25 consecutive words of the global stack from the index 3): `data global[3] 25` can be abbreviated as: `d g 3 25`.

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