how to reserve memory in linux?

[Ming Liu]

Dear sudheer,
About the "mem=xx" argument, I checked it from the website and got such a 
explanation:

This argument has several purposes: The original purpose was to specify the 
amount of installed memory (or a value less than that if you wanted to 
limit the amount of memory available to linux).

>From the argument above, it says "mem=xx" is to specify the amount of 
memory available to Linux. I am not sure if I am right, but I think if I 
use such an argument, that reserved part (that 5MB you mentioned), is not 
available any more for Linux to access it. 

My original meaning is: specify a fixed physical address for a mount of 
memory for reserved use. The point is to make this part of memory fixed in 
the memory of physical address, and then another I/O peripheral could use 
DMA to access such a fixed physical address memory. However if I don't 
reserve such a part, I must let Linux to allocate such a memory part. And 
then Linux will not put it in a fixed physical address and then the 
peripheral will not know where is this part.

Am I right? Thanks for your idea.

Also other ideas are appreciated.

Regards
Ming

>Considering your setup as normal desktop, I hope this can be done by 
>giving
>"mem= xxm"  as the boot arguments.  Say the dmesg in your system 
>shows   "
>495MB LOWMEM available."  .  In the boot arguments you can give 
>mem=490m
>saying the linux to use only 490MB . The rest 5MB can be used later 
>as you
>wish.

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