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HyggeAlien
How do I cheat?
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Joined: 18 Nov 2021
Posts: 6
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 Posted: Tue Dec 07, 2021 2:45 pm Post subject: Why doesn't this auto assemble script work |
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I little over a week I made a post with another problem (called "Help at code injection"), which mostly solved the problem. Although I was told to use vmaxsd, which worked, but not what I needed, so I used vminsd instead, which I think should work, but then CE says that it can't be compiled.
Here's the code I tried using, followed by the error which is thrown:
| Code: |
[Enable]
alloc(newmem,2048,0458935D)
label(returnhere)
label(originalcode)
label(exit)
alloc(mynumber,8)
newmem:
vminsd xmm0,xmm0,[mynumber]
vmovsd [rcx+68],xmm0
originalcode:
vmovsd [rcx+68],xmm0
exit:
jmp returnhere
0458935D:
jmp newmem
returnhere:
[Disable]
dealloc(newmem)
dealloc(mynumber)
0458935D:
vmovsd [rcx+68],xmm0
//Alt: db C5 FB 11 04 19
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| Code: | | Error in line 9 (vminsd xmm0,xmm0,[ffffffffffffffff]) :This instruction can't be compiled |
The guy in the former post didn't know why vminsd didn't work either, so I tried using my past code, but with some changes. Here's that code:
| Code: |
[Enable]
alloc(newmem,2048,02CB51A8)
label(returnhere)
label(originalcode)
label(exit)
alloc(mynumber,8)
mynumber:
dq (double)3.0 ; declare double
newmem:
comisd xmm0,[mynumber] ; compare two double float numbers (I tried cmp and fcom aswell instead of comisd, neither of which worked)
jg originalcode: ; jump if greater than
vmovsd [rcx+rbx],[mynumber] ; move (double) 3
originalcode:
vmovsd [rcx+rbx],xmm0 ; move (double) 3
exit:
jmp returnhere
02CB51A8:
jmp newmem
returnhere:
[Disable]
dealloc(newmem)
02CB51A8:
vmovsd [rcx+rbx],xmm0
//Alt: db C5 FB 11 04 19
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Error:
| Code: |
Error in line 9 (comisd xmm0,[ffffffffffffffff]) :This instruction can't be compiled
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The purpose of the code is to compare xmm0 (which is a value (double) from 0 to 4 with 3, if it's larger than 3, I was to set it to 3, so that it's new maximum value is 3.
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ParkourPenguin
I post too much
 Reputation: 140
Joined: 06 Jul 2014
Posts: 4297
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| Posted: Tue Dec 07, 2021 4:32 pm Post subject: |
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Trying to directly address a memory location like [mynumber] might fail in 64-bit code (see RIP-relative addressing). There's no guarantee mynumber is within 2GiB of that instruction to allow for RIP-relative addressing. That's what the third parameter to alloc is for.
comisd sets the ZF and CF flags in EFLAGS according to the result (also PF but it's only set for unordered results). You need to use the above/below jcc variants to check those flags. The greater/less jcc variants check the ZF, SF, and OF flags.
"jg originalcode" jumps if greater than. Jumping means the cpu will change the path of execution and start executing code at that location. In this case, you trying to jump if xmm0 > 3 skips you trying to write 3 to that address. This effectively means the larger value between 3 and xmm0 gets written to the address.
In general instructions can't directly move from one memory location to another: i.e. "vmovsd [rcx+rbx],[mynumber]" won't work. You need to use a register, e.g. xmm0, as an intermediary.
Directly after you try to move 3 into that memory location, you still execute the original code that writes the original value to the memory location, undoing your write.
What's wrong with foregoing AVX and using SSE2 (minsd)? It should work AFAIK.
https://forum.cheatengine.org/viewtopic.php?p=5775332#5775332
And as I've said in that previous thread, you should probably be using an aobscan.
_________________
I don't know where I'm going, but I'll figure it out when I get there. |
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HyggeAlien
How do I cheat?
![]() Reputation: 0
Joined: 18 Nov 2021
Posts: 6
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| Posted: Wed Dec 08, 2021 11:25 am Post subject: |
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English is not my primary language, sorry for the trouble that has caused. What do you mean by foregoing AVX? And minsd doesn't work for some reason, it wont compile, or is that simply because of that thing where: | Quote: | | In general instructions can't directly move from one memory location to another: i.e. "vmovsd [rcx+rbx],[mynumber]" won't work. You need to use a register, e.g. xmm0, as an intermediary. |
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ParkourPenguin
I post too much
Reputation: 140
Joined: 06 Jul 2014
Posts: 4297
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| Posted: Wed Dec 08, 2021 12:44 pm Post subject: |
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| HyggeAlien wrote: | | What do you mean by foregoing AVX? |
vminsd, vmaxsd, vmovsd, etc. are instructions that come from AVX- the Advanced Vector Extensions. minsd, maxsd, movsd, etc. are instructions that come from one of the SSE instruction sets (minsd is SSE2).
The game is using AVX. You should try to use whatever the game is using, but since CE can't assemble the AVX instruction vminsd, you should try to use the SSE2 instruction minsd instead.
| HyggeAlien wrote: | | And minsd doesn't work for some reason, it wont compile |
This assembles fine for me attached to the 64-bit tutorial:
| Code: | globalalloc(foo,4096)
label(mynumber)
foo:
minsd xmm0,[mynumber]
movsd [rcx+rbx],xmm0
align 10 cc
mynumber:
dq (double)3.0 |
If it doesn't work for you, it's probably because mynumber is too far away for the instruction to access it. Use the third alloc parameter, or make mynumber a label with an align before it like you see in the example above.
_________________
I don't know where I'm going, but I'll figure it out when I get there. |
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