First post, by Anonymous Coward
- Rank
- l33t++
I was wondering if anyone has had any luck getting Cyrix 5x86 working on older 486 boards using an interposer with a voltage regulator.
I've tried 3 or 4 boards mostly based on the 06/06/92 AMI HiFlex "color" BIOS, and the typical behaviour is that the monitor does not get a signal from the display card and the system stays silent.
There is a trick that can be used to get the system booting up though. If your interposer has a jumper for "2.5X" multiplier mode, you can boot the system up in "2X" mode. This only seems to work on Cyrix 5x86 chips that are 2X/3X capable. I have tried a few 3X/4X models, and this trick never works on them.
I've tried a couple of things to fix the problem, but haven't had much success. On one of my boards, I was able to switch to a MR-BIOS from 1994 that is Cyrix DX2/DX4 aware. The 5x86 works without issue. So in my opinion, the problem is BIOS related, not hardware related. In the Cyrix 5x86 databook there are a couple of modifications that are recommended to certain CPU pins for adapting older boards. I tested this with my '92 AMIBIOS boards and it made no difference. Anyway, it seems going with the MR-BIOS is the easiest solution IF it's available for your chipset.
Unfortunately, two boards I like do not use supported chipsets, so I am out of luck there. However, I was able to track down v4.50 of Award BIOS. This is one one of the earlier versions that doesn't have the black screen with energy star logo. According to the machine code, the BIOS knows about the Cyrix SLC/DLC, 486S and 486S2. Unfortunately the behaviour of this BIOS with the Cyrix 5x86 is exactly the same as with the AMIBIOS. I remember at some point using a POST card to figure out what was going on, but didn't come to any real conclusions.
The nice thing about Award BIOS is that you can use MODBIN, unlike with AMIBIOS where AMIBCP is missing in action. Thankfully V4.50 is still modern enough to work with the modbin4 utility. The only thing of interest I saw in there was registers for adjusting SLC/DLC CPUs. I had a theory that perhaps the reason the Cyrix 5x86 wasn't booting was because the board might be trying to set it up according to the SLC/DLC registers. I consulted the databooks and did note some overlap in registers between the two CPUs. Register C3h on the 5x86 for example contains the bit for turning on Linear Burst mode. Award BIOS sets this to "enabled" by default. On boards that don't support it, that would lock up the system. I made an adjustment to disable it, but sadly that made no difference.
Anyone have any ideas about why so many older BIOSes reject the Cyrix 5x86?
"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium
