XP running on a 486 cpu

**Dietmar** · April 2, 2024

@pappyN4

Oh Waaaoh :cheerleader: , this is a crazy nice idea.

It just mean, when you look at original ntoskrnl.exe from win2000, you see only that version with cmpxchg8b.

But when during Setup of Win2000 the installer noticed, that it has to live on a 486 cpu,

it patches all about cmpxchg8b. And not only in ntoskrnl.exe. Also ntdll.dll and each other file from the Setup.

This I will test tomrrow

Dietmar

PS: By the way I noticed my mistake in my new cmpxchg8b Emulator:

The opcode cmpxchg [pointer to 32 bit in memory], REG

changes the 32bit at the adress pointer in memory only, if the 32bit in memory are identic with EAX.

Only then the content of REG is written to those 32 bit in memory.

**Dietmar** · April 2, 2024

20 minutes ago, pappyN4 said:

@pappyN4 Otherwise I would try for AVX on x64 using x86 as guide.

Ask @Mov AX, 0xDEAD for this

Dietmar

Edited April 2, 2024 by Dietmar

**pappyN4** · April 3, 2024

11 minutes ago, Dietmar said:

@pappyN4 Otherwise I would try for AVX on x64 using x86 as guide.

Ask @Mov AX, 0xDEAD for this

Dietmar

Stability of OS is higher priority. I have been testing a BSOD problem for a while now that usually happens every 5 of 100 boots. But I have been able to recreate very easily doing certain thing in certain game. Server 2003x86 seems fine, just x64 has problem. Hopefully with BSOD dumps someone smarter than me can figure out why

**user57** · April 3, 2024

i heared chappell died a few months ago, sad story we could still need him
reading chappells writing it says that there once was a solution that dont use the command if not supported by cpu
that dont neccesary say if you just use a different one from a different OS version that it just work - maybe - maybe not

why would it has to be that other cmpxchg command

the linux one is not perfect - depending on what the other routines do, the linux one might work, but certainly its not 100 % correct, while mine is

the linux one looks almost the same to the one i posted up, but it dont compare the 64 bits for the false result (maybe the linux solution dont need,
but again mine is correct the linux one is not)
so why not "just the right one"

doing it a other way cause more commands and maybe fixes, there are certainly multiple solutions

https://www.felixcloutier.com/x86/cmpxchg
the description might be wrong this time, the description here unlike cmpxchg8b it always compares EAX with the memory location
the description actually dont tell that a other register then EAX can be choosen

well this time your code might work

but you rather trying to fix the results, the ZF non reaction is set to just go back that is ok but you have to do this in every function like this then

also that makes 2 times locks xchg and 2 times lock cmpxchg

you do cmpxchg for the atomic question ?

if you have to change 64 bit at once then it might be atomic for the 64 bits, doing 64 bit in 1 step

just having the lock prefix dont change it to a 64 bit mov

**j7n** · April 3, 2024

On 4/1/2024 at 7:18 PM, Dietmar said:

CMPXCHG [EBP], EAX instruction compares the value in the EAX register with the value at the memory address pointed to by EBP. If they are equal, the value in EAX is stored at that memory address

Can you explain the purpose of this function? I don't understand why it writes the same value from a register into the memory location. The memory already contains that value (when they are equal).

**Dietmar** · April 3, 2024

@j7n

This is not the correct use of this opcode:

In the real cmpxchg8b [EBP], there is the first check, if the value in memory (low 32 bit) is the same as in EAX.

When yes, those lower 32 bit are changed against the value in EBX.

So, using CMPXCHG [EBP], EBX offers exact this functionality,

but only for the lower 32bit of the 64 bit in memory

Dietmar

Edited April 3, 2024 by Dietmar

**user57** · April 3, 2024

well i would think a different offset is possible

but if they are equal it exchange that offset with ECX and EBX (it overworks those)

if you know about logical circuits you might know why

but exactly this is why i wanted to say he dont need that command at least i dont see a reason for that

the function itself seems to compare:

"if (this offset 64 bit entry still has the same value as EAX and EDX) -> change that offset with ECX and EBX (then it actually has a 64 bit changed value there)"

functions descriptions:

https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-exinterlockedflushslist

https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-exinterlockedpopentryslist

https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-exinterlockedpushentrylist

"If there were entries on the specified list, ExInterlockedFlushSList returns a pointer to the first SLIST_ENTRY structure that was an entry on the list; otherwise, it returns NULL."

this opens the question why they used this command to return an pointer offset to the first SLIST_ENTRY structure and changing that SLIST_ENTRY (what is a internal windows structure)

about that "atomical"

it "suppose to be" a "non interruptable doing"

therefore dietmar probaly deleted the interrupt flag (so no interrupts - actually it still do that - but thats a other story) (the other is the "lock" command)

the next question what comes to mind is changing 64 bit at once (non interrupt doing), not stepwise that that cmpxchg8b actually do (even tho its 32 bits - it just use 2 registers)

BUT i never seen that to be needed for 64 bits, that far to small to have an interaction, not even interrupts if they are changed up (and those are constandly used) have that problem

https://www.quora.com/What-is-the-meaning-of-atomic-in-programming

we also talking about a function here , so the function itself might actually be "atomical" if it solves its job, because the function has a start and a end to solve this step

we are not in c++ that might has a code "atom (this)" in assembly you have to write the real instruction that physical do so (if that was a problem we might have some more answers for now)

dietmar just said that he wants to remove cmpxchg8b with a working alternativ code, it might be a little road but over time we will find this

i highly suspect that list is for threading/multicore

**Dietmar** · April 3, 2024

Here is next try for to reach am atomic compare and exchange for to emulate CMPXCHG8B

Dietmar

ExInterlockedFlushSList proc near
  
    push    ebx
    push    ebp
    pushf
    cli
    xor     ebx, ebx
    mov     ebp, ecx 


    mov     edx, [ebp+4]    
    mov     eax, [ebp]      

    or      eax, eax
    jz      .done
    mov     ecx, edx
    mov     cx, bx

    lock cmpxchg [ebp], ebx
     
      push     eax
      mov      eax, edx
      lock cmpxchg [ebp+4], ecx
      pop      eax
     
.done:
    sti
    popf
    pop     ebp
    pop     ebx
    ret

ExInterlockedFlushSList endp

Edited April 3, 2024 by Dietmar

**roytam1** · April 3, 2024

1 hour ago, Dietmar said:
Here is next try for to reach am atomic compare and exchange for to emulate CMPXCHG8B

Dietmar
(...)   

    or      eax, eax
    jz      .done
    mov     ecx, edx
    mov     cx, bx <-- why this uses 16bit registers?

    lock cmpxchg [ebp], ebx
     
(...)

**Dietmar** · April 3, 2024

@roytam1

In win2000, the whole list was set to 0.

But in XP SP3, ECX becomes a special structure:

ECX = ab cd 00 00, where ab is the highest byte from the 64 bit in memory and cd the next following.

And this 16 bit operation kills the 2 following bytes from ECX, because EBX = 00 00 00 00 and so bx = 00 00

Dietmar

**roytam1** · April 3, 2024

15 hours ago, pappyN4 said:

I did look at win2000 ntoskrnl and some of the older pre release ones. They all have cmpxchg8b also. But if windows 2000 is able to install and run on 486, then it must be bypassed somehow for it to work. According to Chappell site,

yeah it does. if you search `cpuid` in ntoskrnl.exe 5.0.2195.1, you can find one in `sub_553EB4`:

INIT:0055415C                 mov     eax, 1
INIT:00554161                 cpuid
INIT:00554163                 test    edx, 100h
INIT:00554169                 jz      short loc_554177
INIT:0055416B                 or      ds:dword_47DEEC, 80h
INIT:00554175                 jmp     short loc_5541EA
INIT:00554177 ; ---------------------------------------------------------------------------
INIT:00554177
INIT:00554177 loc_554177:                             ; CODE XREF: sub_553EB4+29Ej
INIT:00554177                                         ; sub_553EB4+2B5j
INIT:00554177                 lea     eax, ExInterlockedCompareExchange64
INIT:0055417D                 lea     ecx, sub_40078C
INIT:00554183                 mov     byte ptr [eax], 0E9h
INIT:00554186                 lea     edx, [eax+5]
INIT:00554189                 sub     ecx, edx
INIT:0055418B                 mov     [eax+1], ecx
INIT:0055418E                 lea     eax, ExInterlockedPopEntrySList
INIT:00554194                 lea     ecx, loc_4006F0
INIT:0055419A                 mov     byte ptr [eax], 0E9h
INIT:0055419D                 lea     edx, [eax+5]
INIT:005541A0                 sub     ecx, edx
INIT:005541A2                 mov     [eax+1], ecx
INIT:005541A5                 lea     eax, ExInterlockedPushEntrySList
INIT:005541AB                 lea     ecx, loc_400704
INIT:005541B1                 mov     byte ptr [eax], 0E9h
INIT:005541B4                 lea     edx, [eax+5]
INIT:005541B7                 sub     ecx, edx
INIT:005541B9                 mov     [eax+1], ecx
INIT:005541BC                 lea     eax, ExInterlockedFlushSList
INIT:005541C2                 lea     ecx, loc_400714
INIT:005541C8                 mov     byte ptr [eax], 0E9h
INIT:005541CB                 lea     edx, [eax+5]
INIT:005541CE                 sub     ecx, edx
INIT:005541D0                 mov     [eax+1], ecx
INIT:005541D3                 lea     eax, sub_402352
INIT:005541D9                 lea     ecx, ExInterlockedAddLargeInteger
INIT:005541DF                 mov     byte ptr [eax], 0E9h
INIT:005541E2                 lea     edx, [eax+5]
INIT:005541E5                 sub     ecx, edx
INIT:005541E7                 mov     [eax+1], ecx

and 2nd `lea` is the `cmpxchg8b`-less version of upper one.

**Dietmar** · April 3, 2024

@roytam1

Can you make a complete ExInterlockedFlushSList in Hex Code from it?

Because of the a lot of jumps, you do not see, how they make it

Dietmar

**roytam1** · April 3, 2024

50 minutes ago, Dietmar said:

@roytam1

Can you make a complete ExInterlockedFlushSList in Hex Code from it?

Because of the a lot of jumps, you do not see, how they make it

Dietmar

which version? the listing I post above is Win2000 RTM (5.0.2195.1), which may not suit XP's usage.

**Dietmar** · April 3, 2024

@roytam1 This is not so much important.

I have also win2000 SP4. It is only for to get the idea

Dietmar

**Dietmar** · April 3, 2024

We have a new, working Emulation for CMPXCHG8B :cheerleader:

Dietmar

53 55 9C FA 33 DB 8B E9 8B 55 04 8B 45 00 0B C0 74 13 8B CA 66 89 D9 F0 0F B1 5D 00 50 8B C2 F0 0F B1 4D 04 58 FB 9D 5D 5B 90 90 90 90 C3

.data:004762B2 ; Exported entry   7. ExInterlockedFlushSList
.data:004762B2
.data:004762B2 ; =============== S U B R O U T I N E =======================================
.data:004762B2
.data:004762B2
.data:004762B2                 public ExInterlockedFlushSList
.data:004762B2 ExInterlockedFlushSList proc near       ; CODE XREF: sub_45F0DF:loc_45F0F7p
.data:004762B2                                         ; DATA XREF: .edata:off_5AC2A8o
.data:004762B2                 push    ebx
.data:004762B3                 push    ebp
.data:004762B4                 pushf
.data:004762B5                 cli
.data:004762B6                 xor     ebx, ebx
.data:004762B8                 mov     ebp, ecx
.data:004762BA                 mov     edx, [ebp+4]
.data:004762BD                 mov     eax, [ebp+0]
.data:004762C0                 or      eax, eax
.data:004762C2                 jz      short loc_4762D7
.data:004762C4                 mov     ecx, edx
.data:004762C6                 mov     cx, bx
.data:004762C9                 lock cmpxchg [ebp+0], ebx
.data:004762CE                 push    eax
.data:004762CF                 mov     eax, edx
.data:004762D1                 lock cmpxchg [ebp+4], ecx
.data:004762D6                 pop     eax
.data:004762D7
.data:004762D7 loc_4762D7:                             ; CODE XREF: ExInterlockedFlushSList+10j
.data:004762D7                 sti
.data:004762D8                 popf
.data:004762D9                 pop     ebp
.data:004762DA                 pop     ebx
.data:004762DB                 nop
.data:004762DC                 nop
.data:004762DD                 nop
.data:004762DE                 nop
.data:004762DF                 retn
.data:004762DF ExInterlockedFlushSList endp
.data:004762DF
.data:004762DF ; ---------------------------------------------------------------------------

Edited April 3, 2024 by Dietmar

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