Question

So I’m just learning Forth and was curious if anyone could help me understand how memory management generally works. At the moment I only have (some) experience with the C stack-vs-heap paradigm.

From what I understand, one can allocate in the Dictionary, or on the heap. Is the Dictionary faster/preferred like the stack in C? But unlike in C, there aren’t scopes and automatic stack reclamation, so I’m wondering if one only uses the dictionary for global data structures (if at all).

As far as the heap goes, is it pretty much like C? Is heap management a standard (ANS) concept, or is it implementation-defined?

Answer 1

It is not Dictionary, or on the heap – the equivalent of the heap is the dictionary. However, with the severe limitation that it acts more like a stack than a heap – new words are added to the end of the dictionary (allocation by ALLOT and freeing by FORGET or FREE (but freeing all newer words – acting more like multiple POPs)).

An implementation can control the memory layout and thus implement a traditional heap (or garbage collection). An example is A FORTH implementation of the Heap Data Structure for Memory Management (1984). Another implementation is Dynamic Memory Heaps for Quartus Forth (2000).

A lot is implementation dependent or extensions. For instance, the memory layout is often with the two block buffers (location by BLOCK and TIB), the text input buffer and values and low-level/primitive functions of the language, in the lowest portion, dictionary in the middle (growing upwards) and the return stack and the parameter stack at the top ¹.

The address of the first available byte above the dictionary is returned by HERE (it changes as the dictionary expands).

There is also a scratchpad area above the dictionary (address returned by PAD) for temporarily storing data. The scratchpad area can be regarded as free memory.

The preferred mode of operation is to use the stack as much as possible instead of local variables or a heap.

¹ p. 286 (about a particular edition of Forth, MMSFORTH) in chapter "FORTH’s Memory, Dictionary, and Vocabularies", Forth: A text and a reference. Mahlon G. Kelly and Nicholas Spies. ISBN 0-13-326349-5 / 0-13-326331-2 (pbk.). 1986 by Prentice-Hall.