I'm trying to learn redcode, because it looks fun to make a bot.
For those who don't know what redcode is, here's a short explanation. It's an ASM-like language, but far more easy and stripped. It is used to write little programs that need to shut down other programs in 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
The basic strategy 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 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?
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.