14.2.2 Direct or Indirect Threaded?

Threaded forth code consists of references to primitives (simple machine code routines like +) and to non-primitives (e.g., colon definitions, variables, constants); for a specific class of non-primitives (e.g., variables) there is one code routine (e.g., dovar), but each variable needs a separate reference to its data.

Traditionally Forth has been implemented as indirect threaded code, because this allows to use only one cell to reference a non-primitive (basically you point to the data, and find the code address there).

However, threaded code in Gforth (since 0.6.0) uses two cells for non-primitives, one for the code address, and one for the data address; the data pointer is an immediate argument for the virtual machine instruction represented by the code address. We call this primitive-centric threaded code, because all code addresses point to simple primitives. E.g., for a variable, the code address is for lit (also used for integer literals like 99).

Primitive-centric threaded code allows us to use (faster) direct threading as dispatch method, completely portably (direct threaded code in Gforth before 0.6.0 required architecture-specific code). It also eliminates the performance problems related to I-cache consistency that 386 implementations have with direct threaded code, and allows additional optimizations.

There is a catch, however: the xt parameter of execute can occupy only one cell, so how do we pass non-primitives with their code and data addresses to them? Our answer is to use indirect threaded dispatch for execute and other words that use a single-cell xt. So, normal threaded code in colon definitions uses direct threading, and execute and similar words, which dispatch to xts on the data stack, use indirect threaded code. We call this hybrid direct/indirect threaded code.

The engines gforth and gforth-fast use hybrid direct/indirect threaded code. This means that with these engines you cannot use , to compile an xt. Instead, you have to use compile,.

If you want to compile xts with ,, use gforth-itc. This engine uses plain old indirect threaded code. It still compiles in a primitive-centric style, so you cannot use compile, instead of , (e.g., for producing tables of xts with ] word1 word2 ... [). If you want to do that, you have to use gforth-itc and execute ' , is compile,. Your program can check if it is running on a hybrid direct/indirect threaded engine or a pure indirect threaded engine with threading-method (see Threading Words).