CODEWITHSUNDEEP
assemblynasmemu8086corewars
Electric Ice Wolf
Electric Ice Wolf

Reputation: 1

CoreWars safe challenge help cracking safe

I have this safe1:

START:
    PUSH AX
    MOV word [0D5h], 0
    MOV CX, 2           
    looper:
    add [0D5h], AX      
    MOV AX, [0D5h]      
    MUL CX         
    INC CX      
    cmp word [0D5h], 0D287h
    jne looper
    POP AX
    JMP START

and I'm stuck on how to find AX, because I can set [0D5h] to 0D287h whenever to break the inner loop so I know that the framework will probably be something like:

start:
    ;... get ax
    mov word [0D5h], 0D287h
    jmp start

At least that's what I understand, so does anyone have any ideas?

Footnote 1: safe challenges
For those unfamiliar with the safe challenge, I'll give a basic explanation first:
You get a safe which is a NASM compiled assembly 8086 code that does arithmetic actions and also loops forever, for a key to win it needs to run more than the safe, which means it has to also loop forever but also stop the safe's loop. How you may ask well, the key and safe are separate in everything but the memory so for example if you wanted to stop this safe:

start:
    mov [0DAh], ax
    jmp start

the key for this safe is:

start:
    mov bx, [0DAh]
    mov byte [bx], 0cch
    jmp start

Upvotes: 0

Views: 92

Answers (1)

Sep Roland
Sep Roland

Reputation: 39471

I have never understood what these 'safe challenges' are about, but I immediately saw next 2 solutions that stop the safe's inner loop, and then repeat ad infinitum:

  • AX = 53895 (D287h) where jne looper is never taken
  • AX = 17965 (462Dh) where jne looper is taken 1 time

The word value at [0D5h] successively becomes x, 3x, 12x, 60x, 360x, ...
and D287h happens to be divisible by 1 and 3.

Upvotes: 1

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