SWI-Prolog has a number of memory areas which are only enlarged to a
certain limit. The default sizes for these areas should suffice for most
applications, but big applications may require larger ones. They are
modified by command line options. The table below shows these areas. The
first column gives the option name to modify the size of the area. The
option character is immediately followed by a number and optionally by a
k or m. With k or no unit
indicator, the value is interpreted in Kbytes (1024 bytes), with m,
the value is interpreted in Mbytes (1024 × 1024 bytes).
The local-, global- and trail-stack are limited to 128 Mbytes on 32 bit processors, or more in general to 2 ** bits-per-long - 5 bytes.
| Option | Default | Area name | Description |
| -L | 2M | local stack | The local stack is used to store the execution environments of procedure invocations. The space for an environment is reclaimed when it fails, exits without leaving choice points, the alternatives are cut of with the !/0 predicate or no choice points have been created since the invocation and the last subclause is started (tail recursion optimisation). |
| -G | 4M | global stack | The global stack is used to store terms created during Prolog's execution. Terms on this stack will be reclaimed by backtracking to a point before the term was created or by garbage collection (provided the term is no longer referenced). |
| -T | 4M | trail stack | The trail stack is used to store assignments during execution. Entries on this stack remain alive until backtracking before the point of creation or the garbage collector determines they are nor needed any longer. |
| -A | 1M | argument stack | The argument stack is used to store one of the intermediate code interpreter's registers. The amount of space needed on this stack is determined entirely by the depth in which terms are nested in the clauses that constitute the program. Overflow is most likely when using long strings in a clause. |
With the heap, we refer to the memory area used by malloc() and friends. SWI-Prolog uses the area to store atoms, functors, predicates and their clauses, records and other dynamic data. As of SWI-Prolog 2.8.5, no limits are imposed on the addresses returned by malloc() and friends.
On some machines, the runtime stacks described above are allocated using `sparse allocation'. Virtual space upto the limit is claimed at startup and committed and released while the area grows and shrinks. On Win32 platform this is realised using VirtualAlloc() and friends. On Unix systems this is realised using mmap().
max_tagged_integer feature are represented more
efficiently.
The boot compiler (see -b option) does not support
the module system. As large parts of the system are written in Prolog
itself we need some way to avoid name clashes with the user's
predicates, database keys, etc. Like Edinburgh C-Prolog Pereira,
1986 all predicates, database keys, etc. that should be
hidden from the user start with a dollar ($) sign (see style_check/1).
The compiler uses the special functor $VAR$/1 while
analysing the clause to compile. Using this functor in a program causes
unpredictable behaviour of the compiler and resulting program.