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TFORTH

Overview

TFORTH is my custom version of FORTH. It started as a programming exercise that grew into a somewhat usable language. This version of FORTH is a bit odd in that it has no compiling mode at all. Not even bytecode compiling. Everything is 100% interpreted. So, it's essentially a fancy text parser. It also doesn't try to be ANSI compliant at all.

That being said, it does support a lot of common FORTH things like you would expect, including: custom function definitions, calling external functions via function pointers, full access to the system's memory map, input/output, etc.


 

Variables

There are some big differences, under-the-hood, with this version of FORTH. For one, there are no variables. Instead, I define variables as functions with a index to the "variable page," a page in memory dedicated to temporary storage. So, for example, 
 
5 0 !

would assign the literal value 5 to index 0 in the variable page. So, to declare a "variable," I would do the following 

: X 0 ; 

This would create a function called X which pushes the literal value 0 onto the stack. So, when you call: 

5 X !

it does the same operation as above. The reason I didn't implement the VARIABLE keyword is that, under-the-hood, the lookup process would be the same as if it was a function.

Functions

Functions, when defined, are stored as nodes in a linked list. The node itself has the following format:

1 byte  -   length of function name
n bytes -   function name string
1 byte  -   length of entire function def, used for calculating next node
n bytes -   string content of function, ';' terminated
 
So, as shown above, functions themselves are stored into memory as string literals. When the engine is instructed to execute a user-defined function, it stores the state of the engine and switches to parsing the string within the linked list. When the ';' function is called, it acts like a return function and reloads the previous state of the engine. Functions can be deleted with the DELETE keyword. This deletes the function and restores the consumed memory by shifting the linked list down as necessary. 
 
But, because nothing is compiled, the act of creating functions is more of a convenience than a requirement.  
  

Extended Memory (EXM)

As of writing, my version of FORTH works on 8-bit numbers. These can be represented as signed, unsigned, or hexadecimal numbers, but it is still limited to 8-bits. This becomes an issue when you want to access memory or functions in 16-bit address space. So, as a quick fix, I added in some extra functions and engine-specific variables.
 
EXML@           - Get EXtendedMemory L
EXMH@           - Get ExtendedMemory H
EXML!           - Set EXtendedMemory L
EXMH!           - Set ExtendedMemory H
EXM!            - Write value to (EXML, EXMH)
EXM@            - Read value from (EXML, EXMH)
EXMP++          - Increment (EXML, EXMH)
EXMP--          - Deincrement (EXML, EXMH)
EXMCALL         - Call subroutine at (EXML, EXMH)
ARGA!           - Set register A before exmcall
ARGX!           - Set register X before exmcall
ARGY!           - Set register Y before exmcall  
 
As shown, this kinda just bolts-on 16-bit memory functionality to this 8-bit system. As an example, if I had a function at $CE12 that I wanted to call from FORTH, I could do the following:
 
$ CE EXMH!      - Set EXMH to $CE
$ 12 EXML!      - Set EXML to $12
EXMCALL         - Call function at (EXML, EXMH)
 
In machine code, this effectively translates to:
 
lda #$CE
sta EXMH
lda #$12
sta EXML
pha
phx
phy
lda ARGA
ldx ARGX
ldy ARGY
jsr (EXML)
...
  
While this is a rather slow process, it allows the programmer to call external functions or read/write to the full 16-bit address space. 
 

Going Forward

Because this was just a programming exercise, I don't have any plans to make this ANSI compliant, if that's even possible, or more usable.  That said, I have found it useful in development of this computer. And, it is nifty to write in, what is essentially, a scripting language for a 65C02 computer. 
 

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