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 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

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