Implementing Forth in Redcode

I'm currently working on a threaded Forth interpreter in Redcode. The first version should provide about 20 key Forth instructions in 50 lines of code.

Threading

The Forth program will be stored as a list of subroutine calls, for example:

        dat    lit     ; push 1111
        dat    1111
        dat    lit     ; push 1234
        dat    1234
        dat    plus    ; add TOS to 2OS
        dat    udot    ; display TOS

A more compact representation would be to store addresses in both the a-field and b-field. Unfortunately this would be at the expense of more complex flow control:

        dat    lit,      1111
        dat    lit,      1234
        dat    plus,     udot

I'm open to suggestions how the interpreter can use the compact representation without causing too many difficulties.

Instruction Set

I'm planning to support the following instructions in version 1. Have I missed anything important, or have I included something which can be left out:

Ins	Description
-	subtract TOS from 2OS
+	add TOS to 2OS
*	multiply TOS by 2OS
U.	print TOS as an unsigned number
SPACE	print a space
DUP	copy TOS
DROP	remove TOS
ABS	replace TOS with its absolute value
NEGATE	replace TOS with -TOS
!	2OS is stored at the address in TOS
+!	2OS is added to the address in TOS
@	fetch the value at the address in TOS
=	TRUE if TOS=2OS, else FALSE
SWAP	exchange TOS with 2OS
DEPTH	number of elements on stack
BEGIN	start of BEGIN .. UNTIL structure
UNTIL	if TRUE, return to matching BEGIN
DO	start of DO .. LOOP structure
LOOP	inc counter, jump to DO if below limit

Signed vs Unsigned

Unfortunately numbers in Forth are signed and numbers in Redcode are unsigned. This affects a number of instructions, including division and comparison. Would it be worth the extra code to support unsigned numbers in Redcode Forth?

Example Code

The following example interprets +, U. and literals. New instructions can easily be added in Redcode. Support for calling new instuctions written in Forth needs to be added:

        org    next

stack   dat    0,         0

; LIT - place the next value on the stack

lit     mov.b  }ip,       <stack
        jmp    next

; + - remove 2OS and TOS and put their sum on stack

plus    add.b  >stack,    @stack
        jmp    next

; U. - remove and display TOS as an unsigned number

udot    mov.b  @stack,    <stack
        div    #10,       >stack
        mod    #10,       @stack
        add    #48,       @stack
        add    #1,        udcount
        jmn    udot,      <stack
        add    #1,        stack
udloop  sts    >stack,    0
udcount djn    udloop,    #0
        sts.a  #32,       0
        jmp    next

; --------------------------------

next
ip      jmp    @prog,     }ip

; --------------------------------

prog
        dat    lit     ; push 1111
        dat    1111
        dat    lit     ; push 1234
        dat    1234
        dat    plus    ; add TOS to 2OS
        dat    udot    ; display TOS

        end

Underload Interpreter in Redcode

Underload is a stack-based esoteric programming language designed by ais523. A set of 8 single character instructions operate on a stack of variable length strings:

Ins	Description
:	copy the top stack entry
!	drop the top stack entry
~	swap the top two stack entries
*	join the top two stack entries
S	display then drop the top stack entry
^	drop then run the top stack entry
a	enclose the top stack entry in parentheses
( )	add a new stack entry

A combination of different factors make an interpreter for Underload an interesting project to tackle in Redcode:

• parsing single character instructions is easy, I don't like writing parsers
• handling variable length data can be tricky
• no operands / no side effects to worry about
• only 8 instructions, the size of the interpreter should be reasonable
• ^ is the only means of flow control

The final code weighs in at a mere 79 instructions. Here's the final version of the Underload Interpreter. If you have any improvements to suggest, please leave a comment below.

Brainf*** Interpreter in Redcode

Brainf*** is an esoteric programming language developed by Urban Müller. There are 8 instructions in brainf***:

Ins	Macro	Description
<	lt	move pointer left
>	gt	move pointer right
+	inc	increment memory cell at pointer
-	dec	decrement memory cell at pointer
.	out	output a character from the cell at pointer
,	in	input a character and store in cell at pointer
[	op	jump past matching ] if cell at pointer is 0
]	cl	jump to matching [ if cell at pointer not 0

Brainf*** operates on an array of memory cells and has a pointer into the array. Each instruction translates directly into Redcode. Before executing the brainf*** program, the interpreter calculates and stores the offset for [ and ] to avoid repeated searching.

Here's the code:

        lt equ sub #1, ptr
        gt equ add #1, ptr
        inc equ add #1, @ptr
        dec equ sub #1, @ptr
        out equ sts @ptr, 0
        in equ lds @ptr, 0
        op equ jmz 0, @ptr
        cl equ jmn 2, @ptr

        org    seek

ptr     mov.ba #0,        #prog

find    sne.a  #2,        *ptr
        sub    #1,        count
        sne.a  #0,        }ptr
        jmp    find,      >count
count   jmn    find,      #0

        mov.a  ptr,       @ptr
        sub.ba ptr,       >ptr
        mov.ba ptr,       {ptr
        sub.a  ptr,       *ptr

seek    jmn.a  seek,      >ptr
        jmn    ptr,       <ptr

prog

Redcode Interpreter for the RSSB Single Instruction Computer

The RSSB single instruction computer is a minimal, Turing complete virtual machine. The instruction used is Reverse Subtract and Skip if Borrow. RSSB subtracts the accumulator from the contents of a memory location and stores the result in both. The next instruction will be skipped if the accumulator was greater than the value in memory.

Jumps can be implemented by manipulating the instruction pointer at memory location 0. Other special locations are as follows:

1. accumulator
2. always contains 0
3. input
4. output

Here's the code for the interpreter. I've included a Hello World program written in RSSB. If you think you can reduce the size of Hello World, please let me know how.

        org    rssbint

rssbint mov.ba >ip,       ip    ; load next instruction

        mov    #0,        zero  ; set zero

        sne.a  #3,        ip    ; handle input
        lds    in,        0

        sne.a  #4,        ip    ; handle output
        sts    acc,       0
        mov    acc,       out
        add    out,       out

        slt    *ip,       acc   ; set flag for skip
        mov    #2,        skip

        sub.b  acc,       *ip   ; acc, *ip = *ip - acc
        mov.b  *ip,       acc

skip    djn    noskip,    #1    ; skip if required
        nop    >ip,       >skip

noskip  slt    #7900,     @ip   ; protect interpreter

        jmn    rssbint,   ip    ; loop until ip = 0

; --------------------------------------

ip      dat    5      ; instruction pointer
acc     dat    0      ; accumulator
zero    dat    0      ; always 0
in      dat    0      ; character input
out     dat    0      ; character output

; --------------------------------------

loop    dat    -ip+   acc       ; acc = character from ptr
ptr     dat    -ip+   hello        

        dat    -ip+   out       ; display character

        dat    -ip+   zero      ; acc = -acc

        dat    -ip+   zero      ; always skipped

        dat    -ip+   sum       ; subtract acc from sum

        dat    -ip+   ip        ; skipped if sum is negative,
                                ; otherwise jump to 0

one     dat    -ip+   acc       ; subtract 1 from ptr
        dat    -ip+   one
        dat    -ip+   ptr

        dat    -ip+   acc       ; jump to loop
        dat    -ip+   loopoff
        dat    -ip+   ip
loopoff dat    -loop

sum     dat    -1116

        dat    33         ; '!'
        dat    100        ; 'd'
        dat    108        ; 'l'
        dat    114        ; 'r'
        dat    111        ; 'o'
        dat    87         ; 'W'
        dat    32         ; ' '
        dat    44         ; ','
        dat    111        ; 'o'
        dat    108        ; 'l'
        dat    108        ; 'l'
        dat    101        ; 'e'
hello   dat    72         ; 'H'

        end