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Maths co processor question

limboy777

Experienced Member
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Apr 21, 2012
Messages
64
Hi

I have a 386 that was upgraded to a 486. The original spec was cyrix 386 sx25 upgraded using a clip on cyrix 486 srx 50.

I have a spare math processor slot.

Does the bus speed have to match on the co-processor (i.e I am assuming the speed should be multiples of 25) and I may have read somewhere that the speed has to be equal or higher than the processor i.e should be 50mhz?

As the motherboard is 386 but upgraded to a 486, will there be any problems in using the 387 co processor. This is what I am thinking about but think the pricing is a but high.

http://www.ebay.co.uk/itm/US83S87-S...631?pt=LH_DefaultDomain_0&hash=item5ae5d3c4ff

Another question I have is can I use a video card in addition on this computer (im thinking of an isa video card).

Any help much appreciated.
 

krebizfan

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Which motherboard, existing video system, and OS are being used? The answers could differ depending on that information.

387: Check the Cyrix upgrade manual. In general, the 387 would only need to be a 25 MHz model matching the bus speed but some systems restriced 387 speed further. You can buy a faster and more expensive chip but it would do nothing. Note: the Cyrix chip may well emulate floating point faster than the external chip you install will be able to operate. Put another way, adding the 387 might make your system slower.

Video: Depending on your system. With most 386 era systems with stripped down motherboard video, you can disable the motherboard video and use an ISA card instead. Some won't let you; Packard Bell was evil in its forcing the usage of the lackluster included video. Multiple VGA or better cards was seldom supported.
 

limboy777

Experienced Member
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Apr 21, 2012
Messages
64
Hi thanks for your responses

It is an Amstrad 386 sx 25 running window 3.1 for workgroups. 4mb ram.

Video must be built into the motherboard (dont know what it is).

Where can I get the manual, Ive had a look online typing in cyrix 486 srx 50

Thanks in advance
 

vwestlife

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Even though the CPU has been upgraded to a clock-doubled 50 MHz 486, the motherboard still sees it as if it was a 25 MHz 386SX. So therefore, you can still use a 387SX-25 math co-processor.

I think Cyrix and maybe some other companies did sell a 387SX chip that was rebranded as a "487SLC" (or "4S87", or something like that), designed to go together with the 386SX-based 486SLC series of CPUs, but that was really just for advertising purposes; the chip was still 387SX-compatible internally.
 

krebizfan

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Calling what came with the Cyrix upgrade chip a manual might be stretching the point but the one I saw had a short pamphlet with it. I am hoping that would provide accurate information on setting up the coprocessor with the upgrade chip. Some of the CPU upgrades required changes from the normal expected behaviour.

Some Amstrad 386 motherboards had the ability to turn off motherboard video. I remember there used to be a number of web sites with complete jumper information for Amstrad systems but I can't find them anymore in searches.

My best suggestion is to make a new thread requesting both Amstrad and Cyrix documentation. A more descriptive thread title might cause users with knowledge on the topic to notice especially as the European membership checks this site in their morning.
 

vwestlife

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if you upgraded to a 486, the math co processor might actually be on the 486 chip. If you have a SX 486 then its not, but a DX 486 should have a built in NPU.

The Cyrix 486SRx2 upgrade does not have a built-in co-processor. It is really just a 486SLC2 chip in a special package designed to clip on top of a 386SX chip.
 

limboy777

Experienced Member
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Apr 21, 2012
Messages
64
The Cyrix 486SRx2 upgrade does not have a built-in co-processor. It is really just a 486SLC2 chip in a special package designed to clip on top of a 386SX chip.

Thank you all for replying. Ive tried to answer any queries and provide more info below;


Yes its a clip on upgrade. Video is 256k as standard upgradeable to 512k.

So just to confirm, adding a 387 co processor will not slow it down and may speed up programs that would benefit i.e CAD. (does a co processor speed up Microsoft Excel etc?)

Finally one more noob question, I am looking for a lead that enables me to connect two hard drives to this computer. I believe the maximum that a 386/dos could handle was around 500mb. My computer currently has 40mb. Is the maximum the combination of the two hard drives or of each hard drive. I want to keep the 40mb drive as it has the software for the 1k cache for the clip on 486. (I dont have the software and guess it will be hard to find).

Which cable do I need (assumming a cabe with two ide drive connectors and two molex connectors). I will upload a picture of the drives/connectors tomorrow.
 

Stone

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1) Each hard drive.

2) If you're talking about a dual HD ribbon cable I have lots of new ones. The molex connectors are from the power supply.
 

limboy777

Experienced Member
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Apr 21, 2012
Messages
64
Hi Stone

Thanks for your response

I am based in the Uk so postage would be expensive? Do I need seperate molex connector that splits into two?
 

Stone

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I don't know -- you'll have to take a look. Most power supplies have multiple molex connectors. Look for a free one. :smile: If you need another I also have 'Y'' power adapters. Postage shouldn't be all that much (definately less than $10) as these items aren't very heavy.​
 

vwestlife

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So just to confirm, adding a 387 co processor will not slow it down and may speed up programs that would benefit i.e CAD. (does a co processor speed up Microsoft Excel etc?)

That is correct. With the clock-doubled CPU upgrade, the rest of the motherboard is still running at only 25 MHz; only the internal core of the CPU is running at 50 MHz. And most, if not all, spreadsheets do take advantage of a math co-processor; I know Lotus 1-2-3 definitely does.
 

limboy777

Experienced Member
Joined
Apr 21, 2012
Messages
64
That is correct. With the clock-doubled CPU upgrade, the rest of the motherboard is still running at only 25 MHz; only the internal core of the CPU is running at 50 MHz. And most, if not all, spreadsheets do take advantage of a math co-processor; I know Lotus 1-2-3 definitely does.

Thanks westlife. I will put in a wanted ad for this processor, hopefully someone will sell cheaper than the one on ebay!

Stone - I am based in the Uk so posting from the US will be expensive. It will be cheaper to buy from the uk. Thanks for your offer though:D
 
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