Sunday, 25 January 2009

Brainf*** Interpreter in Redcode

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

<ltmove pointer left
>gtmove pointer right
+incincrement memory cell at pointer
-decdecrement memory cell at pointer
.outoutput a character from the cell at pointer
,ininput a character and store in cell at pointer
[opjump past matching ] if cell at pointer is 0
]cljump 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 #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 ptr, >ptr ptr, {ptr
sub.a ptr, *ptr

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


Sunday, 18 January 2009

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 >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'


Friday, 9 January 2009

The Corewar DynaHill

Christian Schmidt has announced the opening of a new Corewar hill with a ranking system similar to Japanese Sumo. The Corewar DynaHill is of unlimited size. New warriors enter at the bottom and at certain time intervals each warrior fights 15 neighbours.

Depending on the number of wins / losses, each warrior moves either up or down a number of positions. Eventually, the warrior will be unable to climb higher, possibly becoming King of the DynaHill!

'94draft and limited process warriors co-exist on the hill. If a '94draft warrior battles against a limited process warrior, a random number of processes is chosen between 40 and 120.

The use of PIN and MAXPROCESSES is forbidden. The maximum length is 200.

For more information, take a look at The Corewar DynaHill FAQ.

Tuesday, 6 January 2009

Call / Return Macros in Redcode, a Possible Solution

Here's one solution to the call / ret problem in Redcode. Unfortunately the offsets in multi-line-equates can be slightly out. In this solution, the offsets are adjusted the first time the call is executed.  Here's how it works:

call routine compiles as follows:
        mov    #2-return, <stack ; save return address
jmp callsub, 1+routine ; jump to callsub which adjusts the address
; b-field pointer is either correct or out by 1
callsub then adjusts the pointer if necessary and moves it to the a-field of the jmp:
         mov    #2-return, <stack ; save return address
jmp routine, 1+routine ; jump to routine, a-field is correct
The jmp now contains the correct offset and jumps directly to routine in future.

Here's the code:
stack   dat    0

call equ mov #2-return, <stack
equ jmp callsub, 1+

ret equ jmp return

return mov.b >stack, #0
jmp @return

callsub mov.b @stack, #0
add #return-callsub, callsub <callsub, @callsub
slt.b @callsub, #CORESIZE/2
jmp @callsub
djn.a @callsub, @callsub

Sunday, 4 January 2009

Call / Return Macros in Redcode

For a while I've been trying to implement call / ret macros in Redcode so I can call subroutines. The obvious solution is as follows and doesn't work correctly:
; this code is buggy

stack dat 0

call equ mov #2-return, <stack
equ jmp

ret equ jmp return

return mov.b >stack, #0
jmp @return
Unfortunately, this only works for forward calls. The address offset is out by 1 for reverse calls and I can't find a simple fix. I've tried the following solutions, but I'm not particularly happy with any of them:
  • adding a nop to the top of all subroutines (extra code)
  • using a macro to automatically adjust the address, e.g. call subroutine adjust (ugly)
  • adjusting the address the first time it is executed (extra code, slower)
Can you suggest a better solution?