Thanks for the Mecer B870 BIOS.
It is perfectly fine, so you saved it correctly. 😀

The checksum of this 64KB AMI BIOS file is correct and it works fine when I tested it in 86Box. But I can see how limited the CPU support is in this 1993 BIOS.

The choice of a replacement Ali 1429 Award BIOS is limited, but the Atrend ATC-1762 VER 2.0 BIOS from 1994 looks promising. I would have liked to use the ATC-1762 Ver: 2.1 BIOS from August 1995, but unfortunately this one is not available on TRW.

I will keep you posted,
Jan

Thank you for the confirmation and the time spent to assist me with this. l’ll send the BIOS image to TRW so that they finally have at least one for this board.

Yes, I suppose for a 1993 board, I couldn’t have expected much.
At least the Intel and AMD DX4 processors that I have do seem to work properly, however the performance is a little underwhelming. There really isn’t much that I can configure in the bios to tune the performance.

I’ll give the Atrend BIOS a try once I receive the chips that I ordered and then I’ll update here again.
Expecting about a month for delivery of those chips unfortunately.

Some BIOSes do not know what to do with your unknown CPU.
It is possible that it won't turn on fastest operating mode like write back although it may enable it on slower processors - as long as the hardware wiring is sufficient. Note that Cyrix processors had write back earlier than Intel ones but with a different layout. If this applies to your motherboard you won't get write back with DX4-processors. To use write back you are limited to Cyrix DX2-80 and perhaps (if there any exists) Cyrix DX4 without the imprint "STANDARD LAYOUT".
It is possible that it is confused by the processor speed. It may guess the FSB then as too fast. Then it unnecessarily may slow down bus, memory and cache dividers or clock speeds.

Intel486dx33 wrote on 2025-04-28, 14:34:

Don’t over clock you might over heat and ruin your motherboard or CPU. It’s NOT worth it because the performance improvement is […]
Show full quote

Don’t over clock you might over heat and ruin your motherboard or CPU.
It’s NOT worth it because the performance improvement is minimal and might make your computer unstable.
If you want the Best performance of a 486 use a DX4-100
Or an AMD 5x86-133 or for the Very Best Performance use an Intel Pentium 83mhz over drive.
64kb of Cache is good enough
8mb to 16mb of RAM is Enough
Windows 95 was Designed to Support the 486dx-33 CPU

In the End. Its Still a 586 cPU
It can’t play MP3’s or Video very well.
You Need a Pentium Class CPU for that.

><((((*>

Which DX4-100 do you mean for Best performance of a 486? There are so many flavours!

An AMD 5x86 is not a 586. It is just an enhanced 486 supporting 4x clock. Just change the clock to 3x and it will be reported as Enhanced AMD 486 DX4. There is no difference to an AMD 486 DX4 with 16 kB L1-Cache then.
A Cyrix 5x86 is a different thing. It has many techniques from a Pentium class CPU incorporated.

Of course your 486 with 64 kB L2 cache cannot play MP3. It is not enough for that!
But it will do it with a lower bitrate, frequency or in mono.
Of course your 486 cannot play full HD video.
But it may play 320x200 with Intel Indeo codec.

Chkcpu wrote on 2025-04-28, 17:51:

Thanks for the Mecer B870 BIOS. It is perfectly fine, so you saved it correctly. :) […]
Show full quote

Thanks for the Mecer B870 BIOS.
It is perfectly fine, so you saved it correctly. 😀

The checksum of this 64KB AMI BIOS file is correct and it works fine when I tested it in 86Box. But I can see how limited the CPU support is in this 1993 BIOS.

The choice of a replacement Ali 1429 Award BIOS is limited, but the Atrend ATC-1762 VER 2.0 BIOS from 1994 looks promising. I would have liked to use the ATC-1762 Ver: 2.1 BIOS from August 1995, but unfortunately this one is not available on TRW.

I will keep you posted,
Jan

Hi Jan,

I finally received the EEPROM chips that I ordered and was able to flash the Atrend ATC-1762 BIOS to one of them and install it into the board.

I can confirm that the BIOS does work on my board and seems to run fine, although I haven't tested all functionality yet.

I'm still struggling to get WB L1 cache working (it is enabled in the BIOS) and have not been able to get my Am5x86 133 to work in 4x clock mode either.
So at this stage, the Atrend BIOS seems to have same functionality as the OEM BIOS, however it is at least an Award BIOS that works on the board, which I believe was the first goal, right?

My board has jumpers labelled for 2x and 3x multipliers but only the 3x option seems to work. That said, it's a 6-pin jumper header and may have other options that aren't printed on the silkscreen.
Not quite sure at the moment how to find the correct multiplier jumpers.

byte_76 wrote on 2025-06-03, 20:14:

I'm still struggling to get WB L1 cache working (it is enabled in the BIOS) and have not been able to get my Am5x86 133 to work in 4x clock mode either.
My board has jumpers labelled for 2x and 3x multipliers but only the 3x option seems to work. That said, it's a 6-pin jumper header and may have other options that aren't printed on the silkscreen.

AMD Socket 1-3 processor's WB function is enabled by jumper. They ignore WB L1 setting in BIOS. The only processors who support this are the Cyrix ones (including ST, Ti and IBM).
Perhaps you use tools like ctcm and speedsys to test cache function.
What happens in 2x (=4x) position?

byte_76 wrote on 2025-06-03, 20:14:

I'm still struggling to get WB L1 cache working (it is enabled in the BIOS)

With Award BIOSes, this often only applies to Cyrix processors (both 486 and 5x86). Those processors have the L1 mode software controllable. On the other hand, late AMD and Intel 486 processors with write-back L1 support (this includes the AMD 5x86) use a processor pin (usually connected to a jumper) to choose the L1 mode. Nevertheless, the BIOS also has to set up the chipset for the Intel/AMD L1WB protocol, and the signals must be routed from the processor socket to the chipset in the correct way. Both the protocol details and the pin positions are different between the L1WB implementation by Cyrix and the one by Intel.

The BIOS typically sets up the chipset for Intel/AMD write-back support only if it recognizes the CPUID of the currently installed processor as "processor with Intel L1WB". The CPUID reported by L1WB capable 486 and 5x86 processors differs depending an the cache mode, and usually also differs depending on the multiplier. The consequence is that old BIOSes often don't recognize the x4 multiplied CPU IDs of the AMD 5x86, which make L1WB fail on those processors in 4x clock mode.

byte_76 wrote on 2025-06-03, 20:14:

My board has jumpers labelled for 2x and 3x multipliers but only the 3x option seems to work.

You should be aware that the earliest AMD 486DX4 processors used a different pin to choose between 2x and 3x than Intel used on their DX4. AMD DX4 processors with markings "NV8T" or "V8T" (rare, old, maybe they don't exist at all?) use the pin for multiplier selection that is used to choose between L1WT and L1WB on later AMD (486 "SV8B" and 5x86) processors. Make sure you use the "Intel DX4 multiplier selection" jumper to choose between 3x and 4x on an 5x86, not the "AMD DX4" multiplier jumper, if the mainboard manual cites different ones.

You may also want to rule out that the L1 WB support might not be wired correctly. For example, my 4DMS-HL3G needs this modification for an Am5x86 to correctly work in WB mode. Certain pins had the Pentium OverDrive version correctly connected to the chipset and/or jumpers, but not the 486 equivalent.

mkarcher wrote on 2025-06-04, 00:03:

With Award BIOSes, this often only applies to Cyrix processors (both 486 and 5x86). Those processors have the L1 mode software c […]
Show full quote

byte_76 wrote on 2025-06-03, 20:14:

I'm still struggling to get WB L1 cache working (it is enabled in the BIOS)

With Award BIOSes, this often only applies to Cyrix processors (both 486 and 5x86). Those processors have the L1 mode software controllable. On the other hand, late AMD and Intel 486 processors with write-back L1 support (this includes the AMD 5x86) use a processor pin (usually connected to a jumper) to choose the L1 mode. Nevertheless, the BIOS also has to set up the chipset for the Intel/AMD L1WB protocol, and the signals must be routed from the processor socket to the chipset in the correct way. Both the protocol details and the pin positions are different between the L1WB implementation by Cyrix and the one by Intel.

The BIOS typically sets up the chipset for Intel/AMD write-back support only if it recognizes the CPUID of the currently installed processor as "processor with Intel L1WB". The CPUID reported by L1WB capable 486 and 5x86 processors differs depending an the cache mode, and usually also differs depending on the multiplier. The consequence is that old BIOSes often don't recognize the x4 multiplied CPU IDs of the AMD 5x86, which make L1WB fail on those processors in 4x clock mode.

byte_76 wrote on 2025-06-03, 20:14:

My board has jumpers labelled for 2x and 3x multipliers but only the 3x option seems to work.

You should be aware that the earliest AMD 486DX4 processors used a different pin to choose between 2x and 3x than Intel used on their DX4. AMD DX4 processors with markings "NV8T" or "V8T" (rare, old, maybe they don't exist at all?) use the pin for multiplier selection that is used to choose between L1WT and L1WB on later AMD (486 "SV8B" and 5x86) processors. Make sure you use the "Intel DX4 multiplier selection" jumper to choose between 3x and 4x on an 5x86, not the "AMD DX4" multiplier jumper, if the mainboard manual cites different ones.

Unfortunately I don’t have the motherboard manual as this is not a very common board.

The jumper settings are only partially printed on the silkscreen but there isn’t enough info to determine jumper settings for newer 486 processors.

In 2x mode, the board does not POST.

At this point, I’m not sure how to determine the correct jumpers for the cache and the clock.

I’m very much a novice with this stuff so I’d be grateful for assistance in walking through the process to figure this out.

Regarding the BIOS, I believe that it does not currently have support for the AMD 5x86 133 but hopefully that can be addressed.

I see that Jan has explained very nicely how to get the Am5x86 into 4x clock mode.

http://www.steunebrink.info/amd5x86.htm

I’ll try this out on my board when I have some time to mess with it again.

Most likely the CLKMUL pin will be connected to the pin for 2x mode on my board.
If that is the case, I’m not sure why it doesn’t POST but I’ll check to be sure.

I have identified CLKMUL pin( R-17), it is indeed the 2x jumper position as printed on the silkscreen but the neighbouring pin is connected to ground through a 1 kΩ SMD resistor, not directly to ground.

Could that be the reason why the system does boot when the jumper is installed in the 2x position, or is it more likely that the lack of support for the CPU in the BIOS results in no POST?

byte_76 wrote on 2025-06-05, 19:55:

I have identified CLKMUL pin( R-17), it is indeed the 2x jumper position as printed on the silkscreen but the neighbouring pin is connected to ground through a 1 kΩ SMD resistor, not directly to ground.

Could that be the reason why the system does boot when the jumper is installed in the 2x position, or is it more likely that the lack of support for the CPU in the BIOS results in no POST?

Hi byte_76,

Okay, the ATC-1762 BIOS works on your B870-I2 board, great!

When you run the Am5x86-133 at 25 or 33MHz FSB, you are not overclocking the CPU when setting the multiplier to x4. I agree that it is the lack of Am5x86 support in the BIOS that prevents POST under these conditions.

I’ve already put the ATC-1762 v2.0 BIOS on my “to be patched” list for Am5x86-133 support. It will take me a while though, but in the meantime you could run the Am5x86 at 3x40MHz if 40MHz FSB is possible on this board.

Jan

Chkcpu wrote on 2025-06-06, 13:49:

Hi byte_76, […]
Show full quote

byte_76 wrote on 2025-06-05, 19:55:

I have identified CLKMUL pin( R-17), it is indeed the 2x jumper position as printed on the silkscreen but the neighbouring pin is connected to ground through a 1 kΩ SMD resistor, not directly to ground.

Could that be the reason why the system does boot when the jumper is installed in the 2x position, or is it more likely that the lack of support for the CPU in the BIOS results in no POST?

Hi byte_76,

Okay, the ATC-1762 BIOS works on your B870-I2 board, great!

When you run the Am5x86-133 at 25 or 33MHz FSB, you are not overclocking the CPU when setting the multiplier to x4. I agree that it is the lack of Am5x86 support in the BIOS that prevents POST under these conditions.

I’ve already put the ATC-1762 v2.0 BIOS on my “to be patched” list for Am5x86-133 support. It will take me a while though, but in the meantime you could run the Am5x86 at 3x40MHz if 40MHz FSB is possible on this board.

Jan

Hi Jan

The board does support 40MHz but I think I'll just run it at DX4 100 speed (which POST just fine) until the patched BIOS is ready.

Thank you very much, I'm looking forward to the patched BIOS.

byte_76 wrote on 2025-06-06, 18:49:

Hi Jan […]
Show full quote

Chkcpu wrote on 2025-06-06, 13:49:

Hi byte_76, […]
Show full quote

byte_76 wrote on 2025-06-05, 19:55:

I have identified CLKMUL pin( R-17), it is indeed the 2x jumper position as printed on the silkscreen but the neighbouring pin is connected to ground through a 1 kΩ SMD resistor, not directly to ground.

Could that be the reason why the system does boot when the jumper is installed in the 2x position, or is it more likely that the lack of support for the CPU in the BIOS results in no POST?

Hi byte_76,

Okay, the ATC-1762 BIOS works on your B870-I2 board, great!

When you run the Am5x86-133 at 25 or 33MHz FSB, you are not overclocking the CPU when setting the multiplier to x4. I agree that it is the lack of Am5x86 support in the BIOS that prevents POST under these conditions.

I’ve already put the ATC-1762 v2.0 BIOS on my “to be patched” list for Am5x86-133 support. It will take me a while though, but in the meantime you could run the Am5x86 at 3x40MHz if 40MHz FSB is possible on this board.

Jan

Hi Jan

The board does support 40MHz but I think I'll just run it at DX4 100 speed (which POST just fine) until the patched BIOS is ready.

Thank you very much, I'm looking forward to the patched BIOS.

Hi byte_76,

In Part 4 of my DIY BIOS Modding guide at Re: Diy modding support for k6+And 120gb hard drives into bios roms I showed some details about patching a 1994 Award BIOS for Am5x86-133 support.
It appears that the VER-2.0 10/1994 ATC-1762 BIOS is an exact fit for this procedure, and I could apply these fixes directly with my hexeditor!
So no need to do the time consuming disassembly and analysis of the BIOS code this time. 😀

The ATC-1762 patch J.1 BIOS is ready now, and I’ve made the following changes:
- Added detection of the Am5x86 and the Enhanced Am486DX4. Both these CPUs will now be displayed as DX4-S, instead of Unknown or an 80486DX2.
- Changed the Busspeed detection to include x4 mode for the Am5x86. This fix will result in correct timings for memory and ISA-bus for each FSB setting in x4 mode, and display the 4x33 setting as 132MHz.
- Fixed the Year 2094 bug
- Fixed the 2GB HDD display limit bug for correct drive size indication up to 8GB
- Changed the "Drive A:" SETUP default from 1.2M to 1.44M.

For all other functions, the patched BIOS is identical to the original VER-2.0.
Here is the patched BIOS:

Note that Cyrix support in this BIOS is very limited. Cx486DX2 should work fine, but proper Cx486DX4 and Cx5x86 support is lacking. I have no fix for that, other then finding a 1995 BIOS for this board.

Let me know how the Am5x86 works now on your B870 board.
Jan

Chkcpu wrote on 2025-06-07, 15:45:

Hi byte_76, […]
Show full quote

byte_76 wrote on 2025-06-06, 18:49:

Hi Jan […]
Show full quote

Chkcpu wrote on 2025-06-06, 13:49:

Hi byte_76, […]
Show full quote

Hi byte_76,

Okay, the ATC-1762 BIOS works on your B870-I2 board, great!

When you run the Am5x86-133 at 25 or 33MHz FSB, you are not overclocking the CPU when setting the multiplier to x4. I agree that it is the lack of Am5x86 support in the BIOS that prevents POST under these conditions.

I’ve already put the ATC-1762 v2.0 BIOS on my “to be patched” list for Am5x86-133 support. It will take me a while though, but in the meantime you could run the Am5x86 at 3x40MHz if 40MHz FSB is possible on this board.

Jan

Hi Jan

The board does support 40MHz but I think I'll just run it at DX4 100 speed (which POST just fine) until the patched BIOS is ready.

Thank you very much, I'm looking forward to the patched BIOS.

Hi byte_76,

In Part 4 of my DIY BIOS Modding guide at Re: Diy modding support for k6+And 120gb hard drives into bios roms I showed some details about patching a 1994 Award BIOS for Am5x86-133 support.
It appears that the VER-2.0 10/1994 ATC-1762 BIOS is an exact fit for this procedure, and I could apply these fixes directly with my hexeditor!
So no need to do the time consuming disassembly and analysis of the BIOS code this time. 😀

The ATC-1762 patch J.1 BIOS is ready now, and I’ve made the following changes:
- Added detection of the Am5x86 and the Enhanced Am486DX4. Both these CPUs will now be displayed as DX4-S, instead of Unknown or an 80486DX2.
- Changed the Busspeed detection to include x4 mode for the Am5x86. This fix will result in correct timings for memory and ISA-bus for each FSB setting in x4 mode, and display the 4x33 setting as 132MHz.
- Fixed the Year 2094 bug
- Fixed the 2GB HDD display limit bug for correct drive size indication up to 8GB
- Changed the "Drive A:" SETUP default from 1.2M to 1.44M.

For all other functions, the patched BIOS is identical to the original VER-2.0.
Here is the patched BIOS:

The attachment ATC1762J.zip is no longer available

Note that Cyrix support in this BIOS is very limited. Cx486DX2 should work fine, but proper Cx486DX4 and Cx5x86 support is lacking. I have no fix for that, other then finding a 1995 BIOS for this board.

Let me know how the Am5x86 works now on your B870 board.
Jan

Hi Jan,

It works!!! 😀

The 2x multiplier that would not POST previously is now working as 4x and the CPU indicates DX4-S at 132MHz on boot screen, but in CHKCPU it shows the correct details for 5x86 P75.

The only thing that I still haven't figured out is the WB cache. It still indicates WT in CHKCPU.

Thank you so much for the time and effort that you spent on this, I'm grateful and very pleased to have my Am5x86 working at the correct clocks on the only VLB board that I have.

Hi byte_76,

Great to hear the patched BIOS works as planned! 😀

However, getting the L1 cache in WB mode is another matter.
The Am5x86 article on my website, that you found earlier, also contains information on how to find the jumper that is connected to the CPU’s WB/WT# pin B-13.

A first step is connecting this pin to Vcc to get the L1 cache in WB, but this will most certainly crash the PC. As you see from the article, a lot of other conditions need to be fulfilled as well, one of which concerns the BIOS.
The 1994 BIOS has only WB enable logic for the Cx486DX(2) and Intel 486DX2WB (P24D), so it doesn’t program the chipset for L1 WB support on any other CPU model, when you set WB in the BIOS Setup.

So another BIOS patch is required, but this will need a full BIOS analysis. Success on this is slim however, as Ali 1429(G) chipset documentation is not available. Also the P24D jumper settings on this board are needed to know how to connect the HITM#, INV, and CACHE# signals between the chipset and CPU.

Not a project with a reasonable chance of success. But when I can find the time, I like to analyze this 1429G chipset L1 WB logic to at least contribute to @Rav’s Universal chipset patcher. project.

Let me know if you ever find the B870 board’s manual with P24D jumper settings. That would help enormously!

Jan

Chkcpu wrote on 2025-06-09, 14:19:

Hi byte_76, […]
Show full quote

Hi byte_76,

Great to hear the patched BIOS works as planned! 😀

However, getting the L1 cache in WB mode is another matter.
The Am5x86 article on my website, that you found earlier, also contains information on how to find the jumper that is connected to the CPU’s WB/WT# pin B-13.

A first step is connecting this pin to Vcc to get the L1 cache in WB, but this will most certainly crash the PC. As you see from the article, a lot of other conditions need to be fulfilled as well, one of which concerns the BIOS.
The 1994 BIOS has only WB enable logic for the Cx486DX(2) and Intel 486DX2WB (P24D), so it doesn’t program the chipset for L1 WB support on any other CPU model, when you set WB in the BIOS Setup.

So another BIOS patch is required, but this will need a full BIOS analysis. Success on this is slim however, as Ali 1429(G) chipset documentation is not available. Also the P24D jumper settings on this board are needed to know how to connect the HITM#, INV, and CACHE# signals between the chipset and CPU.

Not a project with a reasonable chance of success. But when I can find the time, I like to analyze this 1429G chipset L1 WB logic to at least contribute to @Rav’s Universal chipset patcher. project.

Let me know if you ever find the B870 board’s manual with P24D jumper settings. That would help enormously!

Jan

Hi Jan,

I think it is unlikely that I will ever find the manual for this board because it is not a very common board.

I have identified the jumper pin for B-13 but there is no 3.3v or 5V directly next to it. There is however 5.1V on a jumper pin nearby. (eg. if B-13 is jumper pin 1 then 5V is jumper pin 3 and pin 2 does not seem to be connected)

The board currently displays WB cache in CHKCPU when I install my Cyrix DX2-v66 and I think it also previously displayed WB when I installed my AMD DX4-100 SV8B but now it only indicates WT with that CPU.

Should I go ahead and connect jumper B-13 to the 5.1V pin? (Is it not risky to put 5V on that pin? I mean even the CPU voltage is only 3.3V and I'd rather not fry my CPU)

Believe these boards were commonly used in Brother / Kyodai BCR series PCs in the early-mid 90's which might be another route to a manual...I have an older version of the B870-II with an April 93 BIOS which came as part of such a system, but that board only supports uo to P24T class processors (no ob voltage reg & no manual 🙁 )

PC Hoarder Patrol wrote on 2025-06-09, 18:51:

Believe these boards were commonly used in Brother / Kyodai BCR series PCs in the early-mid 90's which might be another route to a manual...I have an older version of the B870-II with an April 93 BIOS which came as part of such a system, but that board only supports uo to P24T class processors (no ob voltage reg & no manual 🙁 )

That’s interesting. How different is your board? Can you post a pic?

Maybe you can add the voltage regulator and any other components that are missing.

I’ve shared the original BIOS from my board earlier in the thread, and to TRW if you want to download it.

byte_76 wrote on 2025-06-09, 16:57:

Hi Jan, […]
Show full quote

Chkcpu wrote on 2025-06-09, 14:19:

Hi byte_76, […]
Show full quote

Hi byte_76,

Great to hear the patched BIOS works as planned! 😀

However, getting the L1 cache in WB mode is another matter.
The Am5x86 article on my website, that you found earlier, also contains information on how to find the jumper that is connected to the CPU’s WB/WT# pin B-13.

A first step is connecting this pin to Vcc to get the L1 cache in WB, but this will most certainly crash the PC. As you see from the article, a lot of other conditions need to be fulfilled as well, one of which concerns the BIOS.
The 1994 BIOS has only WB enable logic for the Cx486DX(2) and Intel 486DX2WB (P24D), so it doesn’t program the chipset for L1 WB support on any other CPU model, when you set WB in the BIOS Setup.

So another BIOS patch is required, but this will need a full BIOS analysis. Success on this is slim however, as Ali 1429(G) chipset documentation is not available. Also the P24D jumper settings on this board are needed to know how to connect the HITM#, INV, and CACHE# signals between the chipset and CPU.

Not a project with a reasonable chance of success. But when I can find the time, I like to analyze this 1429G chipset L1 WB logic to at least contribute to @Rav’s Universal chipset patcher. project.

Let me know if you ever find the B870 board’s manual with P24D jumper settings. That would help enormously!

Jan

Hi Jan,

I think it is unlikely that I will ever find the manual for this board because it is not a very common board.

I have identified the jumper pin for B-13 but there is no 3.3v or 5V directly next to it. There is however 5.1V on a jumper pin nearby. (eg. if B-13 is jumper pin 1 then 5V is jumper pin 3 and pin 2 does not seem to be connected)

The board currently displays WB cache in CHKCPU when I install my Cyrix DX2-v66 and I think it also previously displayed WB when I installed my AMD DX4-100 SV8B but now it only indicates WT with that CPU.

Should I go ahead and connect jumper B-13 to the 5.1V pin? (Is it not risky to put 5V on that pin? I mean even the CPU voltage is only 3.3V and I'd rather not fry my CPU)

Hi byte_76,

The Am5x86 datasheet specifies this 3.45V Vcore CPU with 5V tolerant I/O. Apart from the supply voltage pins, the absolute maximum voltage on any pin is specified as Vcc +2.6V. So as long as you stay below 6.0V on pin B-13, you should be safe.

When you are going ahead with this L1 cache WB test on the Am5x86, try booting from a DOS 5 or 6 floppy. This is the best L1 cache WB coherency test I know, on any WB capable CPU.

Cheers, Jan