Understanding OLD Motherboard Jumpers.

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Understanding OLD Motherboard Jumpers.

Postby BLockOUT » 2017-11-10 @ 14:24

When i tried to build a 486 i took a look at some manuals to see the CPU jumper settings
With new people jumping in the retro stuff, it is kind of difficult to really understand the settings
Lets suppose you are building a 486 motherboard that has jumpers. You are given options like:

1) 486 SX
2) 486 DX
3) 486 DX-SL
4) P24D
5) P24T
6) M6
7) M7
8 ) UMC 486
9) AMD 486 DXL


So lets supponse you are new, you are given those options and its not clear because:

#1) OK thats clear because 486 SX it says on the chip, 486 SX 25mhz
#2) OK, thats clear because 486 DX 33mhz
#3) MMmm 486 DX-SL ? how do you know you have "SL" ????
#4#5#6#7#8#9) P24T ? P24T ? m6? m7 ? umc 486 ???? amd 486 DXL????? what are those?

Lets talk about well known CPUs 486 DX2 80 mhz, 486 DX4 100, IBM 5x86, cyrix 586 . etc, what setting do you choose for those?

Or even better, lets suppose you choose an intel 486 DX4 100mhz with a socketed voltage regulator, what setting would you choose if thats not an option in the manual.
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Re: Understanding OLD Motherboard Jumpers.

Postby BitWrangler » 2017-11-10 @ 14:40

Some actually say SL enhanced on, means they've got power saving features, but I'd also try that for CPU marked 3.3V unmarked they're 5V.

4 and 5 are for intel "overdrive" CPUs, 6 and 7 are for cyrix, probably want M7 for the cyrix 5x86 but memory foggy, 9 you probably want for AMD dx4s at 3.3V and the dx5-133.
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Re: Understanding OLD Motherboard Jumpers.

Postby SW-SSG » 2017-11-10 @ 19:03

BLockOUT wrote:8 ) UMC 486

The relatively uncommon UMC GREEN CPU (U5 series). If you've got one of these, you use that jumper setting.
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Re: Understanding OLD Motherboard Jumpers.

Postby Maleckii » 2017-11-10 @ 19:13

I've run an AMD 5x86-P75 on the 486 DX-SL setting before. I'm not sure there was a difference, since the board has a different jumper set for L1 WB/WT cache.
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Re: Understanding OLD Motherboard Jumpers.

Postby Robin4 » 2017-11-10 @ 19:34

All things you dont know is just googlable.

3) Quad Flat Package of the 486 processor https://en.wikipedia.org/wiki/Quad_Flat_Package (i guess its a cheaper 486 version used in OEM brand systems for mosly soldered to the motherboard, sometimes on a interposer.
4) This 486 processor uses `write- back cache instead of `Write-trough cache (only difference its how it handles the cache on you motherboard.)
5) This 486 processor uses `write- through cache instead of `Write-back cache (only difference its how it handles the cache on you motherboard.)
6) Cyrix type of processor (these where a cheaper alternative for the 486 processors (486 processors where very expensive back them (i guess M6 is like a sort of DX alternative.)
7) Also a Cyrix processor (i guess a alternative for the DX2 versions)
8) A 486 alternative made by UMC.. https://en.wikipedia.org/wiki/UMC_Green_CPU
9) An AMD version for the 486 With a bigger L1 cache.. I guess 16KB instead of 8KB.


Reminder: You have to look at the Core Voltage Rating of the processor... If this is 5volt then they will work on all older 486 motherboards that supports the processor in question.
If the processor is 3.3volt you need a voltage step-back interposer board to use the processor on a 5volt motherboard if the board supports the processor. Or use an New style dial voltage motherboard that supports 5 - 3.3volt rating.
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Re: Understanding OLD Motherboard Jumpers.

Postby BLockOUT » 2017-11-11 @ 00:27

thanks for feedback

what about this option

cpu Intel DX4 100mhz + voltage regulator in the cpu socket.
what jumper option would you choose from the 1 to 9 list i posted?
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Re: Understanding OLD Motherboard Jumpers.

Postby kanecvr » 2017-11-11 @ 00:32

P24T = pentium overdrive (POD5V) - basically a pentium for the 486 socket. Comes in 63 and 83MHz flavors.
P24D = intel 486 overdrive - a DX2 that runs at 5v, designed for older socket 1 motherboards. Full name is i486DX2WB
m6 = a cyrix 486 DX. Full name Cx486S. Comes in 33 or 40MHz flavors.
m7 = a cyrix 486 DX2. Comes in speeds of 40 to 80MHz.
umc 486 - exactly that - a 486 made by UMC.
amd 486 - self explanatory - a 486 made by AMD. They sometimes need different jumper settings then intel DX4 chips since most come with 8kb of write trough L1 cache, but write back versions exist.
DXL - 486 made by amd. Full name is A80486DXL

The 486DX is your garden variaty 486, usually running at 33MHz, but 50MHz versions exist. They are not clock doubled (no multiplier, they run at FSB speed).
The 486DX2 runs at FSB x2. For 33MHz fsb you get 66MHz. Intel made 50 (25x2) and 66Mhz (33x2) versions. Some chips run at 5v, others at 3.3v. AMD and cyrix made 50, 66 and 80MHz versions.
The DX4 runs at FSB x3. The 100MHz intel DX4 runs at 33x3 (100MHz), and so does the cyrix 586-100 and the AMD DX4-100. AMD also made DX4-120 chips that run at 40x3.
586 chips run at 4xFSB (for the AMD 586-133 - 33x4) 3xFSB for the cyrix/ibm 100 and 120Mhz chips (33 or 40 x3) or 2xFSB for some IBM chips (made by cyrix) witch run at 66x2.

To correctly set up a 486 (or pentium / AMD K6 / Cyrix 686 socket 5 or socket 7) cpu, you need to know about the following:

1. CPU voltage - early 486 chips run at 5V. Later chips run at 3.3 or 3.45v (older mainboards only support 5v). Some Cyrix DX2 chips run at 4V. Some (rare) cyrix 586 chips run at 3.65 or 3.7v. Socket 5 pentiums run at 3.3v, while socket 5 AMD k5 chips run at 3.45v. Socket 7 CPUs run on dual-voltage - 3.3v for the I/O, and 2.8v for the CPU core in the case of pentium MMX chips. AMD K6-2 chips run at 2.2, 2.3 or 2.4 volts. Cyrix chips run at 2.3 or 2.9 volts.

2. Front side bus frequency. Usually 33MHz for most 486 chips - so a 100MHz 486 DX4 runs at 33MHz x 3 = 100MHz. A 80MHz AMD or Cyrix DX2 runs at 40MHz x 2 = 80MHz. Pentium chips run at 50, 60 or 66MHz. a 133Mhz pentium 1 runs at 66x2. K6-2 chips run at 66 or 100Mhz - so a 400MHz CPU is 100MHz x 4 or 66MHz x 6.

3. Multiplier. For 486DX chips running at 25, 33 and 50Mhz, there is no multiplier - they run at FSB speed. DX2 chips however have a 2x multiplier. DX4 chips have a 3x multiplier, and some 586 chips like the am5x86-p75 running at 133MHz has a 4x multiplier - however - there is no 4x setting on any 486 board - 4x capable chips interpret 2x multiplier setting as 4x.

4. Cache mode - this tells the motherboard to run the CPUs L1 cache in write back (faster) or write trough mode. Some CPUs only support write trough mode.

To set up a CPU, chose the correct voltage (written on the chip) via jumpers. Then set the FSB speed (usually 33MHz) - different jumpers and then set the multiplier (also by jumpers).
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