foco65/README.md

foco65

Forth cross-compiler targeting 6502 processors. It needs a 6502 cross-assembler (e.g. xasm) as a backend.

Usage

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:

$ 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 executable skeleton. Uses xasm a a backend.

[section] init

[code]
 org $2000
[end-code]

[section] code

: main
  begin
    ...
  again ;

[code]
 run boot
[end-code]

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

[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]

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

: 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] ;

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

create array 16 cells allot

: ++          ( addr -- )
  [label] plus_plus
  dup @ 1+ swap ! ;
...
  array ++
  array 1 cell + ++
...