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PC XT 64-256kb system board jumpers

Chuck(G) said:
Why not buzz them out and see where they go? That would go a long way toward determining their function.
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I don't understand what you mean? As I said before, my technical level of expertise is pretty low. I assume your talking about checking circuits with a multimeter or continuity tester. I would not know where to start!!
 
ibmapc said:
I don't understand what you mean? As I said before, my technical level of expertise is pretty low. I assume your talking about checking circuits with a multimeter or continuity tester. I would not know where to start!!
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A continuity tester is good. You've got one side of each jumper, where does it terminate (IC and pin #). That will allow someone with a schematic to pinpoint the jumper and take a good guess at the function.
 
pearce_jj said:
I'm not sure what you're asking for TBH.
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I'm looking for the prom(HIMEMV2) that replaces the one currently in the u44 socket, or someone that can program one for me. Also need help with device drivers to make use of the available memory above 640k. Evidently, others have had succes with this, but I'm not having any luck finding detailed information. This all started for me when I read an old article by an old IBMer Named Cy Atkinson. the following attachment is hereView attachment 1megXT.txt. Also, MKEMP, wrote earlier in this thread;

"The text document came from the IBMPC discussion forum (kinda like a newsgroup) on the IBM mainframe network; I have a copy that I made when I worked there Once Upon A Time. I also have one of the replacement PROMs obtained from Mr. Atkinson; I contacted him later for another and he replied that he no longer had access to a PROM programmer.
My replacement PROM is in a currently-nonfunctional system board (repairing the board is in the project queue). I picked up a Willem programmer on eBay but haven't used it as my interests have diverged since that time. If there's sufficient interest I'll attempt to duplicate the PROM. If that works I'd be willing to make copies for the price of the parts plus shipping."

I have tried to contact MKEMP as well as Mr. Atkinson, with no response from either. MKEMP also posted the following, which has me very interested;

"I configured the switches to map memory into segments A, D and E, then wrote an XMS driver implementing the UMB functions that allows LOADHI and DEVICEHI directives in CONFIG.SYS. I also wrote a device driver that used up to 64k for a disk cache (write-through for safety, with LRU buffer selection)."

Although I understand the basics, I need a lot of guidence to take on a project like this.
 
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Personally, I'd forget about the bipolar PROM (hard to find and program for most people) and just work up a 20-to-16 pin adapter and put a GAL in there. The equations are pretty simple.
 
Chuck(G) said:
Personally, I'd forget about the bipolar PROM (hard to find and program for most people) and just work up a 20-to-16 pin adapter and put a GAL in there. The equations are pretty simple.
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Well there you go talking over my head again. Do you have the equipment to program a GAL? If I were to buy a programmer, I would still need to know or learn (which gets more difficult every day) how it needs to be programmed to handle the high memory.

Regards,

Greg

PS Why do I need a 20 to 16 pin adapter? Aren't 16 pin GALs availlable?
 
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The ISA varient of the JR/IDE project last time I checked is planned to include a significant RAM; might be an altogether better solution for the 5155?
 
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pearce_jj said:
The ISA varient of the JR/IDE project last time I checked is planned to include a significant RAM; might be an altogether better solution for the 5155?
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Would that be LIM 4? How about device drivers to enable DEVICE HIGH/LOAD HIGH?
 
Chuck(G) said:
Personally, I'd forget about the bipolar PROM (hard to find and program for most people) and just work up a 20-to-16 pin adapter and put a GAL in there. The equations are pretty simple.
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Hey Chuck,
I know you're a busy guy. Could I persuade you to help me to program a GAL for the replacement of the prom in u44? If you don't have the time to do it yourself maybe you could help me with a JEDEC file. I think I could figure out the 20 - 16 pin adapter if you could tell me which pins are needed and where they need to go. Maybe we could work something out if you would need compensation for your time? I'm not very far from you, just a little to the north of Albany, OR. Please PM me if you can help in any way.
I read your Blog "Cloning a PAL/HAL". It just made me realize that I'm in way over my head!!

Thanks in advance,

Greg
 
Easy enough--first of all, let's get a dump of the contents that you'd like to work out. (256 4-bit "nibbles"). Then let's see where we go from there.
 
Chuck(G) said:
Easy enough--first of all, let's get a dump of the contents that you'd like to work out. (256 4-bit "nibbles"). Then let's see where we go from there.
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OK, but I don't have any contents to dump. All I have is the original ROM in socket u44. What I'm looking for is the info to replicate "Himemv2 Custom decoder chip" that Cy Atkinson wrote about in the following document(View attachment 1megXT.txt)to "Decode" memory above 640K. There is a section titled "HIMEMV2 u44 DECODER PROM PROGRAM" But I have no clue about how to use that to program a HAL.
 
So, if I understand the document correctly, the HIMEMV2 chip would be coded thusly (starting at address 00 and going to address FF:

9999FF999999BB99
9999FF9B9999BBBB
...

And the original U44 would be:

9999FFFF99999999
9999EEEE9999EBBB
...

Can someone verify if this is correct?

I can see mapping DRAM to 00000->AFFFF, but beyond that, I can't see how any other remapping is useful, as B000 and up represents space for video adapters, etc. So you gain a grand total of 64K more of contiguous memory.
 
Chuck(G) said:
I can see mapping DRAM to 00000->AFFFF, but beyond that, I can't see how any other remapping is useful, as B000 and up represents space for video adapters, etc. So you gain a grand total of 64K more of contiguous memory.
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Does upper memory have to be contiguous to be useful? I was under the impression that drivers and tsr's could be loaded there to free up base memory, with the right software driver of course. I could be all wet, But for some strange reason, this really has me curious.
 
Well, if you have code to load a driver in fragmented memory, it will work, but DOS doesn't do this naturally.

When the 8086 came out, I considered that one could expand memory simply by decoding the segment tag on a memory access and using a bit of RAM to map a segment to a given area of memory. Theoretically, you could access 4MB simultaneously on an 8088, but it would break the DOS program model.

All in all, you're probably best off using LIM 4.0 add-in memory.
 
Chuck(G) said:
All in all, you're probably best off using LIM 4.0 add-in memory.
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OK I guess I'll start looking for an Intel Above Board, unless you can recomend some other memory board? I'm still curious about filling all the banks with 256k chips and making use of the High memory in some meaningfull way.
 
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