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Upgrading an IBM XT to 640k

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MattCarp

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Apparently there's a hack you can do to the original IBM XT 64-256KB system board that will allow you to convert the system board to a 640K system board.

The hack involves installing a 74LS158 multiplexer chip, adding a jumper to the system board, and then adding the memory chips.

Does anyone have these instructions?

I think the multiplexer goes into an empty chip position silk screens as U84 on the system board. The jumper goes on the processor side of the system board, below the power connector, in a position screened as E2, in the first position.

Those are the only differences between the two system boards that I can tell by comparing a 256K and a 640K system board, side-by-side. Otherwise, all of the components appear to be EXACTLY the same.

Of course, you need to have a 1986 BIOS that can recognize the 640K configuration.

Is this right?


UPDATE: I found an answer here: http://www.textfiles.com/computers/
There's a file (really an e-mail) listed: xt-640k.upd

One part of the hack is to make sure the DIP switches (#3 and #4) are both off, indicating all four banks of memory are in use.

However, the hack does not say anything about updated BIOS being required... is it?
 
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MattCarp said:
However, the hack does not say anything about updated BIOS being required... is it?
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It's the PC you're refering to, not the XT.

The BIOS update isn't really required for the XT. The only thing it adds is INT 15h function calls compatible with the AT's BIOS. Most programs will run on an XT without therse routines.
 
per said:
It's the PC you're refering to, not the XT.
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No. He's referring to the XT.

MattCarp said:
However, the hack does not say anything about updated BIOS being required... is it?
Click to expand...
According to my notes from 80's, the final IBM XT BIOS (dated 05/09/86) is required, but the upgrade procedure listed in the first edition of Mueller's 'Upgrading & Repairing PCs' doesn't include the BIOS.
 
modem7 said:
No. He's referring to the XT.

According to my notes from 80's, the final IBM XT BIOS (dated 05/09/86) is required, but the upgrade procedure listed in the first edition of Mueller's 'Upgrading & Repairing PCs' doesn't include the BIOS.
Click to expand...

I can't see why the eary XT BIOS shouldn't work with the mod. I've examined the BIOS and schematics, and I've not come over anything that seems problematic. Are you sure you don't mix it with anything else?


The PC (5150), couldn't see more than 544Kb without the last BIOS, but I have never heard anything like that about the XT (5160).
 
modem7 said:
According to my notes from 80's, the final IBM XT BIOS (dated 05/09/86) is required, but the upgrade procedure listed in the first edition of Mueller's 'Upgrading & Repairing PCs' doesn't include the BIOS.
Click to expand...
Nothing like an experiment. I pulled out my 640K IBM XT motherboard, removed the 05/09/86 ROMs and in their place inserted 11/08/82 ROMs. The POST counted up to 640K. And so my old notes are wrong.
 
nice work!

nice work!

Thanks for checking that you don't need 1986 BIOS in the XT. A multiplexer, jumper, and chips. that's a great mod to save a slot!
 
Woow, I am goping to try this on my 5155

Woow, I am goping to try this on my 5155

This is very cool, thanks everybody, I am going to try this on my 5155 so I can install an ast 2mb ems card in the free isa slot :)
Will post results here
 
I upgraded my 64K 5150 with a kit from Purple computing (saved me from having to unsolder the first row of 16K chips). No BIOS replacement was necessary. I can't see why upgrading the XT would be any different.
 
I just found a couple of these 74LS158 multiplexer chips lying around so I think I'm gonna fool around with my 64-256K board so I can remove the 384K expansion card from my XT.
 
I did the 640K mod on my 5155 back in 1985 or 86. My Dad learned about it through the IBM PC Club of which he was one of the early members. It was believed at the time that IBM designed the 5160 Motherboard to use the 256K chips but they either weren't available or were to expensive at the time.

While you're at it, you can put 256K chips in all four banks and swap out the PROM in U44 with a custom programmed GAL and you'll have memory above 640K that can be use as UMB. On my 5155, I have 128K starting at D000h used as UMB. This Thread describes the Whole process. If interested, I can provide the custom programmed GAL. Just PM me.
 
Anonymous Coward said:
Is it possible to do a 640k mod to a 5150 motherboard?
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I have this 5150 motherboard with a 640K mod long since in place. It works perfectly, but it obviously wasn't a trivial installation. My knowledge of the system is insufficient to determine the exact nature of this mod, so I included some pictures of it for the curious.
 

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My XT (5160) motherboard type is 64KB-256KB, but RAM is installed as 640KB.
 
fs5500 said:
My XT (5160) motherboard type is 64KB-256KB, but RAM is installed as 640KB.
Click to expand...
Uh, yes....that was mentioned five years ago by Great Hierophant.

(hint: if you are going to bump really old threads you might want to put a bit more content/research into your posts ;) )
 
I am going to add to this bumped-possibly-zombie-thread.

64-256kb Mainboard.

I can still get 41256 from Jameco - the needed chips will be around 65 + shipping.

I can solder blob the E2 jumper, but i this thread i read that a chip needs to be replaced / upgraded. Can this be cleared up?

Also, does the speed of the chip matter? Can i use the 80ns or does it have to be 120ns?
 
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