As I recall CMPXCHG16B is supported by all Intel CPUs that support x64 and the first Intel CPUs to do this are Intel Pentium 4 5x1 (Prescott).

Looking at https://en.wikipedia.org/wiki/List_of_I ... _(90_nm)_2 then the earliest is 12-Jun-2025.

I have just checked a SIV save file and an Intel Pentium 4 541 (Prescott) reports it has CMPXCHG16B .

I also recall that my Gigabyte 8IPE775P does not support Windows x64 so you need to take care in selecting the motherboard.

It's quite easy to figure out :
CMPXCHG16B (or CompareExchange128) - is one of 3 hardware required CPU instructions for Windows 10 64-bit support.
If CPU works with Win10 x64, it must support it.

Unofficial Win10 x64 requirements :
Any Intel CPU made in 65nm node (anything made around 2006 and newer), or AMD CPU with DDR2 memory support (and newer).
^This is due to older 64-bit capable CPUs lacking one or both of the other two required instructions (PrefetchW and LAHF + SAHF)

For older ones (and as mentioned in previous message) :
Find oldest 64-bit capable Prescott (June 2005), and that should be your CPU (Note : It will not work with Win10 x64, since it's 90nm).
AMD only added this instruction in K8 rev. F (a.k.a. AM2).

According to my CPUID dump database, the CMPXCHG16B instruction appeared in the Intel Prescott E0 revision (CPUID 0F41 and later) and AMD Dual-Core Opteron (Santa Rosa JH-F2, CPUID 00040F12 and later, August 2006) / AMD Athlon 64 X2 (Windsor JH-F2, CPUID 00040F32).

As Matt The Cat only asked about CMPXCHG16B I did not take into account the post Windows 7 requirements. I recall I could not run W10 on my Intel Pentium Extreme 840 (Smithfield) 3.20GHz [A0] @ SL8FK

Further looking at https://en.wikipedia.org/wiki/List_of_I ... _(90_nm)_2 then my Smithfield was released on 18-Apr-2005 which is earlier.

agent_x007 wrote on 2026-07-13, 09:22:

It's quite easy to figure out : CMPXCHG16B (or CompareExchange128) - is one of 3 hardware required CPU instructions for Windows […]
Show full quote

It's quite easy to figure out :
CMPXCHG16B (or CompareExchange128) - is one of 3 hardware required CPU instructions for Windows 10 64-bit support.
If CPU works with Win10 x64, it must support it.

Unofficial Win10 x64 requirements :
Any Intel CPU made in 65nm node (anything made around 2006 and newer), or AMD CPU with DDR2 memory support (and newer).
^This is due to older 64-bit capable CPUs lacking one or both of the other two required instructions (PrefetchW and LAHF + SAHF)

For older ones (and as mentioned in previous message) :
Find oldest 64-bit capable Prescott (June 2005), and that should be your CPU (Note : It will not work with Win10 x64, since it's 90nm).
AMD only added this instruction in K8 rev. F (a.k.a. AM2).

https://theretroweb.com/motherboards/s/dell-p … workstation-370

https://www.cpu-world.com/CPUs/Pentium_4/TYPE … dar%20Mill.html

So for windows 10 x64 I would have to go with Cedar Mill Pentium 4?
Would that work on the motherboard if I do the bios updates?

However it doesn't list PrefetchW and LAHF + SAHF on the CPU World page.

red-ray wrote on 2026-07-13, 07:35:

As I recall CMPXCHG16B is supported by all Intel CPUs that support x64 and the first Intel CPUs to do this are Intel Pentium 4 5 […]
Show full quote

As I recall CMPXCHG16B is supported by all Intel CPUs that support x64 and the first Intel CPUs to do this are Intel Pentium 4 5x1 (Prescott).

Looking at https://en.wikipedia.org/wiki/List_of_I ... _(90_nm)_2 then the earliest is 12-Jun-2025.

I have just checked a SIV save file and an Intel Pentium 4 541 (Prescott) reports it has CMPXCHG16B .

I also recall that my Gigabyte 8IPE775P does not support Windows x64 so you need to take care in selecting the motherboard.

Ahh. I only didn't see the 5x1 CPUs on CPU world when narrowing down by year in the prescotts cause they actually didn't have a year label!

The 5x1 Prescott could be what I am looking for?
Windows 10 x64 support would be cool, but it wasn't the original target.

Rather, I have found on my Athlon 64 clawhammer system with Windows 7 x64, 64 bit unity games get an illegal instruction stop cause of CMPXCHG16B missing. Of course there is the concern that x64dbg may then move onto another illegal instruction even if CMPXCHG16B is available. I mean Unity never listed CMPXCHG16B as needed for the x64 unity games, but testing shows its true tested as far back as a 5.1.1 game, so perhaps it would then also need PrefetchW, LAHF und SAHF?
Probably that is something I can also check with x64dbg without actually waiting for it to stop on it...

As I mentioned before though, since it is socket 775, that theoretically let's me go up to the Pentium 4 Cedar Mill if the bios updates would allow it to work.

Matt The Cat wrote on 2026-07-13, 12:13:

https://theretroweb.com/motherboards/s/dell-p … workstation-370 […]
Show full quote

agent_x007 wrote on 2026-07-13, 09:22:

It's quite easy to figure out : CMPXCHG16B (or CompareExchange128) - is one of 3 hardware required CPU instructions for Windows […]
Show full quote

It's quite easy to figure out :
CMPXCHG16B (or CompareExchange128) - is one of 3 hardware required CPU instructions for Windows 10 64-bit support.
If CPU works with Win10 x64, it must support it.

Unofficial Win10 x64 requirements :
Any Intel CPU made in 65nm node (anything made around 2006 and newer), or AMD CPU with DDR2 memory support (and newer).
^This is due to older 64-bit capable CPUs lacking one or both of the other two required instructions (PrefetchW and LAHF + SAHF)

For older ones (and as mentioned in previous message) :
Find oldest 64-bit capable Prescott (June 2005), and that should be your CPU (Note : It will not work with Win10 x64, since it's 90nm).
AMD only added this instruction in K8 rev. F (a.k.a. AM2).

https://theretroweb.com/motherboards/s/dell-p … workstation-370

https://www.cpu-world.com/CPUs/Pentium_4/TYPE … dar%20Mill.html

So for windows 10 x64 I would have to go with Cedar Mill Pentium 4?
Would that work on the motherboard if I do the bios updates?

However it doesn't list PrefetchW and LAHF + SAHF on the CPU World page.

I've seen a stripped down Win11 booting and running on a Cedar Mill, do note itll be exceptionally slow, even Win10 will chug like crazy. (You will need a stripped down version of Win10)

Neither 10 or 11 will be happy running though, the video I saw did show it had a lot of issues since both operating systems are expecting more modern CPUs and while the OS will run many other programs will also have issues with the older CPU.

But if you manage your expectations then I say go for it, good luck finding working Win10/11 drivers that support the older hardware.

If you don't need Win10 then I would stick to Win7 as that OS will be far happier and more stable on the older hardware.

Trashbytes wrote on 2026-07-13, 12:33:

I've seen a stripped down Win11 booting and running on a Cedar Mill, do note itll be exceptionally slow, even Win10 will chug li […]
Show full quote

Matt The Cat wrote on 2026-07-13, 12:13:

https://theretroweb.com/motherboards/s/dell-p … workstation-370 […]
Show full quote

agent_x007 wrote on 2026-07-13, 09:22:

It's quite easy to figure out : CMPXCHG16B (or CompareExchange128) - is one of 3 hardware required CPU instructions for Windows […]
Show full quote

It's quite easy to figure out :
CMPXCHG16B (or CompareExchange128) - is one of 3 hardware required CPU instructions for Windows 10 64-bit support.
If CPU works with Win10 x64, it must support it.

Unofficial Win10 x64 requirements :
Any Intel CPU made in 65nm node (anything made around 2006 and newer), or AMD CPU with DDR2 memory support (and newer).
^This is due to older 64-bit capable CPUs lacking one or both of the other two required instructions (PrefetchW and LAHF + SAHF)

For older ones (and as mentioned in previous message) :
Find oldest 64-bit capable Prescott (June 2005), and that should be your CPU (Note : It will not work with Win10 x64, since it's 90nm).
AMD only added this instruction in K8 rev. F (a.k.a. AM2).

https://theretroweb.com/motherboards/s/dell-p … workstation-370

https://www.cpu-world.com/CPUs/Pentium_4/TYPE … dar%20Mill.html

So for windows 10 x64 I would have to go with Cedar Mill Pentium 4?
Would that work on the motherboard if I do the bios updates?

However it doesn't list PrefetchW and LAHF + SAHF on the CPU World page.

I've seen a stripped down Win11 booting and running on a Cedar Mill, do note itll be exceptionally slow, even Win10 will chug like crazy. (You will need a stripped down version of Win10)

Neither 10 or 11 will be happy running though, the video I saw did show it had a lot of issues since both operating systems are expecting more modern CPUs and while the OS will run many other programs will also have issues with the older CPU.

But if you manage your expectations then I say go for it, good luck finding working Win10/11 drivers that support the older hardware.

If you don't need Win10 then I would stick to Win7 as that OS will be far happier and more stable on the older hardware.

I am more concerned now if the VRM requirements of the 925x board will handle a cedar mill or if I will have to go with a prescott?
Or perhaps a Pentium D...

The A08 BIOS from July 27, 2006 says it adds support for newer processors.
https://www.dell.com/support/product-details/ … ion-370/drivers

I am interested at this point, what is the oldest socket 775 cpu that can boot Windows 10 x64...? (And I would lean toward dual core if it was released at the same time)

red-ray wrote on 2026-07-13, 07:35:

As I recall CMPXCHG16B is supported by all Intel CPUs that support x64 and the first Intel CPUs to do this are Intel Pentium 4 5 […]
Show full quote

As I recall CMPXCHG16B is supported by all Intel CPUs that support x64 and the first Intel CPUs to do this are Intel Pentium 4 5x1 (Prescott).

Looking at https://en.wikipedia.org/wiki/List_of_I ... _(90_nm)_2 then the earliest is 12-Jun-2025.

I have just checked a SIV save file and an Intel Pentium 4 541 (Prescott) reports it has CMPXCHG16B .

I also recall that my Gigabyte 8IPE775P does not support Windows x64 so you need to take care in selecting the motherboard.

I was talking to someone else online about this and they said that "they support those instructions, but they're disabled when the cpu is in 64-bit mode for whatever reason. This bug was fixed on the 6x1 chips..." and this was in reference to running Windows 10 or 11 x64. But is it true that until the Cedar Mill P4s, CMPXCHG16B didn't work in 64 bit mode?

Matt The Cat wrote on 2026-07-13, 15:06:

I am more concerned now if the VRM requirements of the 925x board will handle a cedar mill or if I will have to go with a prescott?
Or perhaps a Pentium D...

A 925x board won't handle any Pentium D. No way. I tried to run such system very hard but than I was told that Pentium D needs SMP support in chipset but i925x (and i915) is lack it.
I supposed that if i925x can handle Conroe than it can Pentium D too - that is obviously wrong.

This was relevant during Windows 8->8.1 because some 64-bit CPUs were retroactively bumped to being 32-bit for lack of several instructions including cmpxchg16b.
I know you know that already, but the Wikipedia page for 8.1 has a few interesting references. This one https://www.pcworld.com/article/448350/new-wi … -windows-8.html suggests some Intel processors without it at first, but then says the "motherboard" (they mean BIOS) doesn't support the instruction.

The issue was frustrating because 8.1 was treated like a service pack, meaning plain 8 had a super short shelf life. 8 users on affected processors would either have to downgrade to 8.1 32 bit or upgrade (sic) to 7 64 bit and get an extra five years of support.

jakethompson1 wrote on 2026-07-13, 19:47:

This was relevant during Windows 8->8.1 because some 64-bit CPUs were retroactively bumped to being 32-bit for lack of several instructions including cmpxchg16b.
I know you know that already, but the Wikipedia page for 8.1 has a few interesting references. This one https://www.pcworld.com/article/448350/new-wi … -windows-8.html suggests some Intel processors without it at first, but then says the "motherboard" (they mean BIOS) doesn't support the instruction.

The issue was frustrating because 8.1 was treated like a service pack, meaning plain 8 had a super short shelf life. 8 users on affected processors would either have to downgrade to 8.1 32 bit or upgrade (sic) to 7 64 bit and get an extra five years of support.

So Dual core is out of the question since 925x lacks SMP. But then Pentium 4 prescott with EM64T or Pentium 4 Cedar Mill? Are they, and what are they lacking if so to run 8.1 and newer?

Roman555 wrote on 2026-07-13, 19:43:

Matt The Cat wrote on 2026-07-13, 15:06:

I am more concerned now if the VRM requirements of the 925x board will handle a cedar mill or if I will have to go with a prescott?
Or perhaps a Pentium D...

A 925x board won't handle any Pentium D. No way. I tried to run such system very hard but than I was told that Pentium D needs SMP support in chipset but i925x (and i915) is lack it.
I supposed that if i925x can handle Conroe than it can Pentium D too - that is obviously wrong.

No SMP, fair enough. but that still has me wonder would the 65nm cedar mill pentium 4 have motherboard incompatibility with the Dell precision 370 workstation with the i924x? Is the VRM the only concern given that it has a BIOS update from 2006? And you mentioned conroe, are you saying it can or can't run those? Cause that would include core 2 duo if I am not mistaken.

Matt The Cat wrote on 2026-07-13, 21:25:

jakethompson1 wrote on 2026-07-13, 19:47:

This was relevant during Windows 8->8.1 because some 64-bit CPUs were retroactively bumped to being 32-bit for lack of several instructions including cmpxchg16b.
I know you know that already, but the Wikipedia page for 8.1 has a few interesting references. This one https://www.pcworld.com/article/448350/new-wi … -windows-8.html suggests some Intel processors without it at first, but then says the "motherboard" (they mean BIOS) doesn't support the instruction.

The issue was frustrating because 8.1 was treated like a service pack, meaning plain 8 had a super short shelf life. 8 users on affected processors would either have to downgrade to 8.1 32 bit or upgrade (sic) to 7 64 bit and get an extra five years of support.

So Dual core Pentium is out of the question since 925x lacks SMP. But then Pentium 4 prescott with EM64T or Pentium 4 Cedar Mill? Are they, and what are they lacking if so to run 8.1 and newer?

Most 925X chipset based boards don't support P4 Cedar Mill's.
There are few that do, but those are later designs (your OEM MB has little chance of supporting it "out of the box").
However, it's not like 65nm CPUs are expensive to get, you could try to buy a lower end model and see if it works on latest BIOS (with another Pentium 4 6xx as spare in case it doesn't work [that IS NOT a 631/641/651/661 model]).
Note : 925X boards can support Core 2 Duo CPUs (with some pad manipulation and BIOS ucode mods), but not Core 2 Quads (same "lack of SMP" issue as on Pentium Ds).

As for CompareExchange128 support, I don't have my P4 524 coreinfo screenshot on hand, however both Smithfield and Prescott-2M support it under 64-bit Windows :
Prescott-2M :

Smithfield :

I know 32-bit only Prescott's doesn't support it (in E0 stepping) :

Out of the two, Prescott-2M should be supported on your OEM motherboard (and "added CPU support" on last BIOS might be about later revision of Prescott-2M : Rev. "R0" and/or Prescott-1M with "G1" revision).
OEM guys in general were always lazy/late to do BIOS updates on stuff (this changed later), because of this - don't expect all CPUs released in 2006 up to BIOS date point to automaticly work with it.

Matt The Cat wrote on 2026-07-13, 19:21:

But is it true that until the Cedar Mill P4s, CMPXCHG16B didn't work in 64 bit mode?

AFAIK no, I found a SIV save file from a Windows XP x64 Professional V5.02 Build 3790 Service Pack 2 ASUS P5AD2-E-Premium system with an Intel Pentium 4 650 (Prescott 2M) 3.40GHz [N0] @ SL7Z7 which is pre Cedar Mill and it reports CMPXCHG16B Instruction support, but the save file is from 24-Sep-2008 and was generated by SIV.exe (now called SIV32L.exe) which is a 32-bit ASCII build.

I just found a SIV64X save file from a Windows 7 x64 Ultimate V6.01 Build 7601 Service Pack 1 Gigabyte 965P-DS3 system with my Intel Pentium Extreme 840 (Smithfield) 3.20GHz [A0] @ SL8FK which is also pre Cedar Mill and it reports CMPXCHG16B Instruction support, given Smithfield + Prescott are basically the same this supports CMPXCHG16B working in 64-bit mode.

Given it's reported as supported I expect it to work, but the only way to be 100% sure is to run an x64 program that does CMPXCHG16B on such a system and see what happens.

Matt The Cat wrote on 2026-07-13, 00:06:

I am trying to build or buy a system based on the oldest x64 CPU that supports CMPXCHG16B instruction.

red-ray wrote on 2026-07-13, 09:54:

Further looking at https://en.wikipedia.org/wiki/List_of_I ... _(90_nm)_2 then my Smithfield was released on 18-Apr-2005 which is earlier.

Given you initial statement then you need to get a motherboard that supports the Intel Pentium Extreme 840 (Smithfield) 3.20GHz [A0] @ SL8FK.

red-ray wrote on 2026-07-14, 06:50:

AFAIK no, I found a SIV save file from a Windows XP x64 Professional V5.02 Build 3790 Service Pack 2 ASUS P5AD2-E-Premium sy […]
Show full quote

Matt The Cat wrote on 2026-07-13, 19:21:

But is it true that until the Cedar Mill P4s, CMPXCHG16B didn't work in 64 bit mode?

AFAIK no, I found a SIV save file from a Windows XP x64 Professional V5.02 Build 3790 Service Pack 2 ASUS P5AD2-E-Premium system with an Intel Pentium 4 650 (Prescott 2M) 3.40GHz [N0] @ SL7Z7 which is pre Cedar Mill and it reports CMPXCHG16B Instruction support, but the save file is from 24-Sep-2008 and was generated by SIV.exe (now called SIV32L.exe) which is a 32-bit ASCII build.

I just found a SIV64X save file from a Windows 7 x64 Ultimate V6.01 Build 7601 Service Pack 1 Gigabyte 965P-DS3 system with my Intel Pentium Extreme 840 (Smithfield) 3.20GHz [A0] @ SL8FK which is also pre Cedar Mill and it reports CMPXCHG16B Instruction support, given Smithfield + Prescott are basically the same this supports CMPXCHG16B working in 64-bit mode.

Given it's reported as supported I expect it to work, but the only way to be 100% sure is to run an x64 program that does CMPXCHG16B on such a system and see what happens.

Matt The Cat wrote on 2026-07-13, 00:06:

I am trying to build or buy a system based on the oldest x64 CPU that supports CMPXCHG16B instruction.

red-ray wrote on 2026-07-13, 09:54:

Further looking at https://en.wikipedia.org/wiki/List_of_I ... _(90_nm)_2 then my Smithfield was released on 18-Apr-2005 which is earlier.

Given you initial statement then you need to get a motherboard that supports the Intel Pentium Extreme 840 (Smithfield) 3.20GHz [A0] @ SL8FK.

https://www.cpu-world.com/CPUs/Pentium_ ... 000F).html

The socket 775 prescott 630 for example was Feb 21, 2005 and also shows support for EM64T and CMPXCHG16B. So about three months earlier than the smithfield. So assuming there is nothing to suggest that it shouldn't be able to use CMPXCHG16B in 64 bit mode, I think that would be the way to go for targeting the earliest CPU/motherboard combination, especially since the june 2004 925x motherboards. I had a chance to get that Dell Precision 370, but I realized that spending money on these things wont really make my life better. It is just impulses and dopamine. Anyway, I need better habits for myself and I must stay responsible. 'Nough said. At least from me.

My bad, I had Pentium 4 541 (not 524) :

^supports CX16 (which is short for "CMPXCHG16B")

agent_x007 wrote on 2026-07-14, 17:38:

My bad, I had Pentium 4 541 (not 524) :

The plot thickens, it seems that the Intel Pentium 4 541 (Prescott) [G1] supports LAHF and SAHF instructions but the Intel Pentium 4 541 (Prescott) [E0] does not!

It looks like I don't save a SIV Menu->File->Save Local file from a 541 (Prescott) [G1], please will you send me one generated by SIV64X.exe ?