How I lost (and subsequently found) 32 MB of RAM on my Compaq in OpenBSD

My inability to do more-than-simple mathematics at times has really put a cramp in my computing style.

In the case of OpenBSD and the $15 Laptop — the Compaq Armada 7770dmt — it has cost me 32 MB of RAM ever since I upgraded from 64 MB to the maximum 144 MB.

The reason is that in some Compaq's OpenBSD will not address more than 16 MB of RAM without some intervention on the part of the user.

This intervention is described in OpenBSD's installation FAQ. In order to have the system recognize the additional memory in the affected Compaqs, you must create a file called /etc/boot.conf containing one line, which tells the OS about your additional memory.

The example in the OpenBSD FAQ is for a system with 64 MB of RAM but with the aforementioned 16 MB being recognized.

To get the additional 48 MB recognized by the system, the following line should be in your new /etc/boot.conf file:

machine mem +0x3000000@0x1000000

That worked perfectly when I, also had 64 MB of RAM.

But when I added 128 MB to the 16 MB on the motherboard (removing the 48 MB of RAM that the laptop had when I originally got it working), I had to change the value in /etc/boot.conf.

And I didn't do my math correctly. This is what I had:

machine mem +0x6000000@0x1000000

I suspected that the machine wasn't using the entire 144 MB, but I'm not conversant enough in OpenBSD as opposed to Linux to a) figure out exactly how much memory the system was recognizing (a number available in the dmesg output) and how much the system was using and how much was still "free" (which can be seen in the output of the top utility.

I've been looking at OpenBSD's installation FAQ again recently because I'm eager to try the OS on my new/old Power Macintosh G4/466, and I looked at the section on Compaq memory again and realized my error.

I realized that if 48 MB is recognized with +0x3000000, then each +0x1000000 represents 16 MB of additional memory. And following that logic, 48 MB is 16 MB times three, yielding the FAQ example of +0x3000000.

And since I was adding 128 MB, that was 16 MB times ... how many? Eight is the answer. So instead of the +0x6000000, I needed the following line in /etc/boot.conf:

machine mem +0x8000000@0x1000000

I booted OpenBSD 4.2 (I don't have enough space in /usr to upgrade to 4.3 — such is the problem with making that partition only 1 GB in size when it needs to be, as the FAQ says, 5 GB). Then I changed the value in /etc/boot.conf, rebooted and saw the dmesg roll by during the reboot with the entire 144 MB being recognized.

Missing 32 MB all this time represents a very large performance hit, and I'll be anxious to see how gaining access to that memory changes performance under X.

For those keeping score, here's the relevant part of my OpenBSD dmesg AFTER doing the /etc/boot.conf fix:

real mem = 150564864 (143MB)
avail mem = 137732096 (131MB)