Retronaut wrote on 2026-05-03, 11:37:

On boot its only showing 62,940 bytes free, which seems to be an odd number, not even 64kb

A 5150 normally doesn't show any actual RAM count from any ROM boot routine, including during the memory test. If you let it boot to ROM BASIC, it only shows up to the max that BASIC can normally utilize, which it 64KB, but also has reserved a small amount to BASIC. So on a system with 64KB or greater, that exact number of bytes 62,940 free is expected.

Boot to DOS and run a diagnostic program to actually visibly test the amount of memory. If you boot cold with 640KB configured at the switches, if the boot takes around 1 minute with no POST codes printed, that is a sign it does correctly see the installed memory.

Retronaut wrote on 2026-05-03, 11:37:

My next steps, without advice, would be to take out all the RAM in banks 3,4 and try for 128kb. If that fails, swap the ram from bank 3 into bank 2, repeat.
I'm not sure I have a RAM tester that could test this RAM, though I do have a T48 Exgecu chip writer...

No, don't try to test without fully populated banks. You'll trigger another problem based on how the 5150 BIOS works. Just boot, set the card and switches to proper amount, and use a diagnostic program or a program that can utilize up to 640KB.

Retronaut wrote on 2026-05-03, 11:37:

I checked using AI and its advice was that errors on banks 2,3,4 will lead to the machine dropping back to only using bank 1's RAM
So showing close to 64k would indicate this. Of course this is AI, so...
I have tried setting it to 128kb and also 256Kb WITHOUT the Megaplus II card installed, and same deal, 62,940 bytes free.

Despite the answer being commonly available there on the net, AI fails again.

the3dfxdude wrote on 2026-05-03, 13:47:

A 5150 normally doesn't show any actual RAM count from any ROM boot routine, including during the memory test. If you let it boot to ROM BASIC, it only shows up to the max that BASIC can normally utilize, which it 64KB, but also has reserved a small amount to BASIC. So on a system with 64KB or greater, that exact number of bytes 62,940 free is expected.

Well, this in itself is good to know. I had assumed that it was showing TOTAL free RAM, not just that available to Basic. Hrm, thats a bit of a design flat eh...

the3dfxdude wrote on 2026-05-03, 13:47:

Boot to DOS and run a diagnostic program to actually visibly test the amount of memory. If you boot cold with 640KB configured at the switches, if the boot takes around 1 minute with no POST codes printed, that is a sign it does correctly see the installed memory.

The problem is, I have no other PC with a working 5.25" disk drive to write out such a disk. I DO have physical MS DOS 3.3 disks, but I have not tested them yet, as I decided to go straight for a HD install approach, as that way CAN sidestep real media using emulation to make the HD .img. So I wonder, is there a util in MS DOS 3.3 that can show the RAM available, I assume not...

the3dfxdude wrote on 2026-05-03, 13:47:

No, don't try to test without fully populated banks. You'll trigger another problem based on how the 5150 BIOS works. Just boot, set the card and switches to proper amount, and use a diagnostic program or a program that can utilize up to 640KB.

Good point, its funny, I thought, yeah, that bug its an oddity, but its not going to bite me. But now its blocking me from doing some kind of diagnostic... bugger.

So I wonder, is there a util in MS DOS 3.3 that can show the RAM available, I assume not...

While the MEM command didn't exist yet in MS-DOS 3.3, the CHKDSK program printed out the memory besides doing its primary task. (Maybe not all OEM versions)

MagefromAntares wrote on 2026-05-03, 17:22:

While the MEM command didn't exist yet in MS-DOS 3.3, the CHKDSK program printed out the memory besides doing its primary task. (Maybe not all OEM versions)

Well, this was a stroke of luck, but with a nip first.
The owner of the machine, gave me a Floppy disk box (DIGITAL brand!) and it contains.

• Compaq MS DOS 3.3 disks 1+2
• Excel disks (about 5)
• and 2 blank disks

The MS DOS 3.3 disk 1 was my first test, after renovating the drive a little (clean heads) and... its dead, as was disk 2.
Then I tried one of the blank disks, nothing
and then the second blank disk, turned out to have Compaq Inspect on it, a diag program.
I ran this and it shows the machine, without the Magaplus II card really DOES have 256Kb on it, and it appears to be working just fine.
Next stage is to get it to boot, once the Megaplus II card is plugged in...

Glad that you have been able to confirm the motherboard installed memory size!

Unfortunately I have no experience with the Megaplus II card myself. However in it's technical specifications it is mentioned that it has a battery backed clock installed, and as it is an early 80s card, its battery might use a technology which can leak, so it might be worthy to replace the battery with a more modern version that is less prone to leaking. (As I said no direct knowledge of that card, but I did had bad experiences with batteries from that era)

Edit: I have checked and found a reliable looking source mentioning that the battery on that board is a non-chargeable lithium type, those are not prone to leaking afaik, so it most likely will not damage the card, however as it is non-chargeable the clock functionality might be unusable without replacing it.

Retronaut wrote on 2026-05-03, 16:41:

The problem is, I have no other PC with a working 5.25" disk drive to write out such a disk. I DO have physical MS DOS 3.3 disks, but I have not tested them yet, as I decided to go straight for a HD install approach, as that way CAN sidestep real media using emulation to make the HD .img. So I wonder, is there a util in MS DOS 3.3 that can show the RAM available, I assume not...

Per minuszerodegrees:

The POST of the 5150 BIOS stores the total amount of RAM (in KB) that it believes is fitted, into the BIOS Data Area (BDA), spec […]
Show full quote

The POST of the 5150 BIOS stores the total amount of RAM (in KB) that it believes is fitted, into the BIOS Data Area (BDA), specifically the word (little endian) at 0040:0013.
One way to read that amount is to:
1. Get into BASIC by removing any hard drive, and then boot without a boot floppy, then
2. In BASIC, enter the following two lines (in the order shown):
def seg = &h40
print peek(&h14)*256 + peek(&h13)

Note, as I mentioned previously, if you don't properly fit all the banks, then some BIOS are bugged and will store the wrong value at 0040:0013.