After successfully modding Z170, Z270 BIOS for Coffee Lake, following the guide:

[GUIDE] Coffee Lake CPUs on Skylake and Kaby Lake motherboards

The Hyper-threading (HT) might not work with 6-cores CPUs (i7-8700K). The problem is that the the BIOS of the boards is configured to support maximum of 8 (logical) CPUs. This guide will show you how to change/increase the maximum value of supported CPUs in your BIOS from 8 to 16. We use BIOS of Asus Maximus VIII Impact (M8I) as an example but these steps should work for BIOSes of other Z170, Z270 boards as well. Basic knowledge of UEFITool, hex editor/disassembler is required.

Two steps are required to (fully) support 16 CPUs:

1. patching PEI module CpuMpPei
2. modifying ACPI tables

According to dsanke, it looks like on some boards/BIOS variants additional module(s) needs to be patched (we have no solution for this):

Zitat von dsanke im Beitrag #56

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I've tested lots of motherboards.
Most ASUS 100 series ( Z170/Z170I PRO GAMING , Z170-P ,Z170-A )need BIOS version lower than 3000. And these BIOS do not have CpuMpPei. HT is OK without patch.
If I use BIOS version higher than 3000 , with CpuMpPei patched , it can boot with i7 , but stuck in ASUS logo or just keep rebooting.
ROG M8H BIOS 3802 with CpuMpPei patched work with i7 perfectly.
ROG M9A BIOS 0906 with CpuMpPei patched work with i7 , but newer version (1010 1203 1301 )stuck in ASUS logo ,debug light show 9C.
ROG M9H all BIOS version with CpuMpPei patched , boot with i7 ,keep rebooting , debug light show 91.
ROG STRIX Z270F BIOS 1301 with CpuMpPei patched , boot with i7 , stuck in ASUS logo .0906 with CpuMpPei patched works.
All I tested BIOSTAR, ECS ,ASRock,GALAX,SOYO,Maxsun,Colorful,Gigabyte,Clevo BIOS with CpuMpPei patched work with i7 perfectly.
MSI laptop MS-16L1 MS-16L3 and mini PC G65/G65VR work with cfl i7 after CpuMpPei patched.
Lots of MSI 100 series 16M BIOS even can not boot with cfl i5. MSI 8M BIOS can work with i5.
But after CpuMpPei patched , it won't boot with cfl i7.EZ debug light show CPU , DRAM and VGA keep circulating.
According to my test , seems except ASUS and MSI , the other manufacturer's motherboard all can work with cfl i7 now.

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file offset   hex                            asm
00000992      8d8cc698040400                 lea         ecx, [esi+eax*8+00040498]
...
000009c4      8db800000400                   lea         edi, [eax+00040000]
...
00000a4b      bec0010000                     mov         esi, 0x1c0
 

with:

file offset   hex                            asm
00000992      8d8cc658080800                 lea         ecx, [esi+eax*8+00080858]
...
000009c4      8db800000800                   lea         edi, [eax+00080000]
...
00000a4b      be80030000                     mov         esi, 0x380
 

At file offset 0x992 (this location may/will be different for different version of BIOS), there is a code responsible for allocating space/memory for some structures describing CPUs.
Some technical details/math here - assuming:

"n" is the count of maximum supported CPUs
"m" is largest monitor-line size in bytes, stored in eax
"k" is some constant stored in esi

    n * (0x8000 + 0x38 + m) + 0x2d8 + k

    8 * (0x8000 + 0x38 + m) + 0x2d8 + k = k + m *8 + 40498

    10 * (0x8000 + 0x38 + m) + 0x2d8 + k = k + m * 10 + 80658

    lea         ecx, [esi+eax*10+00080658]

    lea         ecx, [esi+eax*8+00080858]

    40000 -> 80000
      1c0 -> 380

2. modifying ACPI tables

We need to make these steps to fix ACPI tables:

1. locate and extract ACPI tables from BIOS
2. disassemble ACPI tables
3. increase number of supported CPUs in ACPI tables
4. compile modified tables
5. replace/insert modified tables to BIOS

For reading BIOS we will use UEFITool NE alpha (it has integrated hex view), this version cannot save/modify BIOS, use older version for this. For decompiling/recompiling ACPI tables we will use iASL tool from ACPICA.org

2.1 locate and extract ACPI tables from BIOS

The simple way - assuming that BIOS device name for CPU number 8 is "\_PR.CPU7" - we start by searching string/text "CPU7" in the body of our BIOS using UEFITool:

/6B5C8FE5-70DD-4E17-BFF4-D21C26586EB3/
 |__ 08_6B5C8FE5-70DD-4E17-BFF4-D21C26586EB3.raw  "Ther_Rvp"
 |__ 09_6B5C8FE5-70DD-4E17-BFF4-D21C26586EB3.raw  "Ther_Sds"

/C118F50D-391D-45F4-B3D3-11BC931AA56D/
 |__ 01_C118F50D-391D-45F4-B3D3-11BC931AA56D.raw  "A M I "

/C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3/
 |__ 01_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "ApCst"
 |__ 02_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "ApHwp"
 |__ 03_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "ApIst"
 |__ 04_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "ApTst"
 |__ 06_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "Cpu0Hwp"
 |__ 09_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "CpuSsdt"
 |__ 10_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "CtdpB"
 |__ 11_C38FB0E2-0C43-49C9-B544-9B17AA4DCBA3.raw  "HwpLvt"

CPU8 .. CPUF
STS8 .. STSF
PDC8 .. PDCF
CAPA .. CAPF

Name (DEVY, Package (0x2E)
            {
                Package (0x03)
                {
                    "\\_PR.CPU0", 
                    Zero, 
                    Package (0x02)
                    {
                        Zero, 
                        Package (0x02)
                        {
                            0xFF, 
                            Zero
                        }
                    }
                }, 
 
                Package (0x03)
                {
                    "\\_PR.CPU1",
 

We need to add 8 new packages (for "CPU8" .. "CPUF") into this "DEVY" package. In this case, we do not want to insert the new packages for "CPU8" .. "CPUF" right after packages for "CPU1" .. "CPU7". If we do this, we will change the order of all remaining entries of the "DEVY" package, and we would need to change/modify all references to items in the "DEVY" package. Better solution is to add new packages (for "CPU8" .. "CPUF") at the end of the list. Using file compare tool on original and modified file should make this clear:

Maximus VIII Impact + i7-8700K with multiplier of 47:



Power management - downclocking/downvolting is also working (with 8700K it can save you cca 10W when idle):

### Updated in May 18th 2021:
I got the way to fully unlock 16 threads on all these mainboards:
[GUIDE] Additional for fully unlock 16 threads for 22nm PCH Intel 100/200/300s mainboards.

Great thanks to @s.napi !
Succeed in fixing my Gigabyte B150M DS3H DDR3 .
I didn't do the DSDT fix , so the CPU numbers in Device Manager is abnormal.

Can you get the CPU to other frequencies/is turbo mode working? Any problems with system stability (BSOD after resume from sleep)?

Zitat von s.napi im Beitrag #5

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Can you get the CPU to other frequencies/is turbo mode working? Any problems with system stability (BSOD after resume from sleep)?

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congratulations. very impressive what you did.

€dit: can you name a dissasembler tool for windows? only tool that I can found, and able to use it, was an online dissasembler.

Congratulations, very impressive work!
One question - after the mod, are the core ratio multiplier settings for core 5 and 6 now visible in bios? Or is it the case that just as before only 4 cores are adjustable, however syncing all cores syncs the set multiplier speed also on cores 5 and 6?
Can you also upload you ready modded bios?
I also have M8I but with i5-8600k and would love to give it a try.

@mtothaj no those are bios specific options. Sync all cores will still only apply up to 4c. It's easiest to just use 125bclk though. With the 8x fclk multiplier, it'll still run at 1000mhz. Would allow up to 5375 for 8700k and 5125 for 8600k. Otherwise, 133bclk works fine as well (slightly overclocked fclk at 1066) if you need higher clocks.

@ziddey would it be possible to insert a module from another mainboard to get the core adjustment for all 6 cores working? For the asrock z170 mocf there is a patched bios out with the possibility to change all 6 cores. I didn´t know how it´s coded into the bios, so maybe my question is stupid.

@mtothaj, @oldirdey, @ziddey
Actually, there is also fix for sync all cores (for M8I). You can't choose ratio for 4,5,6 cores individually, in BIOS, but if you choose "Sync All Cores", all cores will be synced now. It will work from one up to eight cores. One byte patch in module C654E340-2DB5-4896-BBC4-6EA45CFEB434. Here is a link to patched (16 cpu Coffee Lake compatible) M8I (v3801) BIOS capsule. Use Flashback method. Presence of Coffee Lake compatible BIOS (or at least ME region) is required - Flashback will not flash ME region.

@s.napi many thanks for this. I have just tried to flash via bios flashback using a number of different usb drives but no luck - the flashback light flashes for a few moments but then just stays lit. This is more or less my experience with bios flashback when trying to flash the 6C / Z370 SKU bios mod via bios flashback - never got it to work and always had to use SPI programmer.
I will need to disassemble the machine and try to flash via SPI programmer, first extracting bin from .cap and adding my board data and will report back.

EDIT: Finally I succeeded using bios flashback - for some reason 'problem' was with pendrive - the branded ones like Adata or Kingston did not want to work (tried 4 different ones from reputable brands) but the cheapest, nastiest one I had ended up working fine.
Bios works great with i5-8600k - All core enhancement syncs all 6 cores so no more need to use XTU. Many thanks for this :)

I understand that in future it will be necessary to add new microcode of the 9 gen CPU's like i9-9900k etc assuming there will be no new hurdles / obstacles stoping them from working on older boards.

I had the same problem a time ago. But I can't remember what the cause was now (usb disk in wrong port, bad format of cap file..). Anyway, you can use any other software flash method (afudos, afuwin...). In case of SPI flashing, dont forget to replace BIOS header/ME region of posted BIOS file.

@s.napi awesome thank you. I compared the ocmr_pei modules and see that you've changed one byte from F4 to F0. I found the same pattern in my bios (asus z170 pro gaming/aura) and will give it a change as well. Can you explain what's actually going on here?

Thanks to @s.napi again ,you have saved ASUS old motherboards.
Could you please help me have a look at other manufacturer's motherboard and BIOS?
I can't find such module like ASUS.

Update : Found mov ecx,1ADh wrmsr in SiInit PEI module too.