Datasets:
HDTenEightyP
/
CompuServe

Dataset card Data Studio Files
xet
 Community
text
stringlengths
0
99.6k
;CODE.
When ;CODE is executed (for example, by typing 10 CONSTANT TEN), it will
REWRITE THE CODE FIELD OF THE WORD BEING DEFINED! This is extremely
important to remember. All Forth words have code fields that define their
run time behaviour, with colon definitions all sharing the same code field,
variables sharing the same code field (different, of course, from that of
colon definitions and constants) and so on. When CREATE executes in our
CONSTANT definition, it creates a code field that contains the address of a
routine which will push the address of the word being defined to the stack.
When ;CODE executes in our CONSTANT definition, it will search out this code
field, and re-write it with the address of the machine language routine
which immediately follows ;CODE. This means that the only run time behaviour
of a word which contains a ;CODE termination is determined by you. Even
though there is a CREATE in the lo-level definition of CONSTANT, it will not
deposit the address of the parameter field on the stack, because ;CODE has
changed the code field to point to our machine language routine. An
illustration may make this clearer:
Header created by CREATE:
**************
* Link Field *
**************
* TEN * Name field - stores ASCII characters of defined name.
**************
* $0900 * $0900 = address of routine to push parameter field on stack
**************
* 10 * Parameter field - stores value of constant
**************
Note that $0900 is just an example address - don't try jumping to it in your
own code.
When a word defined by CREATE is executed, the machine language routine at
$0900 (the address in the code field of the word) will be executed. This
routine is what causes the run-time behaviour of CREATE defined words - it
pushes the address of the parameter field to the stack. The above
illustration shows the state of the dictionary after the CREATE and , part
of CONSTANT have been executed. Now, suppose that the address of the first
instruction following ;CODE in CONSTANT ( the 2 # LDY, ) is at $8000 . When
;CODE executes in the course of defining a new constant, the header will
look like this:
Header after ;CODE has been executed.
**************
* Link Field *
**************
* TEN * Name field - stores ASCII characters of defined name.
**************
* $8000 * $8000 = address of your machine language routine
**************
* 10 * Parameter field - stores value of constant
**************
So, as always, when Forth executes TEN, the first thing to be executed is
the address of the routine in the code field, which no longer points to the
CREATE run time code, but to your machine language code.
Hope that is all clear enough. Now, to the particulars of the ;CODE part of
our low level CONSTANT definition (note that the following is implementation
dependent. Most present day Forths use an implementation along these lines,
but not all.)
Forth depends on two registers for its performance, the IP and the W
register. When Forth is implemented on processors with more registers,
typically a processor register will be used for these Forth system
registers. However, there are not enough on the 6502, so zero page locations
have been used instead. W will contain the address of the code field of the
word currently being executed. For example, if the code field of TEN is
located at $9000, the W register will contain $9000, or the address of the
address of the routine to be executed. The ;CODE part of CONSTANT uses this
fact to access the parameter field of the current word. Since the code field
of an address, is always two bytes long, the first byte of the parameter
field is located at $9002. Given this information, it becomes a simple
matter of indirectly indexing from W. Loading the Y register with 2, and
then performing W )Y LDA, (LDA (W),Y in conventional assembler) will get the
first byte of the constants value. The next byte is at $9003, so we simply
increment Y, and indirectly index again to get the next byte. The rest of
the definition consists of setting up for PUSH, which was covered earlier.
Phew! Hope that was clear enough. I think you can see that indexing from W
can get you any byte in a definitions parameter field. W 1- contains an
indirect JMP instruction, so you can also vector control to other routines
by storing the address of the address of the routine in W, and then doing a
W 1- JMP, . This technique is rarely used, however.
There is an intimate relationship between the IP register and the W
register, and an effective use of these registers depends on a clear
understanding of how they work in the Forth system as a whole. Both of them
are changed by NEXT. When NEXT executes, the IP will point to an address
which contains the code field address of the next word to be executed by
Forth. NEXT fetches this address, and stores it in W. It then bumps the IP
(which stands for Interpretive Pointer) by two, so the next time around it
will be pointing to the next word to be executed. Finally, NEXT does a JMP
to W 1-, which causes an indirect jump to the address pointed to by W.
The IP is most useful for accessing things like inline data structures,
(character strings, and the like.) Since the IP is incremented by NEXT, the
code fragment:
IP )Y LDA, PHA, INY, IP )Y LDA, PUSH JMP,