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Simple Division Question?

 
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Humam2104
How do I cheat?
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Joined: 27 May 2022
Posts: 2
PostPosted: Fri May 27, 2022 11:14 pm    Post subject: Simple Division Question? Reply with quote
I'm creating a simple function that divides damage

The original line of code is:
Code:
  mov [rbx+000003E8],eax

Replacing this line of code in the memory viewer removes all damage.
My idea is divide the damage by 2 for example (damage and health are integers)

But the game crashes whenever I try this simple division

Code:

DamageAI:

  mov ax, [eax]
  mov bx, 2
  div bx //causes crash, skipping this line wouldn't cause a crash
  mov [rbx+000003E8],eax
  jmp return


Thank you. I hope someone helps me with this because most guides I've seen deal with xmm values and I already used divss before once. But I'm having difficulty dealing with integer divisions.
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ParkourPenguin
I post too much
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Joined: 06 Jul 2014
Posts: 4297
PostPosted: Sat May 28, 2022 12:21 am    Post subject: Reply with quote
Why are you using 16-bit registers? The original code makes it look like a 32-bit value.

You're modifying rbx without backing it up. (bx is the lower 16 bits of rbx)
You need to back up rdx too. It needs to be set to 0 prior to the div, and the remainder of the div gets written to dx.

Anyway, if you're dividing by a power of 2, shift right:
Code:
newmem:
  shr eax,1
  mov [rbx+3E8],eax
  jmp return

General integer division:
Code:
label(divisor)

newmem:
  push rcx
  push rdx
  mov ecx,[divisor]
  xor edx,edx
  div ecx
  pop rdx
  pop rcx
  mov [rbx+3E8],eax
  jmp return

align 4 CC
divisor:
  dd 2

Integer division using floating point arithmetic:
Code:
label(divisor)

newmem:
  pxor xmm0,xmm0
  cvtsi2ss xmm0,eax
  divss xmm0,[divisor]
  cvttss2si eax,xmm0
  mov [rbx+3E8],eax
  jmp return

align 4 CC
divisor:
  dd (float)2.0

All this is assuming eax is the damage being done and not your actual health.
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