Could it have something to do with the "antivirus" option in bios getting turned on, or misbehaving?

Since my board boots from a floppy, here is a photo of my jumper configuration for the off chance something is wrong. My board is configured for a cyrix 5x86c at 2x60 so the fsb/divider/multiplier (black jumper)may differ, but the rest should be the same for an am5x86.

feipoa wrote on Yesterday, 02:37:

For anyone who owns a PC Chips M918 v1.2 motherboard, were you able to boot from a 1.44M floppy diskette? When I try to boot t […]
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For anyone who owns a PC Chips M918 v1.2 motherboard, were you able to boot from a 1.44M floppy diskette? When I try to boot to the floppy disk, it reads the disk for about 4 seconds, then stops reading. The floppy drive light stays on, but nothing else happens. If I boot from the HDD, I can read the floppy disk within DOS just fine.

I have tried two different floppy cables, two different brands of floppy drives, and two different brands of boot diskettes (brand A was a w95 startup disk and brand B was Memtest 4.20). These boot disks work fine on every other system I've used. The floppy drives work fine on all other motherboard's I tested. I've tried all 4 M918 BIOS revisions.

There's likely something going on with motherboard hardware or BIOS. I tried swapping the UM8663AF chip, but it didn't help. I reflowed the floppy pins and measured the 10K pull-ups on the floppy header.

Full list of floppy drives tested:
Mitsumi D359T5
Mitsumi D63119
Mitsumi D359M3D
Epsom SMD-300 (F401819729)
Panasonic JU-256A798PC
Panasonic JU-257A137P
Panasonic JU-256A198P
Teac FD-235HF

Has anyone else run into this issue? How did you fix it? Thanks!

😀 Usual 486 mobo with write-back CPU cache problem

Link (sorry in russian) https://phantom.sannata.org/viewtopic.php?t=9954&hilit=4045

I got the same on some VIA and OPTI mobos
Try to catch correct BIOS or CPU type jumpers settings 😀

Hi feipoa,

I agree with Babasha that this looks like an L1 cache WB issue, either due to incorrect jumper setting or lack of BIOS support.
You can easily test this by setting the L1 cache WT/WB jumper to the WT position. I believe this is JP29 1-2 closed on the M918.
You can use my CHKCPU tool to confirm the L1 Cache mode.

If L1 cache in WT mode fixes the floppy boot issue, then you know there is an L1 cache coherency problem in WB mode.
In my old Am5x86 upgrade article, you can read more about this issue.
http://www.steunebrink.info/amd5x86.htm

Cheers, Jan

Thanks for your replies. I agree there seems to be some sort of L1 cache coherency problem, however I've yet to pin it down for the Cyrix 5x86. I did find a solution for the Am5x86.

I tried altering JP29 per chkcpu's recollection, but it didn't help. Next, I decided to trace out where CPU pin WB/WT' goes. The WB/WT' pin goes to JP30 pins 14 and 10. If I place a jumper on JP30 7-14, it forces the Am5x86 in to WT mode. This is expected because JP30 pin 7 goes to GND, and pin 14 is WB/WT'. However, JP30 pin 14 also has a 10 K-ohm pull-up to 5 V (on the motherboard). The Am5x86 CPU datasheet states that WB/WT' has a weak pull-down to GND, so how can removing the jumper from pin 7-14 set WB/WT' high (into WB mode)? The internal pull-down resistor must be of substantially higher resistance than 10K.

Although CHKCPU indicated that the Am5x86 was running in L1: Write-Back mode, the floppy still could not be booted to. I tried to force WB/WT' to 5 V to enable WB, similar to how the M919 does it, but this didn't resolve the boot floppy issue. These are the two convenient points on the motherboard I used to force the WB/WT' pin:

Next, I traced out the other jumper header pin which went to WB/WT', namely JP30 pin 10. The mating end of jumper, JP30 pin 3, goes to the CPU pin SRESET. When I set a jumper on JP30 pins 3-10, I am now able to boot from a floppy diskette! Success at last, but why? Note that there aren't any L1: WB/WT options in the BIOS for the Am5x86. The jumper shown in blue with the red arrow is the one which resolved floppy booting with the Am5x86:

Unfortunately, the jumper on JP30 pin 3-10 did not help booting when a write-back Cyrix 5x86 was installed. M918 BIOS versions 6/2/1995 and 7/21/1995 both have a Cyrix L1: WB/WT BIOS option. If I set L1:WT, the M918 w/Cyrix is able to boot from a floppy. For BIOS versions 11/28/1995 and 1/1/1996, there is no user configurable BIOS option for Cyrix L1:WT/WB - it's always on WB mode. CHKCPU indicates write-back mode, or write-through mode if I sent BIOS to WT. This is where I am stuck now. Thinking a QFP lead might have desoldered, I've reflowed the chipsets after adding flux, but didn't help.

Upon further testing, if I boot from HDD w/Cyrix and try to read the floppy, the DOS can display the floppy's directory, but will hang-up when pressing <ENTER> to see page 2 of the directory contents. It was the Am5x86 which didn't hang-up at this point in testing. Nonetheless, I tried all 4 BIOS versions but could not get a Cyrix 5x86 to boot from a floppy diskette with write-back mode enabled. I even tried installing L2 cache, thinking some funny business might be going on because this motherboard often came with fake L2 cache, but installing L2 installed didn't help resolve the issue w/floppy boots. I tried Cyrix CPU revisions S1R3 and S0R5.

For anyone else with an M918 who has floppy-boot success with a Cyrix 5x86, could you share a photo of your jumper configuration? Similarly, if you are also able to floppy-boot with an Am5x86 and do not have JP30 3-10 jumpered, could you share a photo of your jumpers?

Hmm there is definitely something wrong with that motherboard then.

Nor my running board nor my parts board had the blue jumper on and both booted from floppy with both cyrix or amd

Interestingly, I seem to remember BobocoCz mentioning that he was unable to switch L1 (I think) to WB no mater what he did when he used an am5x86 on the 918. It's sounding like there may be some sort of problem with the circuitry responsible.

I read through a whack of conversation you had with Pshipkov on this board at one point re cache etc, yikes, no wonder this one gets a bad reputation and ends up back in the closet time and time again, seems like the only consistent experience is inconsistencies and frustration, I am actually amazed that mine is so-far-so-good this time around

This may seem an odd suggestion - can you get to DOS from a HD boot (Real DOS or W9x restart in DOS)

If yes, try ImageDisk and see what it can read and what diagnostics it reports.

Set floppy type in BIOS to "none".

ImageDisk talks directly to the floppy controler and does not use BIOS to access the drive... Might at least provide a bit more information/insight.

bertrammatrix wrote on Yesterday, 16:37:

Interestingly, I seem to remember BobocoCz mentioning that he was unable to switch L1 (I think) to WB no mater what he did when he used an am5x86 on the 918. It's sounding like there may be some sort of problem with the circuitry responsible.

I don't think BobocoCz shared an image of his jumpers. Maybe he had a jumper on JP30 7-14, which forces WT mode.

bertrammatrix wrote on Yesterday, 16:37:

I read through a whack of conversation you had with Pshipkov on this board at one point re cache etc, yikes, no wonder this one gets a bad reputation and ends up back in the closet time and time again, seems like the only consistent experience is inconsistencies and frustration, I am actually amazed that mine is so-far-so-good this time around

Try running it in Windows 9x with a Cyrix 5x86 and some late 90's software. I'm not sure how much more patience I have with this motherboard. I had wanted to replicate BobocoCz's results with the Cyrix at 150 MHz, but if L1:WB isn't working well, then I see little purpose. However, you say he couldn't use L1:WB, so I'm not sure how meaningful this replication will be.

DaveDDS wrote on Yesterday, 16:58:

This may seem an odd suggestion - can you get to DOS from a HD boot (Real DOS or W9x restart in DOS) […]
Show full quote

This may seem an odd suggestion - can you get to DOS from a HD boot (Real DOS or W9x restart in DOS)

If yes, try ImageDisk and see what it can read and what diagnostics it reports.

Set floppy type in BIOS to "none".

ImageDisk talks directly to the floppy controler and does not use BIOS to access the drive... Might at least provide a bit more information/insight.

Yes, I can boot from the HDD just fine. I've never used ImageDisk before, but I assume it is v1.20 from here: http://dunfield.classiccmp.org/img/index.htm What do you want me to run after I extract the files: TESTFDC.COM ?

Yes, my site will always have the latest (1.20 is right)

Big thing about ImageDisk is it was designed to read any disk the PC hardware could read which includes many non-pc formats, and all PCs are not created equal. Many PCs can't do oddball formats, things like single-density, sectors not 512 bytes, non-pc sectors/track etc.

Testfdc is to figure out what your PC can do, most useful to help pick the best one to use... not so much to detect/diagnosed hatdware problems.

But .. it can't hurt.. the whole idea is to have it accessed/used by non-BIOS - any extra information you can glean may help figure things out.