foco65
======

Forth cross-compiler targeting 6502 processors. It needs a 6502
cross-assembler (e.g. [xasm](http://xasm.atari.org/)) as a backend.

Usage
-----

'''sh
foco65 [OPTIONS] INPUT-FILE
'''

OPTIONS:

    -h                           display help
    -p ADDR,--pstack-bottom=ADDR parameter stack bottom address
    -s SECTS,--sections=SECTS    specify comma separated list of sections
                                 default: init,boot,data,code
    -S INT,--pstack-SIZE=INT     parameter stack size

Example:

```sh
$ foco65 foo.forth > foo.asx
```

Words
-----

`:` `@` `0=` `1-` `1+` `2/` `2*` `2@` `2!` `and` `c!` `c@` `cmove` `count` `d-` `d+` `d=` `do` `drop` `dup` `i` `j` `loop`
`+loop` `lshift` `<=` `<` `>=` `>` `-` `+` `or` `over` `rshift` `rsp` `sp` `swap` `unloop` `u<` `u>` `while` `<>`
`=` `!` `[` `]` `[code]` `[end-code]` `cell` `cells` `not` `[section]` `variable` `2variable`
`constant` `create` `,` `c,` `,'` `'` `,"` `"` `allot` `lit` `\` `(` `recursive` `[label]` `*` `/`

Internals
---------

Type: indirect-threaded.

Cell size is 16 bits.

Parameter stack pointer is kept in register `X`.
Parameter stack pointer is decremented before write (stack push) and
incremented after read (stack pop).

Hardware stack (page $01) is used as return stack.

Instruction pointer and other work registers are kept in
page zero occupying total of 12 bytes.

After initializing interpreter internal state user-defined word
`main` is executed.

Generated code is placed into sections which are output in the
order: `init`, `boot`, `data`, `code` or other - specified by the user.

Examples
--------

Atari XL/XE example: display character table as 16x16 array.
Uses xasm as a backend.

<pre><code>
[section] init

[code]
 org $3000
[end-code]

[section] code

$230 constant dladr

variable screen
variable cursor
variable line

: cursor-next   ( -- u )
  cursor @ dup 1+ cursor ! ;

: put-char      ( c -- )
  cursor-next c! ;

: set-cursor    ( u -- )
  screen @ + cursor ! ;
  
: main
  dladr @ 4 + @ screen !

  0 line !
  16 0 do
    line @ set-cursor
    line @ 40 + line !
    16 0 do
      i j 4 lshift or put-char
    loop
  loop

  begin again ;

[code]
 run boot
[end-code]
</code></pre>

Atari XL/XE example of defining word in assembly:
wait for keypress and push the pressed key's code
on the parameter stack.

<pre><code>
: get-char    ( -- c )
[code]
 lda #0
 dex
 sta pstack,x
 stx w
 jsr do_gc
 ldx w
 dex
 sta pstack,x
 jmp next
do_gc
 lda $E425
 pha
 lda $E424
 pha
 rts
[end-code] ;
</code></pre>

Increase cell at the given address. Shows defining words not
being a valid assembler label.

<pre><code>
create array 16 cells allot

: ++          ( addr -- )
  [label] plus_plus
  dup @ 1+ swap ! ;
...
  array ++
  array 1 cell + ++
...
</code></pre>