• Please review our updated Terms and Rules here

The world of the 486 processor..

R

Robin4

Veteran Member
Joined
Sep 25, 2011
Messages
535
I have some questions about 486 processor, and system components..
I had never played with 486 stuff before. Most things i know about it.. I can only say that my dad had a 486 system, and on my school whe had also 486 computers.. I only know that in 1997 or so i got some 486 stuff to mess with.
I only could tell that the motherboard or processor was dead because the system never worked.. After that i never played with 486 stuff again.. So it is not very difficult, but my knowledge off 486 stuff has a limit.. So i dont know not all the things about this computer stuff..

I know about the different kind of sockets, i know about platforms with different voltages.. But what i didnt thought about have to do with those caches like write trough and write back..
For that question i found this website: http://www.computercraft.com/docs/old486.shtml

computercraft said:
If you're tempted to use a PENTIUM or one of the exotic 586s, then investigate if your system allows write-back cache. The 486s used "write-through" cache where the CPU writes data to memory external to the processor. With write-back cache, the CPU uses memory designed into the chip - a much faster method. Some 486s have a jumper to enable write-back cache. It's not unusual for this jumper to fail to work!
Click to expand...

I know how i can indentify a write back or write processor, but thats not my question..

I see that some boards have a jumper to set it to write trough or write back. Does that means that boards that dont having a jumper like this wouldnt support the write-back cache on the processor at all.. Or are there boards that just enable write-back cache on the processor without setting a jumper.. And how do i know when a system uses the write-back cache.


I also red about this:

computercraft said:
It's not unusual for this jumper to fail to work!
Click to expand...

Can someone explain why is this? What do they mean about this.. Do i need to see more like if i put a write-through processor on the motherboard and set it to write back that the system wouldnt boot up?



Do i have to worry if i would buy a write-trough processor or a write back one (preformances wise) I dont know if it would make any sense if i build a 486 system that it would better to go for a write back type one, because the 66 write trough version is allready slower compared with a DX4 100 or 120mhz version.
 
The comment about cache jumpers not having any effect refers to the fact that a number of motherboard manufacturers installed fake cache. Look at the board and see black plastic thingies in the correct place but that is all they are black plastic things that look like chips but can't do anything. Often, on these boards, there are no trace lines to connect the cache sockets with anything else so putting in real cache chips won't do anything. Yes, the manufacturer spent $10 to pretend the board was worth another $100 but skimped out on 50 cents that would have made the board useful.

See http://www.redhill.net.au/b/b-bad.html for examples.
 
While waiting for more answer(s), I can share my limited observation first.

To use i486DX2 WB on my ASUS PVI-SP3 mobo, I need to change multiple jumpers to a different settings than normal i486DX2. Thereafter, I can easily switch on/off write-back mode in the BIOS. The bad news is, the performance advantage may not always as significant in practice. I did a computational intensive test which is to render a 3D object in AutoDesk 3DS. The total time difference between WT and WB is less than 1~2%. Other people may have different observations. But I'm still virtually comfortable with it because it makes me a psychological winner in a virtual i486DX2 competition :mrgreen:

One more thing, if I install a normal i486DX2 on the mobo with the jumpers still in WB position but the BIOS L1 option is set to WT, the system can boot without problem but the performance dropped to 60~70%. Does it necessarily mean, P24 core and P24D core require slightly different working condition even if they are all in WT mode?
 
Last edited:
Robin4 said:
... And how do i know when a system uses the write-back cache.
Click to expand...

IIRC, given a WB capable 486, it may have two different CPUIDs corresponding respectly to WT mode and WB mode. Many utilities, such as Speedsys, can detect and display current CPUID, for example:
i486DX2-66: 0436h = WT; 0470h = WB
Dx4-100: 0483h = WT; 0490h => WB
......
 
In case you want to avoid issues you can skip building your own rig. You could buy a 486 from a reputable company back in the day like IBM / Compaq / Gateway 2000. That way it is plug & play (most of the time) and documentation / drivers / replacement parts are relatively easy to come by / pretty common. :)

Obviously part of the fun of retro computing to many people is (re)building their ultimate machine from a specific era (like a heavy / maxed out 80486) but to me that would cause too many headaches. ;)
 
PeterLI said:
Obviously part of the fun of retro computing to many people is (re)building their ultimate machine from a specific era (like a heavy / maxed out 80486) but to me that would cause too many headaches. ;)
Click to expand...
I guess if it didn't, it would boring/demoralizing. Happened to me plenty of times in the past. I used eg to do hardware voltage modifications in graphic cards (~2002-2006 era). At the time I thought I wanted the manufacturers to allow some GPU voltage control, rather than forcing you to hard mod the card. But, when they finally did (2008+ era), I found the whole thing so boring that I never bothered even to overclock the card (not to mention overvolt it).
 
Right. The only modifications I performed were very modest. I used to tweak the BIOS aggressively in the 286/386/486/586 clone period (usually did not work well: systems crash a lot). The one thing that worked reasonably well was buying QA failed CPUs cheap (e.g. Pentium 75 / 90 / 120 / 150) and over clocking those to the next step (e.g. QA passed) CPUs. Ensuring sufficient cooling worked really well for those.

I never really played around with tweaking video cards.
 
krebizfan said:
The comment about cache jumpers not having any effect refers to the fact that a number of motherboard manufacturers installed fake cache. Look at the board and see black plastic thingies in the correct place but that is all they are black plastic things that look like chips but can't do anything. Often, on these boards, there are no trace lines to connect the cache sockets with anything else so putting in real cache chips won't do anything. Yes, the manufacturer spent $10 to pretend the board was worth another $100 but skimped out on 50 cents that would have made the board useful.

See http://www.redhill.net.au/b/b-bad.html for examples.
Click to expand...

Hello i didnt mentioned about a jumper about the cache on the motherboard. I mean that sometimes there a jumper on the motherboard with a name like P24D WT / P24D WB. I mean that its the jumper for config the processor setting.

Dca2 said:
While waiting for more answer(s), I can share my limited observation first.

To use i486DX2 WB on my ASUS PVI-SP3 mobo, I need to change multiple jumpers to a different settings than normal i486DX2. Thereafter, I can easily switch on/off write-back mode in the BIOS. The bad news is, the performance advantage may not always as significant in practice. I did a computational intensive test which is to render a 3D object in AutoDesk 3DS. The total time difference between WT and WB is less than 1~2%. Other people may have different observations. But I'm still virtually comfortable with it because it makes me a psychological winner in a virtual i486DX2 competition :mrgreen:

One more thing, if I install a normal i486DX2 on the mobo with the jumpers still in WB position but the BIOS L1 option is set to WT, the system can boot without problem but the performance dropped to 60~70%. Does it necessarily mean, P24 core and P24D core require slightly different working condition even if they are all in WT mode?
Click to expand...

Hello my main question is: Do the board need that P24D WT / P24D WB jumper to use Write back processors on it?? Or is this just different on which board you or i have? So i like to know are all 486 socket 3 motherboards compatible with write-back 486s..
I know that PCI version have better support for it then VLB boards.. But i only like the use VLBs if i can.
Anyway thanks for the test you did to compare, i think this is usefull information. But what kind of software have benefit from that write back cache, or thought manufacturers that write-back cache could have benefit. But in practice you hadnt much benefit of it.. So i really like to know `when` WB cache would have be advanced performances in real life..


PeterLI said:
In case you want to avoid issues you can skip building your own rig. You could buy a 486 from a reputable company back in the day like IBM / Compaq / Gateway 2000. That way it is plug & play (most of the time) and documentation / drivers / replacement parts are relatively easy to come by / pretty common. :)

Obviously part of the fun of retro computing to many people is (re)building their ultimate machine from a specific era (like a heavy / maxed out 80486) but to me that would cause too many headaches. ;)
Click to expand...

No i dont want to skip my own build retro computer because in my country there arent much old electronics anymore available, because off those scrapers.. They only want to earn money, so everthing that looks junk to them is just money in there eyes.
Otherwise i have to buy from the USA.. Then i have the problem: High prices, high shipping cost, extra customs fee.. Then price / quality wise this isnt the best route to follow for european people.
And those build system do mostly have cheap components inside.. Why i made this thread isnt because of headaches, but because i just want to have more information on what iam doing..
And then iam not just someone who just only wants to play on the most budget computer out there. I really love this old hardware to work with, play games with, and building the machine i really like to see.. I not a ebayer that buys stuff for a limit amount off money to spend. For my its just i hobby and to relive those old days. If i can mostly i try to find old cases i my own area (second hand sites) sometime you just have to pay if you think the price is worth the item.
 
Last edited:
All 486 socket 3 motherboards don't have a choice between write through & write back caches - it depends on how old your VLB board is.

I have 2 such boards, one from 1993 and the other a year later. Neither have any jumpers to choose between the 2 cache schemes. The 1993 board has an AMI BIOS and there is no mention of write through or write back, but the 1994 board has an AWARD BIOS and you can configure write through/write back in it.

As a matter of interest, the 1993 board has a 486DX-33, a Promise VLB caching HD controller and an STB Powergraph VLB graphics card.
The 1994 board has a 486DX4-100, another Promise VLB caching HD controller and a Diamond Stealth 32 VLB graphics card.
 
So if iam right, the older boards doesnt have this particular jumper.. But new ones could possible have?

But is it possible to use a WB version 486 processor on a older board? How would it be enabled? Would it just go automaticilly?
 
Check the chipsets. Generally anything released in 1994 has Writeback L1/L2 cache and Advanced Power Management support. The SiS 8c471 appears to be their first VLB chip set that supports these features and MANY motherboards used it. Almost all of these boards trumped up their "Green PC" abilities so they should be easy to ID.
 
You must log in or register to reply here.
Back
Top