The WAM debugger

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.