Cheat Engine Forum Index Cheat Engine
The Official Site of Cheat Engine
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 


Push/Pop Floats the correct way... [Closed]

 
Post new topic   Reply to topic    Cheat Engine Forum Index -> Cheat Engine
View previous topic :: View next topic  
Author Message
paul44
Expert Cheater
Reputation: 3

Joined: 20 Jul 2017
Posts: 217
PostPosted: Thu Apr 02, 2026 11:32 am    Post subject: Push/Pop Floats the correct way... [Closed] Reply with quote
Goal: "update" an existing float stack value.
=> details: see here [ https://ibb.co/album/ccBpXF ] (pics in reverse order)
1. the current float_value on the stack = 66 (which i'd like to change to 100)
2. what i do now, is pop - in this case - that 1st value - then push/load the new value...

Is this the proper way to do it ? And whatIf you'd like to update st(3) here instead ?
and yep, did some googling here, but could not find a single page describing exactly what i "wanted" to do (sort of). Most of them refer to the FLD instruction.

ps:
> When you break and follow through the action, you'll notice that the stack then shuffles one value down; and the 1st value wounds up at st(7).
> the FLD then pushes the new value to st(0) and "drops" st(7) entirely

Last edited by paul44 on Mon Apr 06, 2026 12:09 pm; edited 1 time in total
Back to top
View user's profile Send private message
ParkourPenguin
I post too much
Reputation: 155

Joined: 06 Jul 2014
Posts: 4756
PostPosted: Thu Apr 02, 2026 1:36 pm    Post subject: Reply with quote
paul44 wrote:
1. the current float_value on the stack = 66 (which i'd like to change to 100)
2. what i do now, is pop - in this case - that 1st value - then push/load the new value...

Is this the proper way to do it ? And whatIf you'd like to update st(3) here instead ?
I think that's fine.
For modifying ST(3), you can use `fxch st(3)` to swap ST(0) and ST(3), do the same pop old value / push new value, and `fxch st(3)` again to put the values back in place before you return to the original code.

paul44 wrote:
ps:
> When you break and follow through the action, you'll notice that the stack then shuffles one value down; and the 1st value wounds up at st(7).
> the FLD then pushes the new value to st(0) and "drops" st(7) entirely

Virtually, the x87 FPU operates as a stack. You can push values onto it and pop values off. When you pop a value off the stack, it is removed from the stack and can't be accessed again.

Physically, the x87 FPU has 8 data registers that are used to store values. There's a pointer that points to the register that's currently the top of the stack. "Pushing" a value corresponds to decrementing this pointer and loading the value from memory into the register, while "popping" a value corresponds to incrementing this pointer and storing the value from the register into memory. The pointer will "wrap around" like a ring buffer when needed.

There's also a "tag" associated with each of the 8 data registers that indicates whether a register is in use or empty. You generally can't access empty registers through normal means.

Example:
Code:
reg  tag  value
7   Empty   0
6   Empty   0
5   Empty   0 <- TOP
4   Empty   0
3   Empty   0
2   Empty   0
1   Empty   0
0   Empty   0

fld [value1]
fld [value2]
fld [value3]

reg  tag  value
7   Empty   0
6   Empty   0
5   Empty   0
4   Number  1 ST(2)
3   Number  2 ST(1)
2   Number  3 ST(0) <- TOP
1   Empty   0
0   Empty   0

fstp st(0)
fstp st(0)

reg  tag  value
7   Empty   0
6   Empty   0
5   Empty   0
4   Number  1 ST(0) <- TOP
3   Empty   2
2   Empty   3
1   Empty   0
0   Empty   0

This loads 3 values then pops 2 values. Even though the values were popped and are now "empty", they can still be accessed through other means such as debuggers like CE. CE doesn't account for the tag at all, however, and simply displays each register as ST(0) - ST(7) relative to the TOP pointer. This is why you see st(0) end up as st(7) after you pop it.

Pushing a new value would overwrite what CE displays as ST(7). This overwrites the old data and "drops it entirely."

For more information, see Intel's Software Developer's Manual Volume 1: Basic Architecture, chapter 8 "Programming with the x87 FPU".
_________________
I don't know where I'm going, but I'll figure it out when I get there.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    Cheat Engine Forum Index -> Cheat Engine All times are GMT - 6 Hours
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum


Powered by phpBB © 2001, 2005 phpBB Group

CE Wiki   IRC (#CEF)   Twitter
Third party websites