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One thing you must remember is that you have to explicitly leave a CODE
definition by doing a JMP, to another code level routine. This is probably
the single most common error made by newcomers. Possibly it is caused by
making a false analogy between the higher level ; and the code level
END-CODE. While the Forth word ; does in fact get you back to where you came
from, END-CODE does not. In fact, END-CODE does nothing at all at run-time.
You can exit a CODE definition by doing a JMP, to any of the following:
NEXT POP POPTWO PUSH or PUT . These are described below:
NEXT
NEXT is commonly called the address interpreter. It is the word that is
responsible for the execution of all Forth words. ALL words in Forth
ultimately end up here. Doing a NEXT JMP, will cause the current code
definition to stop and return to the word that called it. All of the
following exit points end with a jump to NEXT . In what follows, remember
that a "stack element" refers to a 16 bit quantity -- i.e. two bytes.
POP
POP first drops the first element of the stack, and then jumps to NEXT.
Same as DROP in hi-level forth.
POPTWO
POPTWO drops the top two elements from the stack, and then jumps to NEXT.
Same as 2DROP in hi-level.
PUSH
PUSH lets you leave a result on the top of the stack. PUSH expects the
low byte of the new top of stack to be on the return stack, and the high
in the accumulator. PUSH will leave these as the new top of the parameter
stack (the former top will then be the second element). PUSH then calls
NEXT. Since this routine is somewhat more complicated than the others here
is a typical sequence:
PHA, ( push low byte to return stack )
TYA, ( assume high byte is saved in Y register, move it to the A reg)
PUSH JMP, END-CODE ( parameters set, so jump to PUSH)
PUT
PUT replaces the top of the stack with a new value. You setup PUT in
the same way as you setup for PUSH - the new lobyte is pushed to the return
stack, and the new hibyte is in the accumulator before the call to PUT. The
only difference between the two is that PUT replaces the present top of the
stack, while PUSH creates a new top of stack.
Register Usage:
Since Forth uses two stacks, and the 6502 CPU only implements one
hardware stack, the parameter stack must be "artificially" maintained. In
this implementation (as in most) it is located in the zero page, and the X
register is used as the parameter stack pointer. The machine stack is
Forth's return stack. Therefore instructions which affect the X register or
the machine stack should be used with care. At entry to a CODE defintion,
the X register points to the top byte of the parameter stack. This stack
grows downward in memory, so decrementing the X register will make room
for another element on the stack, and incrementing the stack pointer will
remove an element from the stack. Here is a diagram which shows the
situation when two elements are on the stack:
Hi Memory
**************
* hibyte2 *
* lobyte2 *
**************
* hibyte1 *
X --> * lobyte1 * Top of Stack
**************
Notice that the two bytes which make up one stack entry are stored in the
usual 6502 order, with the lobyte lower in memory. To remove the top element
of the stack, we can define the code word DROP:
CODE DROP ( N -- ) INX, INX, NEXT JMP, END-CODE
Which in fact, is exactly the the way DROP is defined in the BFC. After DROP
has been executed, the stack looks like this:
Hi Memory
*************
* hibyte2 *
X --> * lobyte2 * New top of Stack
*************
We will return to this topic later, when we talk about accessing the
parameter stack in more detail. For now, the main point is to remember that
when Forth starts executing your code definition, the X register will
contain a pointer to the top of the stack. You can use the X register to
access the stack, or to remove elements from the stack, but when your code
definition is finished, other Forth words, and the Forth system itself is
going to expect the X register to contain a valid stack pointer, so don't
change it wantonly. You should also remember that since each stack entry is
two bytes, only even multiples of the INX, or DEX, instruction make sense.
(I.E. INX, INX, INX, INX, not INX, INX, INX, - the first will drop two stack
elements, while the second will drop 1 and 1/2 stack elements - and cause
the Forth system to behave oddly.)
Both the Accumulator (A reg) and the Y register may be freely used. The A
register will contain garbage, and must be initialized, but the Y register
is guaranteed to be 0 on entry to your code, and you may take advantage of
this fact or not, as you wish. Here is a short code definition that will
leave the value of -1 (Forth's canonical TRUE flag) on the stack. It uses
the PUSH routine described earlier.