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IBM 5150 flaky with DOS 5

B

BillO

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Sep 15, 2022
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HI All,

Season's greetings!

I have a an 2nd gen IBM 5150 (came with 256K RAM). In it is the standard floppy controller, a Trident VGA card, a MicroRAM card (for 640K) and a XT-CF Lite V4.1.

It boots DOS 5 fine. I will run GWBASIC just fine. It will load EDIT, DOSSHELL and TurboBASIC but if I try to do anything with them (like load a program in TB, or create a directory with DOSSHELL) the system gets unstable and can lock-up or reset or a variety of other less than pleasant behavior. Sometimes when it locks-up it will respond to Ctrl-Alt-Del and sometimes it won't.

I have run CheckIT on it and done all the tests which it passes. Usually. One time I let the memory test run overnight and by mooring the machine was locked up but no error was reported.

I don't recall anything like this with DOS 3.3, but then I didn't have EDIT, DOSSHELL or even TB.

Any ideas?
 
It does sound like that with DOS 5, you are 'exercising' the computer more. If there was a known instability issue with the IBM 5150 and DOS 5 combination, surely we would have heard about it by now.

This sounds like an intermittent hardware problem to me.

Have you tried re-seating the RAM chips in banks 1,2 and 3 ?
Have you tried replace-with-known-good the RAM chips in banks 1,2 and 3 ?
 
MicroRAM Rev 1.2 has problems with IBM PC 5150 / 5160. Up to Rev 1.1 worked fine, but Rev 1.2 caused problems.
And DOS 5 means MS-DOS 5.0, right?
DOS Shell for IBM PC DOS needs 384KB of RAM
 
DOS 5.0 requires 512k of RAM

System requirements: DOS 2.11 or later, a PC with an 8088 or higher processor, 512 KB of RAM, and a low-density 3.5" or 5.25" floppy disk drive. A Microsoft Mouse is optional.
 
OP has 640K with MicroRAM installed. Which makes it the suspect since it is providing memory from 256K to 640K.

The actual memory usage of DOS 5.0 is only 18K more than DOS 3.3. I think the 512K "requirement" is just for running QBasic with some breathing room.
 
That’s an interesting chart. I didn’t realize how much better the later versions of PC-DOS were about using memory efficiently. Too bad he didn’t do DR-DOS too.
 
fs5500 said:
MicroRAM Rev 1.2 has problems with IBM PC 5150 / 5160. Up to Rev 1.1 worked fine, but Rev 1.2 caused problems.
And DOS 5 means MS-DOS 5.0, right?
DOS Shell for IBM PC DOS needs 384KB of RAM
Click to expand...
Yes, MS-DOS 5.0.

Mine is a Rev 1.2. Does that mean I'm knackered?
 
So far I have removed every chip in a socket on the MB and the I/O cards, cleaned the leads and re-inserted them with a little deoxit. No change.

The next thing I will try is to remove the RAM in blocks 1, 2 and 3 and use the MicroRAM to supply all the memory from 64K to 640K.
 
Well, tried taking out all the removeable memory and just using the MicroRAM, but no joy.

Is there another RAM card I should get? It seems the MicroRAM may indeed be the problem. Maybe a sticky thread should be started to warn people away from it. I'll contact teh seller, but I'm sure he must already be aware of this and is still selling it, so perhaps not the most honest person.
 
BillO said:
I'll contact teh seller, but I'm sure he must already be aware of this and is still selling it, so perhaps not the most honest person.
Click to expand...
It is too early to finger point.

BillO said:
Well, tried taking out all the removeable memory and just using the MicroRAM, but no joy.
Click to expand...
• IBM 5150 motherboard of type '64KB-256KB' (4 banks of 64 KB)
• RAM chips removed from motherboard RAM banks 1/2/3 (0 is intact because the chips are soldered in)
• RAM on MicroRAM set to start at address 64K

That configuration is invalid.

In an IBM 5150, there is no way to disable the motherboard's RAM banks - they are permanently enabled. And removing the motherboard's RAM chips does not work because the 5150 motherboard still 'drives' (puts voltage onto) the data bus. So in the scenario that you have just described, one ends up with the problem of data bus contention (see [here]) happening for the address range of 64K to 256K.
 
Well, the behavior is exactly the same as what I tried before.

I do know what bus contention is. So are you saying there are buffer chips from the on-board ram to the system data bus?

I'll have to look at the schematic. Is that available on your site?

So assuming you are correct, what would you try next?

Keep in mind, the machine runs perfectly fine without the MicroRAM while running MS-DOS 3.3. It will not run MS-DOS 5 with just the 256K. Is that a clue?

I do not have other sets of RAM chips to try, and since these are very hard to find "new" I am not likely to find known good ones.


EDIT: Okay, I see U12 is enabled for RAM addresses up to 256K. Bummer.

So, what can I try next?
 
Last edited:
BillO said:
I do know what bus contention is. So are you saying there are buffer chips from the on-board ram to the system data bus?

I'll have to look at the schematic. Is that available on your site?
Click to expand...
Yes.
Page 1-42 of the APR84 edition of the IBM 5150 technical Reference (available at [here]).
See chip U12.

BillO said:
So assuming you are correct, what would you try next?
Click to expand...
Let me have a think.
 
BillO said:
Keep in mind, the machine runs perfectly fine without the MicroRAM while running MS-DOS 3.3. It will not run MS-DOS 5 with just the 256K. Is that a clue?
Click to expand...
MS-DOS 3.3 with only 256K motherboard RAM present: Stable
MS-DOS 5 with only 256K motherboard RAM present: Unstable

Can you confirm that the version of MS-DOS is the only changed variable ?

BillO said:
It will not run MS-DOS 5 with just the 256K.
Click to expand...
If the instability is present with the MicroRAM absent, why was the MicroRAM under suspicion ?
 
modem7 said:
MS-DOS 3.3 with only 256K motherboard RAM present: Stable
MS-DOS 5 with only 256K motherboard RAM present: Unstable

Can you confirm that the version of MS-DOS is the only changed variable ?


If the instability is present with the MicroRAM absent, why was the MicroRAM under suspicion ?
Click to expand...
It's not so much instability. I won't even load MS-DOS 5 with only 256K. It will access the disk for a few reads then just stops and locks up. So, the only way I can get it to run at all is with the MicroRAM. That means both the DOS and the MicroRAM.

Looks like we were posting at the same time. Yeah, I found U12.

Do you think there would be any value in removing and socketing U12 so that I could disable it and test using the MicroRAM for all the memory?
 
BillO said:
It's not so much instability. I won't even load MS-DOS 5 with only 256K. It will access the disk for a few reads then just stops and locks up.
Click to expand...
I don't think it's insufficient RAM, because my own earlier experiments with MS-DOS 6.22 indicate that it will boot with only 192 KB of RAM.

As an experiment, have you tried removing CONFIG.SYS and AUTOEXEC.BAT ?

BillO said:
Do you think there would be any value in removing and socketing U12 so that I could disable it and test using the MicroRAM for all the memory?
Click to expand...
So, quickly (relatively speaking) ruling out the motherboard RAM. (Of course, there's an assumption that the MicroRAM is fully functional).
It can't hurt.
 
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