Helping understanding RedCode

Question

I'm trying to learn redcode, because it looks fun to make a warrior.

Introduction

For those who don't know what redcode is, here's a short explenation. It's an ASM-like language, but far more easy an stripped. It is used to write little programs that need to shut down other programs in a virtual memory. (See for more info here: http://vyznev.net/corewar/guide.html)

Here's a piece of code:

;redcode
;name Mice
;author Chip Wendell
;strategy paper (replicator)
;history Winner of the 1986 ICWS tournament
Top dat #0, #0
Start   mov #12, Top
Loop    mov @Top, <Target
    djn Loop, Top
    spl @Target,0
Spacer  equ 653
    add #Spacer,Target
    jmz Start, Top
Target  dat #0, #833
    end Start

Problem

The basic stratgy is to replicate itself to another place, and the fork the process. What i don't understand is this rule:

Loop    mov @Top, <Target

I understand the meaning of this line. It says, move the B-Field of target to the line where the B-Field of top points, and decrease the value of the B-Field of target.

When loop is executed for the first time, the first line will be:

Top dat #0, #12

As far as i get, the line with Loop thus means: Move the instruction 12 lines ahead (filled with dat #0, #0) to line 833.

But when this code is executed, the line of code is placed at line 839.

Does someone understand what is happening really?

Answer 1

Okay, that took quite a bit of reading, but here's your answer:

The first instruction, as you correctly surmise, makes Top into DAT #0, #12. Simple enough, but the next instruction is trickier. First, it decrements the B-value of Target (making it 832). Then, it copies the instruction at Top into the location 832 lines ahead relative to Target. That's the key: the indirect addressing mode means the destination is relative to the B-value read for the offset. Now look at the code after compilation, with line numbers in front:

0000 Top    DAT.F  #0        #0       
0001 Start  MOV.AB #12       $-1      
0002 Loop   MOV.I  @-2       <5       
0003        DJN.B  $-1       $-3      
0004        SPL.B  @3        $0       
0005        ADD.AB #653      $2       
0006        JMZ.B  $-5       $-6      
0007 Target DAT.F  #0        #833

As you can see, Target is on line 7, so 832 lines ahead relative to Target is line 839.

Hope that clears it up for you.