Editing Threaded code (section)

===Indirect threading===
Indirect threading uses pointers to locations that in turn point to machine code. The indirect pointer may be followed by operands which are stored in the indirect "block" rather than storing them repeatedly in the thread. Thus, indirect code is often more compact than direct-threaded code.  The indirection typically makes it slower, though usually still faster than bytecode interpreters. Where the handler operands include both values and types, the space savings over direct-threaded code may be significant. Older FORTH systems typically produce indirect-threaded code.

For example, if the goal is to execute "push A, push B, add", the following might be used. Here, <code>ip</code> is initialized to address <code>&thread</code>, each code fragment (<code>push</code>, <code>add</code>) is found by double-indirecting through <code>ip</code> and an indirect block; and any operands to the fragment are found in the indirect block following the fragment's address. This requires keeping the ''current'' subroutine in <code>ip</code>, unlike all previous examples where it contained the ''next'' subroutine to be called.

<syntaxhighlight lang="c">
start:
  ip = &thread  // points to '&i_pushA'
  jump *(*ip)  // follow pointers to 1st instruction of 'push', DO NOT advance ip yet
thread:
  &i_pushA
  &i_pushB
  &i_add
  ...
i_pushA:
  &push
  &A
i_pushB:
  &push
  &B
i_add:
  &add
push:
  *sp++ = *(*ip + 1)  // look 1 past start of indirect block for operand address
  jump *(*++ip)  // advance ip in thread, jump through next indirect block to next subroutine
add:
  addend1 = *--sp
  addend2 = *--sp
  *sp++ = addend1 + addend2
  jump *(*++ip)
</syntaxhighlight>