by kragen | 2025-09-22 15:44:22+00:00
You're describing the outer interpreter in interpretation state; Forth control flow words don't work properly in interpretation state, only in compile state. They're immediate words, so they execute at compile time instead of run time, so they can do arbitrary things to the code being compiled. Here's Mike Perry and Henry Laxen's implementation of the main control-flow words from F83, which is an indirect-threaded Forth:

\ Run Time Code for Control Structures 04OCT83HHL \ \ Run Time Code for Control Structures 05MAR83HHL CODE BRANCH (S -- ) \ BRANCH Performs an unconditional branch. Notice that we LABEL BRAN1 0 [IP] IP MOV NEXT END-CODE \ are using absolute addresses insead of relative ones. (fast) CODE ?BRANCH (S f -- ) \ ?BRANCH Performs a conditional branch. If the top of the AX POP AX AX OR BRAN1 JE IP INC IP INC NEXT END-CODE \ parameter stack in True, take the branch. If not, skip \ over the branch address which is inline. \ Extensible Layer Structures 03Apr84map \ \ Extensible Layer Structures 03Apr84map : ?CONDITION (S f -- ) \ ?CONDITION NOT ABORT" Conditionals Wrong" ; \ Simple compile time error checking. Usually adequate : >MARK (S -- addr ) HERE 0 , ; \ >MARK Set up for a Forward Branch : >RESOLVE (S addr -- ) HERE SWAP ! ; \ >RESOLVE Resolve a Forward Branch : <MARK (S -- addr ) HERE ; \ <MARK Set up for a Backwards Branch : <RESOLVE (S addr -- ) , ; \ <RESOLVE Resolve a Backwards Branch : ?>MARK (S -- f addr ) TRUE >MARK ; \ ?>MARK Set up a forward Branch with Error Checking : ?>RESOLVE (S f addr -- ) SWAP ?CONDITION >RESOLVE ; \ ?>RESOLVE Resolve a forward Branch with Error Checking : ?<MARK (S -- f addr ) TRUE <MARK ; \ ?<MARK Set up for a Backwards Branch with Error Checking : ?<RESOLVE (S f addr -- ) SWAP ?CONDITION <RESOLVE ; \ ?<RESOLVE Resolve a backwards Branch with Error Checking : LEAVE COMPILE (LEAVE) ; IMMEDIATE \ LEAVE and ?LEAVE could be non-immediate in this system, : ?LEAVE COMPILE (?LEAVE) ; IMMEDIATE \ but the 83 standard specifies an immediate LEAVE, so they \ both are for uniformity. \ Extensible Layer Structures 01Oct83map \ \ Extensible Layer Structures 27JUL83HHL : BEGIN ?<MARK ; IMMEDIATE \ These are the compiling words needed to properly compile : THEN ?>RESOLVE ; IMMEDIATE \ the Forth Conditional Structures. Each of them is immediate : DO COMPILE (DO) ?>MARK ; IMMEDIATE \ and they must compile their runtime routines along with : ?DO COMPILE (?DO) ?>MARK ; IMMEDIATE \ whatever addresses they need. A modest amount of error : LOOP \ checking is done. If you want to rip out the error checking COMPILE (LOOP) 2DUP 2+ ?<RESOLVE ?>RESOLVE ; IMMEDIATE \ change the ?> and ?< words to > and < words, and : +LOOP \ all of the 2DUPs to DUPs and the 2SWAPs to SWAPs. The rest COMPILE (+LOOP) 2DUP 2+ ?<RESOLVE ?>RESOLVE ; IMMEDIATE \ should stay the same. : UNTIL COMPILE ?BRANCH ?<RESOLVE ; IMMEDIATE : AGAIN COMPILE BRANCH ?<RESOLVE ; IMMEDIATE : REPEAT 2SWAP [COMPILE] AGAIN [COMPILE] THEN ; IMMEDIATE : IF COMPILE ?BRANCH ?>MARK ; IMMEDIATE : ELSE COMPILE BRANCH ?>MARK 2SWAP ?>RESOLVE ; IMMEDIATE : WHILE [COMPILE] IF ; IMMEDIATE

When the interpreter is toodling along in compile state, compiling a colon definition by stowing pointers one after another (at the pointer here) into the definition of some word you're compiling, and it encounters an if, it sees that if is immediate, and so instead of stowing a pointer to if it just runs it immediately. The definition of if is compile ?branch ?>mark. compile is also an immediate word [correction, no, it's not, see below comment, though the following is still correct]; compile ?branch stows a pointer to ?branch into the colon definition being compiled, and then ?>mark writes a 0 into the entry following the ?branch and pushes true and the address of the 0 on the operand stack, at compile time, with the sequence true here 0 ,. The interpreter toodles along compiling the body of the if and eventually gets to, for example, then, which is also immediate, and is defined as ?>resolve, which overwrites the 0 into the address of the indirect-threaded code that will be compiled following the then. It does this with swap ?condition here swap !. The swap ?condition part aborts with an error if there isn't an unresolved if or similar on the stack to resolve, consuming the true, leaving only the address of the 0 that ?>mark had pushed. So then here swap ! overwrites that 0 with the current value of here.

?branch is a word written in assembly which does a conditional jump in the inner interpreter (the one that interprets the indirect-threaded code); when it's executed, it pops a value off the stack and checks to see if it's zero, and if so, it changes the interpreter's execution pointer ip (which is defined elsewhere as the register si) to the number stored in the threaded code following the pointer to ?branch. If, on the other hand, the value it popped was nonzero, it increments ip twice to skip over that number. (Note that Laxen's comment on ?branch is incorrect in that it reverses the sense of the test.)

All the forward jumps work in pretty much the same way: when you begin a control structure you call ?>mark to write a zero placeholder and push its address, and later on you "resolve" that placeholder by popping its address off the stack and overwriting it with the correct address. leave (break) and ?leave (if (...) break) work slightly differently, but mostly the same.

Backward jumps work the other way around: when you begin a control structure, as in begin, you call ?<mark to save the current address on the stack so that you can jump to it later, which ends up just being true here. Then, to actually compile the jump, for example in until or again, you call ?<resolve, which ends up just being swap ?condition ,—the , pops the jump target address off the stack and compiles it into the indirect threaded code, serving as an argument the ?branch or branch instruction compiled immediately before it.

begin ... while ... repeat is handled, as you can see, by treating the while ... repeat part as an if ... then with an unconditional jump back to the begin jammed in right before the then.

Hopefully this is helpful!

BTW, for the above, I reformatted the block files from the F83 distribution with http://canonical.org/~kragen/sw/dev3/blk2unix.py, which you may find useful if you want to do the same thing.