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PC/XT Motherboard/RAM Issues - Please HELP!

M

marcoguy

Experienced Member
Joined
Feb 12, 2012
Messages
485
Location
Boston
Today I fired up my XT and as it was counting the RAM, it halted. It didn't occur to me until after I had reset it to record the amount of RAM the machine halted while counting. Now, when I turn it on, it only counts 128 Kb of RAM (there is a full 640 inside and the mobo is configured to register it all) but it does load PC-DOS 3.31 off the HD fine (when my SCSI HD actually decides to spin up properly) I would greatly appreciate any help that can be given.
 
So, switches 3 and 4 are set to off, off? For sure? Because if they're set to on, on, the board would stop counting at 128k no matter how much ram is installed.
 
Yes. I am sure they are off, and as for MikeS, I have no idea what that has to do with my issue. If you have a problem with my signature, I'd be glad to discuss it with you via PM.
 
Yeah I really don't care what he thinks of my signature, I just don't want a bunch of off-topic messages in this thread so all I'm saying is that if he really wants to have a conversation about that, don't do it here.
 
It's possible your bad chip is the one right past 128K which is halting the search for any further memory. You'd have to do the math depending on the size of your chips to figure out which number that would be unless you have a RAM diagnostic tool. You could also try piggy backing chips (set it for 384? (whatever the next jumper step down from 640 would be) and that will give you a bank of spares to try swapping out.

Also the first step really would be reseating them all just to see if it's a simple bad connection. I'm not sure if they use the same methodology as a processor of pin 1 having power but if so you might be able to trace a bad dip that way as well.
 
I haven't checked it from DOS yet. I'll do that tomorrow, however, I'm pretty sure it will just say 128k because as soon as the XT loads 128k when it boots it proceeds to load my MACH 20 BIOS and my SCSI card's BIOS after that.
 
marcoguy said:
...as for MikeS, I have no idea what that has to do with my issue. If you have a problem with my signature, I'd be glad to discuss it with you via PM.
Click to expand...
No need for any discussion.

My apologies; I thought it might interest you that it is not true and perhaps you'd want to change it, but I was obviously mistaken and should have known better.

Please forgive my error in judgement.
 
There is a design flaw in the 5160's POST that can cause this problem. Refer to [this] earlier thread for more information.

Based on that, and on the fact that the symptom appeared unprovoked, I'm betting my money on a faulty RAM chip.
 
modem7 said:
SCENARIO #2 - BAD RAM CHIP IN 128K-192K

So let's look at the sitation where the bit 1 chip in the 128K-192K bank is faulty, and faulty in a way that results in it always returning 0.

The HOW-BIG routine will determine that there is RAM up to 128K.
It will go on to see if there is a 64K block of RAM starting at 128K (where our bad RAM is).
The HOW-BIG routine will write AAh to address 20000h and 55h to address 20001h.

1FFFE:
1FFFF:

20000: AA (bit7=1, bit6=0, bit5=1, bit4=0, bit3=1, bit2=0, bit 1=1, bit0=0)
20001: 55 (bit7=0, bit6=1, bit5=0, bit4=1, bit3=0, bit2=1, bit 1=0, bit0=1)
20002:
20003:

When the HOW-BIG routine reads back addresses 20000h/20001h, it will read back A8h/55h (expecting AAh/55h).
Consequently, the HOW-BIG routine then incorrectly assumes that there is no RAM past the 128K address.
Click to expand...

It sounds like I'm having exactly this problem. I'll switch my RAM around later today. Thanks for the help!
 
Stone said:
Nope... if that were the case there would be an error code displayed denoting the hex address of the bad chip.
Click to expand...


I think that error code due to bad RAM chip only happens in IBM BIOS or I'm wrong?

marcoguy only said the machine was a XT, it could be a clone.
I have a XT clone that don't boot if there's problems in RAM chips.
 
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Well, some systems depending on the bios yes can find and error and print out a code. Others are even nice enough to tell you which chip (what a time saver!) (Zenith was one that was really cool about their built-in hardware diagnostics). But I've seen other systems stop counting or error out on the bank upon a bad chip. Doesn't necessarily mean it's the first chip there but again if you properly configure it for less RAM you'll have a bank of spares to piggy back and find out where the culprit likely exists.
 
lucasdaytona said:
marcoguy only said the machine was a XT, it could be a clone.
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Good point. Marcoguy's earlier posts indicate that he has an IBM XT, but he could also have a clone XT.

barythrin said:
But I've seen other systems stop counting ....... on the bank upon a bad chip.
Click to expand...
I can see why. The authors of the BIOS/POST for clone XTs had the same problem to overcome - no switch block SW2 to inform the POST of how much RAM to expect, and no CMOS Setup to store that same amount. Some authors possibly just copied IBM's method of RAM size detection (without enhancement).
 
I don't have a clone. It's an IBM 5160 and also, I not altered the system in any way yet but it now counts to 256k of RAM! This whole problem is very mysterious to me.
 
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