[ltp] The mystery of missing RAM

[Chris Schumann]

On Tue, 18 Mar 2008, David A. Desrosiers wrote:

> On Mon, 2008-03-17 at 21:19 -0700, Bernard Tremblay wrote:
>> Did you check in the BIOS setup if your hole 4Gb was seen.  It should
>> be totally seen by the BIOS. May be there is other stuff that sit on
>> your motherboard (sound card, network card, modem, IBM stuff ?...)
>> and this use some memory from the main board.
>
> I'm skeptical. Why would a modem use physical system RAM? Why would a
> sound card use physical system RAM?

Since everything before the Core 2 is a 32-bit CPU, registers such as the 
address register have 32 bits, meaning the system can use 4GB of total 
address space.

The convention had been to use part of that memory for I/O and the rest 
for system memory. (The old Intel CPUs had a IO/MEM line to separate them, 
but apparently that isn't used on PCs.)

In any case, devices each need some address space to communicate with the 
device, and video cards can use a lot, and there may be conventions that 
use at least 512MB. In any case, a "normal" 32-bit OS can't use it all for 
RAM.

PAE should be able to use the extra 4 bits afforded and have a total 
address space of 64GB, but that is exactly like the old bank switching to 
get more than 1MB RAM on the 386 systems. XMS vs EMS anyone? Some kernels 
(such as Windows Server 2003 32-bit) will use that, but your hardware has 
to support it too.

This thread (http://forum.thinkpads.com/viewtopic.php?t=49703) would seem 
to indicate the T61/p DOES work with some PAE-capable software.

> More-importantly, why did this design change?

I think it's really not a change; just that PCs have never used separated 
I/O and memory spaces, and now that they've really run out of address 
bits, it shows up as a new problem.

> MY T42p has none of these problems, and sees the full 4GB of RAM, runs a
> 32-bit OS, and has an integrated graphics adapter.

The T42p can use only 2GB of RAM total according to every source I can 
find. If yours has 4GB, many people would be thrilled to know how you did 
that.

> Now that I've "upgraded" to an Intel Core 2 Duo, discrete (external)
> graphics, etc. I lose 25% of my physical RAM. I may sell these two
> laptops and go back to my T42p if I can't solve this.

You have many other options: Help the PAE folks get it to work on your 
machine, go back to 3GB of RAM (but lose ~8% performance boost with 
synchronized memory), use a 64-bit kernel.

Chris