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.

Display Signed Number in Base X

The following Redcode displays a signed number is a chosen number base:

dot     slt    @stack,    #1+CORESIZE/2
        sts.a  #45,       0
        slt    @stack,    #1+CORESIZE/2
        mul    #-1,       @stack
udot    mov.b  @stack,    <stack
        div.b  base,      >stack
        mod.b  base,      @stack
        slt    @stack,    #10
        add    #7,        @stack
        add    #48,       @stack
        add    #1,        udcount
        jmn    udot,      <stack
        add    #1,        stack
udloop  sts    >stack,    0
udcount djn    udloop,    #0

base    dat    10

Unfortunately, the code is pretty ugly. Can you suggest a more elegant solution?